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

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.

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.

m6A Methyltransferase METTL3 Reduces Hippocampal Neuron Apoptosis in a Mouse Model of Autism Through the MALAT1/SFRP2/Wnt/β-catenin Axis

OBJECTIVE

Hippocampal neuron apoptosis contributes to autism, while METTL3 has been documented to possess great potentials in neuron apoptosis. Our study probed into the role of METTL3 in neuron apoptosis in autism and to determine the underlying mechanism.

METHODS

Bioinformatics analysis was used to analyze expressed genes in autism samples. Institute of Cancer Research mice were treated with valproic acid to develop autism models. The function of METTL3 in autism-like symptoms in mice was analyzed with behavioral tests and histological examination of their hippocampal tissues. Primary mouse hippocampal neurons were extracted for in vitro studies. Downstream factors of METTL3 were explored and validated.

RESULTS

METTL3, MALAT1, and Wnt/β-catenin signaling were downregulated, while SFRP2 was upregulated in the hippocampal tissues of a mouse model of autism. METTL3 stabilized MALAT1 expression by promoting m6A modification of MALAT1. MALAT1 promoted SFRP2 methylation and led to reduced SFRP2 expression by recruiting DNMT1, DNMT3A, and DNMT3B to the promoter region of SFRP2. Furthermore, SFRP2 facilitated activation of the Wnt/β-catenin signaling. By this mechanism, METTL3 suppressed autism-like symptoms and hippocampal neuron apoptosis.

CONCLUSION

This research suggests that METTL3 can reduce autism-like symptoms and hippocampal neuron apoptosis by regulating the MALAT1/SFRP2/Wnt/β-catenin axis.

Involvement of the Wnt pathway in BVDV cytopathogenic strain replication in primary bovine cells

Background

Bovine viral diarrhea virus 1 (BVDV-1) of the pestivirus genus is an economically crippling virus in the cattle industry; this positive RNA virus causes mucosal disease resulting in reproductive losses and other disease syndromes. The pathogenesis mechanism of the disease caused by BVDV infection is not well understood; for a better understanding of in vivo host BVDV-1 interactions, we conducted a transcriptomic study of infected cells at different times post-infection.

Methods

We compared the permissiveness and cellular response of a BVDV-1 cytopathogenic strain on Madin-Darby Bovine Kidney cells (MDBK) and bovine lung primary cells, a model closer to in vivo infection. Then a RNAseq analysis was realized on the infected bovine lung primary cells, at 10 hpi and 30 hpi (hours post-infection), to identify transcriptomic signatures.

Results

RNAseq analysis on BVDV-1 infected bovine primary cells showed 2,759 and 5,376 differentially expressed genes at respectively 10 hpi and 30 hpi with an absolute Fold Change  ≥ 2. Among the different pathways deregulated, data analysis revealed a deregulation of Wnt signaling pathway, a conserved process that play a critical role in embryogenesis, cellular proliferation, and differentiation as well as in viral responses against viruses such as Influenza or Hepatitis C. We demonstrated here that the deregulation of the Wnt/βcatenin signaling pathway plays a role in viral replication of BVDV cp strain. Interestingly, we showed that the inhibition of this Wnt pathway using two inhibitors, FZM1 and iCRT14, induced a delay in onset of the establishment of a cytopathic effect of primary cells.

Conclusions

Thereby, this study highlighted a role of the Wnt signaling pathway in the BVDV-1 viral replication in bovine cells, suggesting an interesting option to explore as a new therapeutic target.

Stress Hormones Epinephrine and Corticosterone Selectively Reactivate HSV-1 and HSV-2 in Sympathetic and Sensory Neurons

