Independent Cis-Regulatory Modules within the Herpes Simplex Virus 1 Infected Cell Protein 0 (ICP0) Promoter Are Transactivated by Krüppel-like Factor 15 and Glucocorticoid Receptor

Human alpha-herpesvirus 1 (HSV-1) viral replication and reactivation from latency are expedited by the glucocorticoid receptor

Acute human alpha-herpesvirus 1 (HSV-1) infection leads to infection of neurons within trigeminal ganglia (TG), brainstem, and other regions of the central nervous system. Lytic cycle viral gene expression is subsequently silenced, a subset of neurons survive infection, and life-long latency is established. In contrast to lytic infection, the latency-associated transcript (LAT) is the only viral gene product abundantly expressed in latently infected neurons. Stress (acute or chronic), UV light, or heat stress increases the incidence of reactivation from latency in humans and mouse models of infection. Ironically, these divergent reactivation stimuli activate the glucocorticoid receptor (GR). Recent studies revealed GR and Krüppel-like factors (KLF), KLF4 or KLF15 for example, cooperatively transactivate the infected cell protein 0 (ICP0) promoter and cis-regulatory motifs that activate ICP4 and ICP27 promoter activity. GR and KLF4 are "pioneer transcription factors" that specifically bind DNA even when it exists as heterochromatin; consequently, chromatin is remodeled, and transcription is activated. Conversely, a VP16 cis-regulatory motif is transactivated by GR and Slug but not KLF family members. Female mice that express a GR containing a serine → alanine mutation at position 229 (GRS229A) shed significantly lower HSV-1 levels compared with age-matched male GRS229A mice or wild-type parental C57BL/6 mice during reactivation from latency. These observations imply GR and stress-induced cellular transcription factors play an important role during reactivation from latency by activating key viral promoters. GR activation may also enhance virus spread by impairing immune and inflammatory responses.

Viral Appropriation of Specificity Protein 1 (Sp1): The Role of Sp1 in Human Retro- and DNA Viruses in Promoter Activation and Beyond

Specificity protein 1 (Sp1) is a highly ubiquitous transcription factor and one employed by numerous viruses to complete their life cycles. In this review, we start by summarizing the relationships between Sp1 function, DNA binding, and structural motifs. We then describe the role Sp1 plays in transcriptional activation of seven viral families, composed of human retro- and DNA viruses, with a focus on key promoter regions. Additionally, we discuss pathways in common across multiple viruses, highlighting the importance of the cell regulatory role of Sp1. We also describe Sp1-related epigenetic and protein post-translational modifications during viral infection and how they relate to Sp1 binding. Finally, with these insights in mind, we comment on the potential for Sp1-targeting therapies, such as repurposing drugs currently in use in the anti-cancer realm, and what limitations such agents would have as antivirals.

Glucocorticoid Receptor (GR) and Specificity Protein 1 (Sp1) or Sp3 Transactivate the Bovine Alphaherpesvirus 1 (BoHV-1)-Infected Cell Protein 0 Early Promoter

Bovine alphaherpesvirus 1 (BoHV-1) acute infection leads to latently infected sensory neurons in trigeminal ganglia. During lytic infection, the immediate early expression of infected cell protein 0 (bICP0) and bICP4 is regulated by an immediate early transcription unit 1 (IEtu1) promoter. A separate bICP0 early (E) promoter drives bICP0 as an early viral gene, presumably to sustain high levels during productive infection. Notably, bICP0 protein expression is detected before bICP4 during reactivation from latency, suggesting the bICP0 E promoter drives bICP0 protein expression during the early phases of reactivation from latency. The glucocorticoid receptor (GR) and Krüppel-like factor 4 (KLF4) cooperatively transactivate the bICP0 E promoter despite this promoter lacks a consensus GR response element (GRE). KLF and specificity protein (Sp) family members comprise a "super-family" of transcription factors. Consequently, we hypothesized Sp1 and Sp3 transactivated the bICP0 E promoter. These studies revealed GR and Sp3 or Sp1 cooperatively transactivated bICP0 E promoter activity. KLF4 and Sp3, but not Sp1, had an additive effect on bICP0 E promoter activity. Mutating the consensus Sp1 and CACCC binding sites proximal to the TATA box impaired promoter activity more than the Sp1 sites further upstream from the TATA box.

