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

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.

Prevalence of bovine herpes virus-1 among reproductive disorders in cattle and buffaloes in Punjab region of India

Bovine Herpes Virus (BHV-1) is a virus prevalent among cattle and buffaloes which accounts for considerable reproductive failures. This study was undertaken with the objective of studying the prevalence of BHV-1 in reproductive tract infections of cattle and buffaloes in Punjab region in India. A total of 70 reproductive tract samples (like vaginal mucous, cervical mucous, uterine discharges, uterine pus and aborted materials like placenta, caruncles, foetal stomach contents, amniotic fluid and placental fluid) were taken from cattle and buffaloes from various areas of Punjab which were suffering from different reproductive disorders. The samples were screened for the presence of genome of BHV-1using PCR targeting gE gene. Out of 70 samples screened, only one sample was positive for the presence of BHV-1 genome which had an amplicon size of 468 bp, specific to the targeted gene. The study concluded that BHV-1 has very low prevalence among reproductive disorders in cattle and buffaloes in Punjab region, but has increased over last few years, particularly in female cattle and buffaloes.

Specificity protein 1 (Sp1) and glucocorticoid receptor (GR) stimulate bovine alphaherpesvirus 1 (BoHV-1) replication and cooperatively transactivate the immediate early transcription unit 1 promoter

Bovine alphaherpesvirus 1 (BoHV-1) infections cause respiratory tract disorders and suppress immune responses, which can culminate in bacterial pneumonia. Following acute infection, BoHV-1 establishes lifelong latency in sensory neurons present in trigeminal ganglia (TG) and unknown cells in pharyngeal tonsil. Latently infected calves consistently reactivate from latency after an intravenous injection of the synthetic corticosteroid dexamethasone (DEX), which mimics the effects of stress. The immediate early transcription unit 1 (IEtu1) promoter drives expression of infected cell protein 0 (bICP0) and bICP4, two key viral transcriptional regulators. The IEtu1 promoter contains two functional glucocorticoid receptor (GR) response elements (GREs), and this promoter is transactivated by GR, DEX, and certain Krüppel transcription factors that interact with GC-rich motifs, including consensus specificity protein 1 (Sp1) binding sites. Based on these observations, we hypothesized that Sp1 stimulates productive infection and transactivates key BoHV-1 promoters. DEX treatment of latently infected calves increased the number of Sp1+ TG neurons and cells in pharyngeal tonsil indicating that Sp1 expression is induced by stress. Silencing Sp1 protein expression with siRNA or mithramycin A, a drug that preferentially binds GC-rich DNA, significantly reduced BoHV-1 replication. Moreover, BoHV-1 infection of permissive cells increased Sp1 steady-state protein levels. In transient transfection studies, GR and Sp1 cooperatively transactivate IEtu1 promoter activity unless both GREs are mutated. Co-immunoprecipitation studies revealed that GR and Sp1 interact in mouse neuroblastoma cells (Neuro-2A) suggesting this interaction stimulates IEtu1 promoter activity. Collectively, these studies suggested that the cellular transcription factor Sp1 enhances productive infection and stress-induced BoHV-1 reactivation from latency.IMPORTANCEFollowing acute infection, bovine alphaherpesvirus 1 (BoHV-1) establishes lifelong latency in sensory neurons in trigeminal ganglia (TG) and pharyngeal tonsil. The synthetic corticosteroid dexamethasone consistently induces BoHV-1 reactivation from latency. The number of TG neurons and cells in pharyngeal tonsil expressing the cellular transcription factor specificity protein 1 (Sp1) protein increases during early stages of dexamethasone-induced reactivation from latency. Silencing Sp1 expression impairs BoHV-1 replication in permissive cells. Interestingly, mithramycin A, a neuroprotective antibiotic that preferentially binds GC-rich DNA, impairs Sp1 functions and reduces BoHV-1 replication suggesting that it is a potential antiviral drug. The glucocorticoid receptor (GR) and Sp1 cooperatively transactivate the BoHV-1 immediate early transcript unit 1 (IEtu1) promoter, which drives expression of infected cell protein 0 (bICP0) and bICP4. Mithramycin A also reduced Sp1- and GR-mediated transactivation of the IEtu1 promoter. These studies revealed that GR and Sp1 trigger viral gene expression and replication following stressful stimuli.

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.