Herpes simplex viruses 1 and 2 (HSV-1 and HSV-2) establish latency in sensory and autonomic neurons, from which they can reactivate to cause recurrent disease throughout the life of the host. Stress is strongly associated with HSV recurrences in humans and animal models. However, the mechanisms through which stress hormones act on the latent virus to cause reactivation are unknown. We show that the stress hormones epinephrine (EPI) and corticosterone (CORT) induce HSV-1 reactivation selectively in sympathetic neurons, but not sensory or parasympathetic neurons. Activation of multiple adrenergic receptors is necessary for EPI-induced HSV-1 reactivation, while CORT requires the glucocorticoid receptor. In contrast, CORT, but not EPI, induces HSV-2 reactivation in both sensory and sympathetic neurons through either glucocorticoid or mineralocorticoid receptors. Reactivation is dependent on different transcription factors for EPI and CORT, and coincides with rapid changes in viral gene expression, although genes differ for HSV-1 and HSV-2, and temporal kinetics differ for EPI and CORT. Thus, stress-induced reactivation mechanisms are neuron-specific, stimulus-specific and virus-specific. These findings have implications for differences in HSV-1 and HSV-2 recurrent disease patterns and frequencies, as well as development of targeted, more effective antivirals that may act on different responses in different types of neurons.

Regulation of herpes simplex virus type 1 latency-reactivation cycle and ocular disease by cellular signaling pathways

Following acute infection, herpes simplex virus type 1 (HSV-1) establishes life-long latency in sensory and other neurons. Recurrent ocular HSV-1 outbreaks are generally due to reactivation from latency. The HSV-1 latency-reactivation cycle is a complex virus-host relationship. The viral encoded latency-associated transcript (LAT) is abundantly expressed in latency and encodes several micro-RNAs and other small non-coding RNAs, which may regulate expression of key viral and cellular genes. Certain cellular signaling pathways, including Wnt/β-catenin and mTOR pathway, mediate certain aspect of the latency-reactivation cycle. Stress, via activation of the glucocorticoid receptor and other stress induced cellular transcription factors, are predicted to trigger reactivation from latency by stimulating viral gene expression and impairing immune responses and inflammation. These observations suggest stress and certain cellular signaling pathways play key roles in regulating the latency-reactivation cycle and recurrent ocular disease.

β-catenin has potential effects on the expression, subcellular localization, and release of high mobility group box 1 during bovine herpesvirus 1 productive infection in MDBK cell culture

High mobility group box 1 (HMGB1), a ubiquitous DNA-binding protein, can be released into extracellular space and function as a strong proinflammatory cytokine, which plays critical roles in the pathogenesis of various inflammatory diseases. Here, we showed that BoHV-1 productive infection in MDBK cells at later stage significantly increases HMGB1 mRNA expression and the protein release, but decreases the steady-state protein levels. Virus infection increases accumulation of HMGB1 protein in both nucleus and mitochondria, and relocalizes nuclear HMGB1 to assemble in highlighted foci via a confocal microscope assay. Interestingly, β-catenin-specific inhibitor iCRT14 is able to increase HMGB1 transcription and the protein release, and subcellular translocation in virus-infected cells. HMGB1-specific inhibitor, glycyrrhizin, could differentially affect virus gene transcription such as, the viral regulatory protein bICP0, bICP4 and bICP22, as well as glycoprotein gD. In summary, our data provides a novel mechanism that β-catenin signaling may regulate inflammatory response via affecting HMGB1 signaling.

Wnt antagonists suppress herpes simplex virus type 1 productive infection

Following acute infection of mucosal surfaces, herpes simplex virus 1 (HSV-1) establishes life-long latent infections within neurons, including sensory neurons in trigeminal ganglia (TG). Periodically, reactivation from latency occurs resulting in virus transmission and recurrent disease. In the absence of lytic cycle viral transcriptional proteins, host factors are predicted to mediate early stages of reactivation from latency. Previous studies suggested the canonical Wnt/β-catenin signaling pathway promotes productive infection. To further examine how the Wnt/β-catenin signaling pathway enhances productive infection, we examined two antagonists of the Wnt-signaling pathway. KYA1797K enhances formation of the β-catenin destruction complex, resulting in β-catenin degradation. Conversely, iCRT14 inhibits β-catenin dependent transcription by interfering with β-catenin interactions with T-cell factor/lymphoid enhancer factor (TCF)/Lef family of cellular transcription factors and interferes with TCF/Lef binding to DNA. iCRT14 and KYA1797K significantly inhibited HSV-1 productive infection in human and mouse neuronal cells and monkey kidney cells (VERO). Although iCRT14 was only effective when present throughout infection, delayed addition or early removal of KYA1797K did not significantly reduce its antiviral properties. KYA1797K had no effect on virus entry or penetration indicating it impairs certain aspects of viral replication. These studies demonstrated β-catenin promotes HSV-1 productive infection and indicate antagonists of the canonical Wnt/β-catenin signaling pathway may be effective anti-HSV therapeutic agents.