Glucocorticoid receptor and specificity protein 1 (Sp1) or Sp3 transactivate HSV-1 ICP0 promoter sequences but a GC-rich binding antibiotic, Mithramycin A, impairs reactivation from latency

Glucocorticoid receptor (GR) activation enhances Human alpha-herpes virus 1 (HSV-1) replication and explant-induced reactivation from latency. Furthermore, GR and Krüppel-like factor 15 (KLF15) cooperatively transactivate cis-regulatory modules (CRMs) that drive expression of infected cell protein 0 (ICP0), ICP4, and ICP27. KLF and specificity protein (Sp) family members bind GC-rich or C-rich sequences and belong to the same super-family of transcription factors. Based on these observations, we hypothesized CRMs spanning the ICP0 promoter are transactivated by GR and Sp1 or Sp3. CRM-A (-800 to -635), CRM-B (-485 to -635), and CRM-D (-232 to -24), but not CRM-C, were significantly transactivated by GR, DEX, and Sp1 or Sp3 in mouse neuroblastoma cells (Neuro-2A). Mutagenesis of Sp1/Sp3 binding sites were important for transactivation of CRM-A and CRM-B. Chromatin immunoprecipitation studies revealed significantly higher levels of GR occupied ICP0 promoter sequences when Sp1 or Sp3 was over-expressed suggesting these transcriptions factors recruit GR to ICP0 CRM sequences. Mithramycin A, an antibiotic that preferentially binds GC-rich DNA and impairs Sp1/Sp3 dependent transactivation and reduced virus shedding during reactivation from latency in mice latently infected with HSV-1. These studies indicate GR and certain stress-induced cellular transcription factors preferentially bind GC rich DNA, which stimulates HSV-1 gene expression and reactivation from latency in trigeminal ganglia of latently infected mice.

The Distal Promoter of the B438L Gene of African Swine Fever Virus Is Responsible for the Transcription of the Alternatively Spliced B169L

The B169L protein (pB169L) of African swine fever virus (ASFV) is a structural protein with an unidentified function during the virus replication. The sequences of the B169L gene and the downstream B438L gene are separated by short intergenic regions. However, the regulatory mode of the gene transcription remains unknown. Here, we identified two distinct promoter regions and two transcription start sites (TSSs) located upstream of the open reading frame (ORF) of B438L. Using the promoter reporter system, we demonstrated that the cis activity of the ORF proximal promoter exhibited significantly higher levels compared with that of the distal promoter located in the B169L gene. Furthermore, transfection with the plasmids with two different promoters for B438L could initiate the transcription and expression of the B438L gene in HEK293T cells, and the cis activity of the ORF proximal promoter also displayed higher activities compared with the distal promoter. Interestingly, the B438L distal promoter also initiated the transcription of the alternatively spliced B169L mRNA (B169L mRNA2) encoding a truncated pB169L (tpB169L) (amino acids 92-169), and the gene transcription efficiency was increased upon mutation of the initiation codon located upstream of the alternatively spliced B169L gene. Taken together, we demonstrated that the distal promoter of B438L gene initiates the transcription of both the B438L mRNA and B169L mRNA2. Comprehensive analysis of the transcriptional regulatory mode of the B438L gene is beneficial for the understanding of the association of B438L protein and pB169L and the construction of the gene-deleted ASFV.

Glucocorticoid receptor and specificity protein 1 (Sp1) or Sp3, but not the antibiotic Mithramycin A, stimulates human alphaherpesvirus 1 (HSV-1) replication