Stress Triggers Expression of Bovine Herpesvirus 1 Infected Cell Protein 4 (bICP4) RNA during Early Stages of Reactivation from Latency in Pharyngeal Tonsil

Bovine herpesvirus 1 (BoHV-1), an important pathogen of cattle, establishes lifelong latency in sensory neurons within trigeminal ganglia (TG) after acute infection. The BoHV-1 latency-reactivation cycle, like other alphaherpesvirinae subfamily members, is essential for viral persistence and transmission. Notably, cells within pharyngeal tonsil (PT) also support a quiescent or latent BoHV-1 infection. The synthetic corticosteroid dexamethasone, which mimics the effects of stress, consistently induces BoHV-1 reactivation from latency allowing early stages of viral reactivation to be examined in the natural host. Based on previous studies, we hypothesized that stress-induced cellular factors trigger expression of key viral transcriptional regulatory genes. To explore this hypothesis, RNA-sequencing studies compared viral gene expression in PT during early stages of dexamethasone-induced reactivation from latency. Strikingly, RNA encoding infected cell protein 4 (bICP4), which is translated into an essential viral transcriptional regulatory protein, was detected 30 min after dexamethasone treatment. Ninety minutes after dexamethasone treatment bICP4 and, to a lesser extent, bICP0 RNA were detected in PT. All lytic cycle viral transcripts were detected within 3 h after dexamethasone treatment. Surprisingly, the latency related (LR) gene, the only viral gene abundantly expressed in latently infected TG neurons, was not detected in PT during latency. In TG neurons, bICP0 and the viral tegument protein VP16 are expressed before bICP4 during reactivation, suggesting distinct viral regulatory genes mediate reactivation from latency in PT versus TG neurons. Finally, these studies confirm PT is a biologically relevant site for BoHV-1 latency, reactivation from latency, and virus transmission. IMPORTANCE BoHV-1, a neurotropic herpesvirus, establishes, maintains, and reactivates from latency in neurons. BoHV-1 DNA is also detected in pharyngeal tonsil (PT) from latently infected calves. RNA-sequencing studies revealed the viral infected cell protein 4 (bICP4) RNA was expressed in PT of latently infected calves within 30 min after dexamethasone was used to initiate reactivation. As expected, bICP4 RNA was not detected during latency. All lytic cycle viral genes were expressed within 3 h after dexamethasone treatment. Conversely, bICP0 and the viral tegument protein VP16 are expressed prior to bICP4 in trigeminal ganglionic neurons during reactivation. The viral latency related gene, which is abundantly expressed in latently infected neurons, was not abundantly expressed in PT during latency. These studies provide new evidence PT is a biologically relevant site for BoHV-1 latency and reactivation. Finally, we predict other alphaherpesvirinae subfamily members utilize PT as a site for latency and reactivation.

A Pioneer Transcription Factor and Type I Nuclear Hormone Receptors Synergistically Activate the Bovine Herpesvirus 1 Infected Cell Protein 0 (ICP0) Early Promoter

Following bovine herpesvirus 1 (BoHV-1) acute infection of ocular, oral, or nasal cavities, sensory neurons within trigeminal ganglia are an important site for latency. Stress, as mimicked by the synthetic corticosteroid dexamethasone, consistently induces reactivation from latency. Expression of two key viral transcriptional regulatory proteins, BoHV-1 infected cell protein 0 (bICP0) and bICP4, are regulated by sequences within the immediate early promoter (IEtu1). A separate early promoter also drives bICP0 expression, presumably to ensure sufficient levels of this important transcriptional regulatory protein. Productive infection and bICP0 early promoter activity are cooperatively transactivated by Krüppel-like factor 4 (KLF4) and a type I nuclear hormone receptor (NHR), androgen receptor, glucocorticoid receptor, or progesterone receptor. The bICP0 early promoter contains three separate transcriptional enhancers that mediate cooperative transactivation. In contrast to the IEtu1 promoter, the bICP0 early promoter lacks consensus type I NHR binding sites. Consequently, we hypothesized that KLF4 and Sp1 binding sites are essential for type I NHR and KLF4 to transactivate the bICP0 promoter. Mutating KLF4 and Sp1 binding sites in each enhancer domain significantly reduced transactivation by KLF4 and a type I NHR. Chromatin immunoprecipitation (ChIP) studies demonstrated that occupancy of bICP0 early promoter sequences by KLF4 and type I NHR is significantly reduced when KLF4 and/or Sp1 binding sites are mutated. These studies suggest that cooperative transactivation of the bICP0 E promoter by type I NHRs and a stress-induced pioneer transcription factor (KLF4) promote viral replication and spread in neurons or nonneural cells in reproductive tissue. IMPORTANCE Understanding how stressful stimuli and changes in the cellular milieu mediate viral replication and gene expression in the natural host is important for developing therapeutic strategies that impair virus transmission and disease. For example, bovine herpesvirus 1 (BoHV-1) reactivation from latency is consistently induced by the synthetic corticosteroid dexamethasone, which mimics the effects of stress. Furthermore, BoHV-1 infection increases the incidence of abortion in pregnant cows, suggesting that sex hormones stimulate viral growth in certain tissues. Previous studies revealed that type I nuclear hormone receptors (NHRs) (androgen, glucocorticoid, or progesterone) and a pioneer transcription factor, Krüppel-like factor 4 (KLF4), cooperatively transactivate the BoHV-1 infected cell protein 0 (bICP0) early promoter. Transactivation was mediated by Sp1 and/or KLF4 consensus binding sites within the three transcriptional enhancers. These studies underscore the complexity by which BoHV-1 exploits type I NHR fluctuations to enhance viral gene expression, replication, and transmission in the natural host.