Inhibition of Stress-Induced Viral Promoters by a Bovine Herpesvirus 1 Non-Coding RNA and the Cellular Transcription Factor, β-Catenin

The ability to establish, maintain, and reactivate from latency in sensory neurons within trigeminal ganglia (TG) is crucial for bovine herpesvirus 1 (BoHV-1) transmission. In contrast to lytic infection, the only viral gene abundantly expressed during latency is the latency-related (LR) gene. The synthetic corticosteroid dexamethasone consistently induces reactivation from latency, in part because the glucocorticoid receptor (GR) transactivates viral promoters that drive expression of key viral transcriptional regulator proteins (bICP0 and bICP4). Within hours after dexamethasone treatment of latently infected calves, LR gene products and β-catenin are not readily detected in TG neurons. Hence, we hypothesized that LR gene products and/or β-catenin restrict GR-mediated transcriptional activation. A plasmid expressing LR RNA sequences that span open reading frame 2 (ORF2-Stop) inhibited GR-mediated transactivation of the BoHV-1 immediate early transcription unit 1 (IEtu1) and mouse mammary tumor virus (MMTV) promoter activity in mouse neuroblastoma cells (Neuro-2A). ORF2-Stop also reduced productive infection and GR steady-state protein levels in transfected Neuro-2A cells. Additional studies revealed that the constitutively active β-catenin mutant reduced the transactivation of the IEtu1 promoter by GR and dexamethasone. Collectively, these studies suggest ORF2 RNA sequences and Wnt/β-catenin signaling pathway actively promote maintenance of latency, in part, by impairing GR-mediated gene expression.

A Dynamic Role of Mastermind-Like 1: A Journey Through the Main (Path)ways Between Development and Cancer

Major signaling pathways, such as Notch, Hedgehog (Hh), Wnt/β-catenin and Hippo, are targeted by a plethora of physiological and pathological stimuli, ultimately resulting in the modulation of genes that act coordinately to establish specific biological processes. Many biological programs are strictly controlled by the assembly of multiprotein complexes into the nucleus, where a regulated recruitment of specific transcription factors and coactivators on gene promoter region leads to different transcriptional outcomes. MAML1 results to be a versatile coactivator, able to set up synergistic interlinking with pivotal signaling cascades and able to coordinate the network of cross-talking pathways. Accordingly, despite its original identification as a component of the Notch signaling pathway, several recent reports suggest a more articulated role for MAML1 protein, showing that it is able to sustain/empower Wnt/β-catenin, Hh and Hippo pathways, in a Notch-independent manner. For this reason, MAML1 may be associated to a molecular “switch”, with the function to control the activation of major signaling pathways, triggering in this way critical biological processes during embryonic and post-natal life. In this review, we summarize the current knowledge about the pleiotropic role played by MAML proteins, in particular MAML1, and we recapitulate how it takes part actively in physiological and pathological signaling networks. On this point, we also discuss the contribution of MAML proteins to malignant transformation. Accordingly, genetic alterations or impaired expression of MAML proteins may lead to a deregulated crosstalk among the pathways, culminating in a series of pathological disorders, including cancer development. Given their central role, a better knowledge of the molecular mechanisms that regulate the interplay of MAML proteins with several signaling pathways involved in tumorigenesis may open up novel opportunities for an attractive molecular targeted anticancer therapy.

Specific Akt Family Members Impair Stress-Mediated Transactivation of Viral Promoters and Enhance Neuronal Differentiation: Important Functions for Maintaining Latency