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.

Regulation of Krüppel-Like Factor 15 Expression by Herpes Simplex Virus Type 1 or Bovine Herpesvirus 1 Productive Infection

Expression of Krüppel-like factor 15 (KLF15), a stress-induced transcription factor, is induced during bovine herpesvirus 1 (BoHV-1) reactivation from latency, and KLF15 stimulates BoHV-1 replication. Transient transfection studies revealed that KLF15 and glucocorticoid receptor (GR) cooperatively transactivate the BoHV-1-immediate-early transcription unit 1 (IEtu1), herpes simplex virus type 1 (HSV-1) infected cell protein 0 (ICP0), and ICP4 promoters. The IEtu1 promoter drives expression of bICP0 and bICP4, two key BoHV-1 transcriptional regulatory proteins. Based on these studies, we hypothesized infection is a stressful stimulus that increases KLF15 expression and enhances productive infection. New studies demonstrated that silencing KLF15 impaired HSV-1 productive infection, and KLF15 steady-state protein levels were increased at late stages of productive infection. KLF15 was primarily localized to the nucleus following infection of cultured cells with HSV-1, but not BoHV-1. When cells were transfected with a KLF15 promoter construct and then infected with HSV-1, promoter activity was significantly increased. The ICP0 gene, and to a lesser extent, bICP0 transactivated the KLF15 promoter in the absence of other viral proteins. In contrast, BoHV-1 or HSV-1 encoded VP16 had no effect on KLF15 promoter activity. Collectively, these studies revealed that HSV-1 and BoHV-1 productive infection increased KLF15 steady-state protein levels, which correlated with increased virus production.

The Role of VP16 in the Life Cycle of Alphaherpesviruses

The protein encoded by the UL48 gene of alphaherpesviruses is named VP16 or alpha-gene-transactivating factor (α-TIF). In the early stage of viral replication, VP16 is an important transactivator that can activate the transcription of viral immediate-early genes, and in the late stage of viral replication, VP16, as a tegument, is involved in viral assembly. This review will explain the mechanism of VP16 acting as α-TIF to activate the transcription of viral immediate-early genes, its role in the transition from viral latency to reactivation, and its effects on viral assembly and maturation. In addition, this review also provides new insights for further research on the life cycle of alphaherpesviruses and the role of VP16 in the viral life cycle.

Pioneer transcription factors, progesterone receptor and Krüppel like transcription factor 4, cooperatively stimulate the bovine herpesvirus 1 ICP0 early promoter and productive late protein expression

Bovine herpesvirus 1 (BoHV-1), including commercially available modified live vaccines, readily infect the fetus and ovaries, which can cause reproductive failure. The BoHV-1 latency-reactivation cycle in sensory neurons further complicates reproductive failure because progesterone sporadically induces reactivation from latency. The progesterone receptor (PR) and Krüppel-like transcription factor 15 (KLF15) cooperatively stimulate productive infection and the immediate early transcription unit 1 (IEtu1) promoter. In addition to the IEtu1 promoter, the bICP0 gene also contains a separate early (E) promoter. In this study, we tested the hypothesis that PR and KLF family members transactivate the bICP0 E promoter. PR and KLF4 stimulated bICP0 E promoter activity and expression of late productive viral protein expression in a cooperative manner. Additional studies revealed three enhancer domains within the bICP0 E promoter were responsive to PR and KLF4. Chromatin immunoprecipitation studies demonstrated PR and KLF4 occupy bICP0 E promoter sequences in transfected Neuro-2A cells and at late times following infection of bovine kidney cells. Co-immunoprecipitation studies indicated PR and KLF4 stably interact with each other. These studies suggest cooperative activation of the bICP0 E promoter by PR and KLF4 correlate with interactions between these pioneer transcription factors.

Typical gene expression profile of pseudorabies virus reactivation from latency in swine trigeminal ganglion

Pseudorabies virus (PRV) establishes a lifelong latent infection in swine trigeminal ganglion (TG) following acute infection. Increased corticosteroid levels, due to stress, increases the incidence of reactivation from latency. Muscle injection combined with intravenous deliver of the synthetic corticosteroid dexamethasone (DEX) consistently induces reactivation from latency in pigs. In this study, PRV-free piglets were infected with PRV. Viral shedding in nasal and ocular swabs demonstrated that PRV infection entered the latent period. The anti-PRV antibody was detected by enzyme-linked immunosorbent assay and the serum neutralization test, which suggested that the PRV could establish latent infection in the presence of humoral immunity. Immunohistochemistry and viral genome detection of TG neurons suggested that PRV was reactivated from latency. Viral gene expressions of IE180, EP0, VP16, and LLT-intron were readily detected at 3-h post-DEX treatment, but gB, a γ1 gene, was not detectable. The differentially expressed phosphorylated proteins of TG neurons were analyzed by ITRAQ coupled with LC-MS/MS, and p-EIF2S2 differentially expression was confirmed by western blot assay. Taken together, our study provides the evidence that typical gene expression in PRV reactivation from latency in TG is disordered compared with known lytic infection in epithelial cells.

Ivermectin Inhibits Bovine Herpesvirus 1 DNA Polymerase Nuclear Import and Interferes with Viral Replication

Bovine herpesvirus1 (BoHV-1) is a major bovine pathogen. Despite several vaccines being available to prevent viral infection, outbreaks are frequent and cause important economic consequences worldwide. The development of new antiviral drugs is therefore highly desirable. In this context, viral genome replication represents a potential target for therapeutic intervention. BoHV-1 genome is a dsDNA molecule whose replication takes place in the nuclei of infected cells and is mediated by a viral encoded DNA polymerase holoenzyme. Here, we studied the physical interaction and subcellular localization of BoHV-1 DNA polymerase subunits in cells for the first time. By means of co-immunoprecipitation and confocal laser scanning microscopy (CLSM) experiments, we could show that the processivity factor of the DNA polymerase pUL42 is capable of being autonomously transported into the nucleus, whereas the catalytic subunit pUL30 is not. Accordingly, a putative classic NLS (cNLS) was identified on pUL42 but not on pUL30. Importantly, both proteins could interact in the absence of other viral proteins and their co-expression resulted in accumulation of UL30 to the cell nucleus. Treatment of cells with Ivermectin, an anti-parasitic drug which has been recently identified as an inhibitor of importin α/β-dependent nuclear transport, reduced UL42 nuclear import and specifically reduced BoHV-1 replication in a dose-dependent manner, while virus attachment and entry into cells were not affected. Therefore, this study provides a new option of antiviral therapy for BoHV-1 infection with Ivermectin.

Two Pioneer Transcription Factors, Krüppel-Like Transcription Factor 4 and Glucocorticoid Receptor, Cooperatively Transactivate the Bovine Herpesvirus 1 ICP0 Early Promoter and Stimulate Productive Infection

An important site for bovine herpesvirus 1 (BoHV-1) latency is sensory neurons within trigeminal ganglia (TG). The synthetic corticosteroid dexamethasone consistently induces BoHV-1 reactivation from latency. Expression of four Krüppel-like transcription factors (KLF), i.e., KLF4, KLF6, PLZF (promyelocytic leukemia zinc finger), and KLF15, are induced in TG neurons early during dexamethasone-induced reactivation. The glucocorticoid receptor (GR) and KLF15 form a feed-forward transcription loop that cooperatively transactivates the BoHV-1 immediate early transcription unit 1 (IEtu1) promoter that drives bovine infected cell protein 0 (bICP0) and bICP4 expression. Since the bICP0 gene also contains a separate early (E) promoter, we tested the hypothesis that GR and KLF family members transactivate the bICP0 E promoter. GR and KLF4, both pioneer transcription factors, cooperated to stimulate bICP0 E promoter activity in a ligand-independent manner in mouse neuroblastoma cells (Neuro-2A). Furthermore, GR and KLF4 stimulated productive infection. Mutating both half GR binding sites did not significantly reduce GR- and KLF4-mediated transactivation of the bICP0 E promoter, suggesting that a novel mechanism exists for transactivation. GR and KLF15 cooperatively stimulated bICP0 activity less efficiently than GR and KL4: however, KLF6, PLZF, and GR had little effect on the bICP0 E promoter. GR, KLF4, and KLF15 occupied bICP0 E promoter sequences in transfected Neuro-2A cells. GR and KLF15, but not KLF4, occupied the bICP0 E promoter at late times during productive infection of bovine cells. Collectively, these studies suggest that cooperative transactivation of the bICP0 E promoter by two pioneer transcription factors (GR and KLF4) correlates with stimulating lytic cycle viral gene expression following stressful stimuli.IMPORTANCE Bovine herpesvirus 1 (BoHV-1), an important bovine pathogen, establishes lifelong latency in sensory neurons. Reactivation from latency is consistently induced by the synthetic corticosteroid dexamethasone. We predict that increased corticosteroid levels activate the glucocorticoid receptor (GR). Consequently, viral gene expression is stimulated by the activated GR. The immediate early transcription unit 1 promoter (IEtu1) drives expression of two viral transcriptional regulatory proteins, bovine infected cell protein 0 (bICP0) and bICP4. Interestingly, a separate early promoter also drives bICP0 expression. Two pioneer transcription factors, GR and Krüppel-like transcription factor 4 (KLF4), cooperatively transactivate the bICP0 early (E) promoter. GR and KLF15 cooperate to stimulate bICP0 E promoter activity but significantly less than GR and KLF4. The bICP0 E promoter contains enhancer-like domains necessary for GR- and KLF4-mediated transactivation that are distinct from those for GR and KLF15. Stress-induced pioneer transcription factors are proposed to activate key viral promoters, including the bICP0 E promoter, during early stages of reactivation from latency.