Neurotropic Alphaherpesvirinae subfamily members such as bovine herpesvirus 1 (BoHV-1) and herpes simplex virus 1 (HSV-1) establish and maintain lifelong latent infections in neurons. Following infection of ocular, oral, or nasal cavities, sensory neurons within trigeminal ganglia (TG) are an important site for latency. Certain external stressors can trigger reactivation from latency, in part because activation of the glucocorticoid receptor (GR) stimulates productive infection and promoters that drive expression of key viral transcriptional regulators. The Akt serine/threonine protein kinase family is linked to maintaining latency. For example, Akt3 is detected in more TG neurons during BoHV-1 latency than in reactivation and uninfected calves. Furthermore, Akt signaling correlates with maintaining HSV-1 latency in certain neuronal models of latency. Finally, an active Akt protein kinase is crucial for the ability of the HSV-1 latency-associated transcript (LAT) to inhibit apoptosis in neuronal cell lines. Consequently, we hypothesized that viral and/or cellular factors impair stress-induced transcription and reduce the incidence of reactivation triggered by low levels of stress. New studies demonstrate that Akt1 and Akt2, but not Akt3, significantly reduced GR-mediated transactivation of the BoHV-1 immediate early transcription unit 1 (IEtu1) promoter, the HSV-1 infected cell protein 0 (ICP0) promoter, and the mouse mammary tumor virus long terminal repeat (MMTV-LTR). Akt3, but not Akt1 or Akt2, significantly enhanced neurite formation in mouse neuroblastoma cells, which correlates with repairing damaged neurons. These studies suggest that unique biological properties of the three Akt family members promote the maintenance of latency in differentiated neurons.IMPORTANCE External stressful stimuli are known to increase the incidence of reactivation of Alphaherpesvirinae subfamily members. Activation of the glucocorticoid receptor (GR) by the synthetic corticosteroid dexamethasone (DEX) stimulates bovine herpesvirus 1 (BoHV-1) and herpes simplex virus 1 (HSV-1) reactivation. Furthermore, GR and dexamethasone stimulate productive infection and promoters that drive expression of viral transcriptional regulators. These observations lead us to predict that stress-induced transcription is impaired by factors abundantly expressed during latency. Interestingly, activation of the Akt family of serine/threonine protein kinases is linked to maintenance of latency. New studies reveal that Akt1 and Ak2, but not Akt3, impaired GR- and dexamethasone-mediated transactivation of the BoHV-1 immediate early transcription unit 1 and HSV-1 ICP0 promoters. Strikingly, Akt3, but not Akt1 or Akt2, stimulated neurite formation in mouse neuroblastoma cells, a requirement for neurogenesis. These studies provide insight into how Akt family members may promote the maintenance of lifelong latency.

β-cantenin is potentially involved in the regulation of c-Jun signaling following bovine herpesvirus 1 infection

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BoHV-1 infection promotes nucleus accumulation of p-c-Jun(S73) and p-β-catenin(S552)

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The association between β-catenin and c-Jun in in nucleus is readily detected following BoHV-1 infection.

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BoHV-1 infection stimulates the expression and activation of c-Jun potentially through β-catenin.

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BoHV-1 infection leads to relocalization of nucleus c-Jun to form specific foci.

C-Jun, activated by various extracellular signals, is important for cell differentiation, proliferation, apoptosis, and inflammatory responses. We have previously reported that bovine herpesvirus 1 (BoHV-1) infection in MDBK cells stimulates the c-Jun NH2-terminal kinase (JNK)/c-Jun cascade for efficient replication. However, the mechanisms regarding the regulation of c-Jun following BoHV-1 infection remain unknown. In this study, we show that virus infection increases accumulation of p-c-Jun(S73) (phosphorylated c-Jun at Ser73) and p-β-catenin(S552) in the nucleus, resulting in relocalized nuclear p-c-Jun(S73) to assemble in highlighted punctum via a confocal microscope assay. An association between β-catenin and c-Jun in the nucleus was readily detected in virus-infected, but not mock-infected cells. Interestingly, β-catenin was found to be involved in the regulation of c-Jun signaling in virus-infected cells as iCRT14, a β-catenin-specific inhibitor that can inhibit β-catenin-dependent transcriptional activity, was able to decrease protein expression and phosphorylation of c-Jun. Furthermore, we suggest that BoHV-1 infection stimulates c-Jun phosphorylation regulated by β-catenin via both c-Jun NH2-terminal kinase (JNK)-dependent and JNK-independent mechanisms. These data add to our knowledge regarding the regulation of c-Jun following virus infection and further support the important roles of β-catenin signaling playing in BoHV-1 infection.