An optimized aptamer-binding viral tegument protein VP8 inhibits the production of Bovine Herpesvirus-1 through blocking nucleocytoplasmic shuttling

Bovine herpesvirus 1 (BoHV-1) is a major pathogen of infectious bovine rhinotracheitis in bovine. Previously, we generated the aptamer IBRV A4 using systemic evolution of ligands by exponential enrichment. This aptamer inhibited infectivity of BoHV-1 by blocking viral particle absorption onto cell membranes. In this study, we found that the major tegument protein VP8 of BoHV-1 was involved in inhibition of infectious virus production by IBRV A4. We improved the affinity of IBRV A4 for VP8 by optimizing aptamer's structure and repeat conformation. An optimized aptamer, IBRV A4.7, was constructed with quadruple binding sites and a new stem-loop structure, which had a stronger binding affinity for VP8 or BoHV-1 than raw aptamer IBRV A4. IBRV A4.7 bound to VP8 with a dissociation constant (Kd) value of 0.2054 ± 0.03948 nM and bound to BoHV-1 with a Kd value of 0.3637 ± 0.05452 nM. Crucially, IBRV A4.7 had improved antiviral activity compared to IBRV A4, with a half-maximal inhibitory concentration of 1.16 ± 0.042 μM. Our results also revealed IBRV A4.7 inhibited BoHV-1 production in MDBK cells through blocking nucleocytoplasmic shuttling of viral VP8 in BoHV-1-infected MDBK cells. In conclusion, the aptamer IBRV A4.7 may have potency in preventing outbreaks in herds due to reactivation of latency.

Antagonizing the Glucocorticoid Receptor Impairs Explant-Induced Reactivation in Mice Latently Infected with Herpes Simplex Virus 1

Herpes simplex virus 1 (HSV-1) establishes lifelong latent infections in neurons. Reactivation from latency can lead to serious recurrent disease, including stromal keratitis, corneal scarring, blindness, and encephalitis. Although numerous studies link stress to an increase in the incidence of reactivation from latency and recurrent disease, the mechanism of action is not well understood. We hypothesized that stress, via corticosteroid-mediated activation of the glucocorticoid receptor (GR), stimulates viral gene expression and productive infection during reactivation from latency. Consequently, we tested whether GR activation by the synthetic corticosteroid dexamethasone influenced virus shedding during reactivation from latency using trigeminal ganglion (TG) explants from Swiss Webster mice latently infected with HSV-1, strain McKrae. TG explants from the latently infected mice shed significantly higher levels of virus when treated with dexamethasone. Conversely, virus shedding from TG explants was significantly impaired when they were incubated with medium containing a GR-specific antagonist (CORT-108297) or stripped fetal bovine serum, which lacks nuclear hormones and other growth factors. TG explants from latently infected, but not uninfected, TG contained significantly more GR-positive neurons following explant when treated with dexamethasone. Strikingly, VP16 protein expression was detected in TG neurons at 8 hours after explant whereas infected-cell protein 0 (ICP0) and ICP4 protein expression was not readily detected until 16 hours after explant. Expression of all three viral regulatory proteins was stimulated by dexamethasone. These studies indicated corticosteroid-mediated GR activation increased the number of TG neurons expressing viral regulatory proteins, which enhanced virus shedding during explant-induced reactivation from latency.IMPORTANCE Herpes simplex virus 1 (HSV-1) establishes lifelong latent infections in neurons within trigeminal ganglia (TG); periodically, reactivation from latency occurs, leading to virus transmission and recurrent disease. Chronic or acute stress increases the frequency of reactivation from latency; how this occurs is not well understood. Here, we demonstrate that the synthetic corticosteroid dexamethasone stimulated explant-induced reactivation from latency. Conversely, a glucocorticoid receptor (GR) antagonist significantly impaired reactivation from latency, indicating that GR activation stimulated explant-induced reactivation. The viral regulatory protein VP16 was readily detected in TG neurons prior to infected-cell protein 0 (ICP0) and ICP4 during explant-induced reactivation. Dexamethasone induced expression of all three viral regulatory proteins following TG explant. Whereas the immunosuppressive properties of corticosteroids would facilitate viral spread during reactivation from latency, these studies indicate GR activation increases the number of TG neurons that express viral regulatory proteins during early stages of explant-induced reactivation.

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.

Synergistic Activation of Bovine Herpesvirus 1 Productive Infection and Viral Regulatory Promoters by the Progesterone Receptor and Krüppel-Like Transcription Factor 15

Bovine herpesvirus 1 (BoHV-1), including modified live vaccines, readily infects the fetus and ovaries, which can lead to reproductive failure. The BoHV-1 latency reactivation cycle in sensory neurons may further complicate reproductive failure in pregnant cows. The immediate early transcription unit 1 (IEtu1) promoter drives expression of important viral transcriptional regulators (bICP0 and bICP4). This promoter contains two functional glucocorticoid receptor (GR) response elements (GREs) that have the potential to stimulate productive infection following stressful stimuli. Since progesterone and the progesterone receptor (PR) can activate many GREs, we hypothesized that the PR and/or progesterone regulates productive infection and viral transcription. New studies demonstrated that progesterone stimulated productive infection. Additional studies revealed the PR and Krüppel-like transcription factor 15 (KLF15) cooperated to stimulate productive infection and IEtu1 promoter activity. IEtu1 promoter activation required both GREs, which correlated with the ability of the PR to interact with wild-type (wt) GREs but not mutant GREs. KLF15 also cooperated with the PR to transactivate the bICP0 early promoter, a promoter that maintains bICP0 protein expression during productive infection. Intergenic viral DNA fragments (less than 400 bp) containing two GREs and putative KLF binding sites present within genes encoding unique long 52 (UL-52; component of DNA primase/helicase complex), Circ, bICP4, and IEtu2 were stimulated by KLF15 and the PR more than 10-fold, suggesting that additional viral promoters are activated by these transcription factors. Collectively, these studies suggest progesterone and the PR promote BoHV-1 spread to reproductive tissues, thus increasing the incidence of reproductive failure.IMPORTANCE Bovine herpesvirus 1 (BoHV-1) is the most frequently diagnosed cause of abortions in pregnant cows and can cause "abortion storms" in susceptible herds. Virulent field strains and even commercially available modified live vaccines can induce abortion, in part because BoHV-1 replicates efficiently in the ovary and corpus luteum. We now demonstrate that progesterone and the progesterone receptor (PR) stimulate productive infection. The BoHV-1 genome contains approximately 100 glucocorticoid receptor (GR) response elements (GREs). Interestingly, the PR can bind and activate many promoters that contain GREs. The PR and Krüppel-like transcription factor 15 (KLF15), which regulate key steps during embryo implantation, cooperate to stimulate productive infection and two viral promoters that drive expression of key viral transcriptional regulators. These studies suggest that the ability of progesterone and the PR to stimulate productive infection has the potential to promote virus spread in reproductive tissue and induce reproductive failure.

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 Cellular Coactivator HCF-1 Is Required for Glucocorticoid Receptor-Mediated Transcription of Bovine Herpesvirus 1 Immediate Early Genes