Genome-wide CRISPR screening reveals genes essential for cell viability and resistance to abiotic and biotic stresses in Bombyx mori

High-throughput genetic screens are powerful methods to interrogate gene function on a genome-wide scale and identify genes responsible to certain stresses. Here, we developed a piggyBac strategy to deliver pooled sgRNA libraries stably into cell lines. We used this strategy to conduct a screen based on genome-wide clustered regularly interspaced short palindromic repeat technology (CRISPR)-Cas9 in Bombyx mori cells. We first constructed a single guide RNA (sgRNA) library containing 94,000 sgRNAs, which targeted 16,571 protein-coding genes. We then generated knockout collections in BmE cells using the piggyBac transposon. We identified 1006 genes that are essential for cell viability under normal growth conditions. Of the identified genes, 82.4% (829 genes) were homologous to essential genes in seven animal species. We also identified 838 genes whose loss facilitated cell growth. Next, we performed context-specific positive screens for resistance to biotic or nonbiotic stresses using temperature and baculovirus separately, which identified several key genes and pathways from each screen. Collectively, our results provide a novel and versatile platform for functional annotations of B. mori genomes and deciphering key genes responsible for various conditions. This study also shows the effectiveness, practicality, and convenience of genome-wide CRISPR screens in nonmodel organisms.

Herpes simplex virus 1 regulates β-catenin expression in TG neurons during the latency-reactivation cycle

When herpes simplex virus 1 (HSV-1) infection is initiated in the ocular, nasal, or oral cavity, sensory neurons within trigeminal ganglia (TG) become infected. Following a burst of viral transcription in TG neurons, lytic cycle viral genes are suppressed and latency is established. The latency-associated transcript (LAT) is the only viral gene abundantly expressed during latency, and LAT expression is important for the latency-reactivation cycle. Reactivation from latency is required for virus transmission and recurrent disease, including encephalitis. The Wnt/β-catenin signaling pathway is differentially expressed in TG during the bovine herpesvirus 1 latency-reactivation cycle. Hence, we hypothesized HSV-1 regulates the Wnt/β-catenin pathway and promotes maintenance of latency because this pathway enhances neuronal survival and axonal repair. New studies revealed β-catenin was expressed in significantly more TG neurons during latency compared to TG from uninfected mice or mice latently infected with a LAT-/- mutant virus. When TG explants were incubated with media containing dexamethasone to stimulate reactivation, significantly fewer β-catenin+ TG neurons were detected. Conversely, TG explants from uninfected mice or mice latently infected with a LAT-/- mutant increased the number of β-catenin+ TG neurons in the presence of DEX relative to samples not treated with DEX. Impairing Wnt signaling with small molecule antagonists reduced virus shedding during explant-induced reactivation. These studies suggested β-catenin was differentially expressed during the latency-reactivation cycle, in part due to LAT expression.

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.

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 canonical Wnt/β-catenin signaling pathway stimulates herpes simplex virus 1 productive infection

The ability of herpes simplex virus 1 (HSV-1) to replicate efficiently in differentiated cells is regulated by cellular factors that stimulate viral gene expression, cell survival, and viral morphogenesis. Activation of the canonical Wnt signaling pathway generally increases β-catenin protein levels, cell survival, and growth in dividing cells suggesting this important signaling pathway regulates productive infection. In this study, we demonstrated that a β-catenin specific small molecule inhibitor (iCRT14) reduced HSV-1 titers approximately 10-fold in primary human lung fibroblasts and Vero cells. Furthermore, β-catenin dependent transcription was increased at late times after infection and as expected iCRT14 reduced β-catenin dependent transcription. Although HSV-1 infection increased β-catenin steady state protein levels approximately 4-fold in Vero cells, there was only a nominal increase in human lung fibroblasts. We hypothesized that VP16 regulates β-catenin dependent transcription because VP16 is a viral regulatory protein expressed at late times after infection. In the absence of other viral proteins, VP16 increased β-catenin dependent transcription and β-catenin steady state protein levels. Collectively, these studies suggested the cellular transcription factor β-catenin stimulates productive infection, in part because VP16 enhances β-catenin dependent transcription.

Molecular Determinants and the Regulation of Human Cytomegalovirus Latency and Reactivation

Human cytomegalovirus (HCMV) is a beta herpesvirus that establishes a life-long persistence in the host, like all herpesviruses, by way of a latent infection. During latency, viral genomes are maintained in a quieted state. Virus replication can be reactivated from latency in response to changes in cellular signaling caused by stress or differentiation. The past decade has brought great insights into the molecular basis of HCMV latency. Here, we review the complex persistence of HCMV with consideration of latent reservoirs, viral determinants and their host interactions, and host signaling and the control of cellular and viral gene expression that contributes to the establishment of and reactivation from latency.