Following productive infection, bovine herpesvirus 1 (BoHV-1) establishes latency in sensory neurons. As in other alphaherpesviruses, expression of BoHV-1 immediate early (IE) genes is regulated by an enhancer complex containing the viral IE activator VP16, the cellular transcription factor Oct-1, and transcriptional coactivator HCF-1, which is assembled on an IE enhancer core element (TAATGARAT). Expression of the IE transcription unit that encodes the viral IE activators bICP0 and bICP4 may also be induced by the activated glucocorticoid receptor (GR) via two glucocorticoid response elements (GREs) located upstream of the enhancer core. Strikingly, lytic infection and reactivation from latency are consistently enhanced by glucocorticoid treatment in vivo As the coactivator HCF-1 is essential for IE gene expression of alphaherpesviruses and recruited by multiple transcription factors, we tested whether HCF-1 is required for glucocorticoid-induced IE gene expression. Depletion of HCF-1 reduced GR-mediated activation of the IE promoter in mouse neuroblastoma cells (Neuro-2A). More importantly, HCF-1-mediated GR activation of the promoter was dependent on the presence of GRE sites but independent of the TAATGARAT enhancer core element. HCF-1 was also recruited to the GRE region of a promoter lacking the enhancer core, consistent with a direct role of the coactivator in mediating GR-induced transcription. Similarly, during productive lytic infection, HCF-1 and GR occupied the IE region containing the GREs. These studies indicate HCF-1 is critical for GR activation of the viral IE genes and suggests that glucocorticoid induction of viral reactivation proceeds via an HCF-1-GR mechanism in the absence of the viral IE activator VP16.IMPORTANCE BoHV-1 transcription is rapidly activated during stress-induced reactivation from latency. The immediate early transcription unit 1 (IEtu1) promoter is regulated by the GR via two GREs. The IEtu1 promoter regulates expression of two viral transcriptional regulatory proteins, infected cell proteins 0 and 4 (bICP0 and bICP4), and thus must be stimulated during reactivation. This study demonstrates that activation of the IEtu1 promoter by the synthetic corticosteroid dexamethasone requires HCF-1. Interestingly, the GRE sites, but not the IE enhancer core element (TAATGARAT), were required for HCF-1-mediated GR promoter activation. The GR and HCF-1 were recruited to the IEtu1 promoter in transfected and infected cells. Collectively, these studies indicate that HCF-1 is critical for GR activation of the viral IE genes and suggest that an HCF-1-GR complex can stimulate the IEtu1 promoter in the absence of the viral IE activator VP16.

The Wnt Signaling Pathway Is Differentially Expressed during the Bovine Herpesvirus 1 Latency-Reactivation Cycle: Evidence That Two Protein Kinases Associated with Neuronal Survival, Akt3 and BMPR2, Are Expressed at Higher Levels during Latency

Sensory neurons in trigeminal ganglia (TG) of calves latently infected with bovine herpesvirus 1 (BoHV-1) abundantly express latency-related (LR) gene products, including a protein (ORF2) and two micro-RNAs. Recent studies in mouse neuroblastoma cells (Neuro-2A) demonstrated ORF2 interacts with β-catenin and a β-catenin coactivator, high-mobility group AT-hook 1 (HMGA1) protein, which correlates with increased β-catenin-dependent transcription and cell survival. β-Catenin and HMGA1 are readily detected in a subset of latently infected TG neurons but not TG neurons from uninfected calves or reactivation from latency. Consequently, we hypothesized that the Wnt/β-catenin signaling pathway is differentially expressed during the latency and reactivation cycle and an active Wnt pathway promotes latency. RNA-sequencing studies revealed that 102 genes associated with the Wnt/β-catenin signaling pathway were differentially expressed in TG during the latency-reactivation cycle in calves. Wnt agonists were generally expressed at higher levels during latency, but these levels decreased during dexamethasone-induced reactivation. The Wnt agonist bone morphogenetic protein receptor 2 (BMPR2) was intriguing because it encodes a serine/threonine receptor kinase that promotes neuronal differentiation and inhibits cell death. Another differentially expressed gene encodes a protein kinase (Akt3), which is significant because Akt activity enhances cell survival and is linked to herpes simplex virus 1 latency and neuronal survival. Additional studies demonstrated ORF2 increased Akt3 steady-state protein levels and interacted with Akt3 in transfected Neuro-2A cells, which correlated with Akt3 activation. Conversely, expression of Wnt antagonists increased during reactivation from latency. Collectively, these studies suggest Wnt signaling cooperates with LR gene products, in particular ORF2, to promote latency.IMPORTANCE Lifelong BoHV-1 latency primarily occurs in sensory neurons. The synthetic corticosteroid dexamethasone consistently induces reactivation from latency in calves. RNA sequencing studies revealed 102 genes associated with the Wnt/β-catenin signaling pathway are differentially regulated during the latency-reactivation cycle. Two protein kinases associated with the Wnt pathway, Akt3 and BMPR2, were expressed at higher levels during latency but were repressed during reactivation. Furthermore, five genes encoding soluble Wnt antagonists and β-catenin-dependent transcription inhibitors were induced during reactivation from latency. These findings are important because Wnt, BMPR2, and Akt3 promote neurogenesis and cell survival, processes crucial for lifelong viral latency. In transfected neuroblastoma cells, a viral protein expressed during latency (ORF2) interacts with and enhances Akt3 protein kinase activity. These findings provide insight into how cellular factors associated with the Wnt signaling pathway cooperate with LR gene products to regulate the BoHV-1 latency-reactivation cycle.

Antiviral Immunotoxin Against Bovine herpesvirus-1: Targeted Inhibition of Viral Replication and Apoptosis of Infected Cell

Bovine herpesvirus 1 (BoHV-1) is a highly contagious viral pathogen which causes infectious bovine rhinotracheitis in cattle worldwide. Currently, there is no antiviral prophylactic treatment available capable of mitigating the disease impact and facilitating recovery from latent infection. In this study, we have engineered a novel recombinant anti-BoHV-1 immunotoxin construct termed "BoScFv-PE38" that consists of a single-chain monoclonal antibody fragment (scFv) fused with an active domain of Pseudomonas exotoxin A as a toxic effector (PE38). The recombinant BoScFv-PE38 immunotoxin expressed in a prokaryotic expression system has specific binding affinity for BoHV-1 glycoprotein D (gD) with a dissociation constant (Kd) of 12.81 nM and for BoHV-1 virus particles with a Kd value of 97.63 nM. We demonstrate that the recombinant BoScFv-PE38 is internalized into MDBK cell compartments that inhibit BoHV-1 replication with a half-maximal inhibitory concentration (IC₅₀) of 4.95 ± 0.33 nM and a selective index (SI) of 456 ± 31. Furthermore, the BoScFv-PE38 exerted a cytotoxic effect through the induction of ATP and ammonia, leading to apoptosis of BoHV-1-infected cells and the inhibition of BoHV-1 replication in MDBK cells. Collectively, we show that BoScFv-PE38 can potentially be employed as a therapeutic agent for the treatment of BoHV-1 infection.

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 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.

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.

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.

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.

Identification of B cells as a major site for cyprinid herpesvirus 3 latency

Cyprinid herpesvirus 3 (CyHV-3), commonly known as koi herpesvirus (KHV), is a member of the Alloherpesviridae, and is a recently discovered emerging herpesvirus that is highly pathogenic for koi and common carp. Our previous study demonstrated that CyHV-3 becomes latent in peripheral white blood cells (WBC). In this study, CyHV-3 latency was further investigated in IgM(+) WBC. The presence of the CyHV-3 genome in IgM(+) WBC was about 20-fold greater than in IgM(-) WBC. To determine whether CyHV-3 expressed genes during latency, transcription from all eight open reading frames (ORFs) in the terminal repeat was investigated in IgM(+) WBC from koi with latent CyHV-3 infection. Only a spliced ORF6 transcript was found to be abundantly expressed in IgM(+) WBC from CyHV-3 latently infected koi. The spliced ORF6 transcript was also detected in vitro during productive infection as early as 1 day postinfection. The ORF6 transcript from in vitro infection begins at -127 bp upstream of the ATG codon and ends +188 bp downstream of the stop codon, +20 bp downstream of the polyadenylation signal. The hypothetical protein of ORF6 contains a consensus sequence with homology to a conserved domain of EBNA-3B and ICP4 from Epstein-Barr virus and herpes simplex virus 1, respectively, both members of the Herpesviridae. This is the first report of latent CyHV-3 in B cells and identification of gene transcription during latency for a member of the Alloherpesviridae.

IMPORTANCE

This is the first demonstration that a member of the Alloherpesviridae, cyprinid herpesvirus 3 (CyHV-3), establishes a latent infection in the B cells of its host, Cyprinus carpio. In addition, this is the first report of identification of gene transcription during latency for a member of Herpesvirales outside Herpesviridae. This is also the first report that the hypothetical protein of latent transcript of CyHV-3 contains a consensus sequence with homology to a conserved domain of EBNA-3B from Epstein-Barr virus and ICP4 from herpes simplex virus 1, which are genes important for latency. These strongly suggest that latency is evolutionally conserved across vertebrates.

Stress-induced cellular transcription factors expressed in trigeminal ganglionic neurons stimulate the herpes simplex virus 1 ICP0 promoter

Alphaherpesvirinae family members can reactivate from latency following stress. The synthetic corticosteroid dexamethasone induces certain cellular transcription factors in murine and bovine trigeminal ganglionic neurons. Three dexamethasone-induced transcription factors, Krüppel-like factor 15, Slug, and SPDEF, stimulated the herpes simplex virus type 1-infected cell protein 0 (ICP0) promoter more than 150-fold. Conversely, other viral promoters (VP16 and ICP4) were not strongly stimulated, suggesting that the ICP0 promoter is preferentially activated by dexamethasone-simulated stress.