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.

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.

Effects of the synthetic corticosteroid dexamethasone on bovine herpesvirus 1 productive infection

Sensory neurons are a primary site for life-long latency of bovine herpesvirus 1 (BoHV-1). The synthetic corticosteroid dexamethasone induces reactivation from latency and productive infection, in part because the BoHV-1 genome contains more than 100 glucocorticoid receptor (GR) responsive elements (GREs). Two GREs in the immediate early transcription unit 1 promoter are required for dexamethasone induction. Recent studies also demonstrated that the serum and glucocorticoid receptor protein kinase (SGK) family stimulated BoHV-1 replication. Consequently, we hypothesized that dexamethasone influences several aspects of productive infection. In this study, we demonstrated that dexamethasone increased expression of the immediate early protein bICP4, certain late transcripts, and UL23 (thymidine kinase) by four hours after infection. SGK1 expression and Akt phosphorylation were also stimulated during early stages of infection and dexamethasone treatment further increased this effect. These studies suggest that stress, as mimicked by dexamethasone treatment, has the potential to stimulate productive infection by multiple pathways.

Potential Role for a β-Catenin Coactivator (High-Mobility Group AT-Hook 1 Protein) during the Latency-Reactivation Cycle of Bovine Herpesvirus 1

The latency-related (LR) RNA encoded by bovine herpesvirus 1 (BoHV-1) is abundantly expressed in latently infected sensory neurons. Although the LR gene encodes several products, ORF2 appears to mediate important steps during the latency-reactivation cycle because a mutant virus containing stop codons at the amino terminus of ORF2 does not reactivate from latency in calves. We recently found that the Wnt/β-catenin signaling pathway is regulated during the BoHV-1 latency-reactivation cycle (Y. Liu, M. Hancock, A. Workman, A. Doster, and C. Jones, J Virol 90:3148-3159, 2016). In the present study, a β-catenin coactivator, high-mobility group AT-hook 1 protein (HMGA1), was detected in significantly more neurons in the trigeminal ganglia of latently infected calves than in those of uninfected calves. Consequently, we hypothesized that HMGA1 cooperates with ORF2 and β-catenin to maintain latency. In support of this hypothesis, coimmunoprecipitation studies demonstrated that ORF2 stably interacts with a complex containing β-catenin and/or HMGA1 in transfected mouse neuroblastoma (Neuro-2A) cells. Confocal microscopy provided evidence that ORF2 was relocalized by HMGA1 and β-catenin in Neuro-2A cells. ORF2 consistently enhanced the ability of HMGA1 to stimulate β-catenin-dependent transcription, suggesting that interactions between ORF2 and a complex containing β-catenin and HMGA1 have functional significance. An ORF2 stop codon mutant, an ORF2 nuclear localization mutant, or a mutant lacking the 5 protein kinase A or C phosphorylation sites interfered with its ability to stimulate β-catenin-dependent transcription. Since the canonical Wnt/β-catenin signaling pathway promotes neurogenesis (synapse formation and remodeling) and inhibits neurodegeneration, interactions between ORF2, HMGA1, and β-catenin may be important for certain aspects of the latency-reactivation cycle.IMPORTANCE The lifelong latency of bovine herpesvirus 1 (BoHV-1) requires that significant numbers of infected sensory neurons survive infection and maintain normal functions. Consequently, we hypothesize that viral products expressed during latency cooperate with neuronal factors to maintain latency. Our studies revealed that a β-catenin coactivator, high-mobility group AT-hook 1 protein (HMGA1), was readily detected in a subset of trigeminal ganglion neurons in latently infected calves but not in uninfected calves. A viral protein (ORF2) expressed in latently infected neurons interacted with β-catenin and HMGA1 in transfected cells, which resulted in the nuclear localization of β-catenin. This interaction correlated with the ability of ORF2 to stimulate the coactivator functions of HMGA1. These findings are significant because the canonical Wnt/β-catenin signaling pathway promotes neurogenesis and inhibits neurodegeneration.

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.