Following acute human alphaherpesvirus 1 (HSV-1) infection of oral-facial mucosal surfaces, sensory neurons in trigeminal ganglia (TG) are important sites for life-long latency. Neurons in the central nervous system, including brainstem, also harbor viral genomes during latency. Periodically, certain cellular stressors trigger reactivation from latency, which can lead to recurrent HSV-1 disease: herpes labialis, herpes stromal keratitis, and encephalitis for example. Activation of the glucocorticoid receptor (GR) by stressful stimuli enhances HSV-1 gene expression, replication, and explant-induced reactivation. GR and certain stress-induced Krüppel like factors (KLF) cooperatively transactivate cis-regulatory modules (CRM) that drive expression of viral transcriptional regulatory proteins (ICP0, ICP4, and ICP27). These CRMs lack GR response elements (GRE); however, specificity protein 1 (Sp1) binding sites are crucial for GR and KLF15 or KLF4 mediated transactivation. Hence, we tested whether Sp1 or Sp3 regulate viral replication and transactivation of the ICP0 promoter. During early stages of explant-induced reactivation from latency, the number of Sp3+ TG neurons were significantly higher relative to TG from latently infected mice. Conversely, Sp1+ TG neurons were only increased in females, but not male mice, during explant-induced reactivation. Sp1 siRNA significantly reduced HSV-1 replication in cultured mouse (Neuro-2A) and monkey (CV-1) cells. Mithramycin A, an antibiotic that has anti-tumor activity preferentially interacts with GC-rich DNA, including Sp1 binding sites, significantly reduced HSV-1 replication indicating it has antiviral activity. GR and Sp1 or Sp3 transactivated the HSV-1 ICP0 promoter in Neuro-2A and CV-1 cells confirming these transcription factors enhance viral replication and gene expression.

Sp1 facilitates continued HSV-1 gene expression in the absence of key viral transactivators

Productive replication of herpes simplex virus (HSV) relies upon a well-ordered transcriptional cascade flowing from immediate-early (IE) to early (E) to late (L) gene products. While several virus-encoded transcriptional activators are involved in this process, IE and E gene promoters also contain multiple binding sites for the ubiquitously expressed cellular transcription factor Sp1. Sp1 has been previously implicated in activating HSV-1 gene transcription downstream of these sites, but why Sp1-binding sites are maintained in the promoters of genes activated by virus-encoded activators remains unclear. We hypothesized that Sp1 enables continued HSV-1 transcription and replication when viral transactivators are limited. We used a depletion-based approach in human foreskin fibroblasts to investigate the specific contribution of Sp1 to the initiation and progression of the HSV-1 lytic gene cascade. We found that Sp1 increased viral transcript levels, protein expression, and replication following infection with VP16- or ICP0-deficient viruses but had little to no effect on rescued viruses or during wild-type (WT) HSV-1 infection. Moreover, Sp1 promoted WT virus transcription and replication following interferon treatment of fibroblasts and thus may contribute to viral immune evasion. Interestingly, we observed reduced expression of Sp1 and Sp1-family transcription factors in differentiated sensory neurons compared to undifferentiated cells, suggesting that reduced Sp1 levels may also contribute to HSV-1 latent infection. Overall, these findings indicate that Sp1 can promote HSV-1 gene expression in the absence of key viral transactivators; thus, HSV-1 may use Sp1 to maintain its gene expression and replication under adverse conditions.IMPORTANCEHerpes simplex virus (HSV) is a common human pathogen that actively replicates in the epithelia but can persist for the lifetime of the infected host via a stable, latent infection in neurons. A key feature of the HSV replication cycle is a complex transcriptional program in which virus and host-cell factors coordinate to regulate expression of the viral gene products necessary for continued viral replication. Multiple binding sites for the cellular transcription factor Sp1 are located in the promoters of HSV-1 genes, but how Sp1 binding contributes to transcription and replication of wild-type virus is not fully understood. In this study, we identified a specific role for Sp1 in maintaining HSV-1 gene transcription under adverse conditions, as when virus-encoded transcriptional activators were absent or limited. Preservation of Sp1-binding sites in HSV-1 gene promoters may thus benefit the virus as it navigates diverse cell types and host-cell conditions during infection.

c-Jun signaling during initial HSV-1 infection modulates latency to enhance later reactivation in addition to directly promoting the progression to full reactivation

Herpes simplex virus-1 (HSV-1) establishes a latent infection in peripheral neurons and periodically reactivates to permit transmission, which can result in clinical manifestations. Viral transactivators required for lytic infection are largely absent during latent infection, and therefore, HSV-1 relies on the co-option of neuronal host signaling pathways to initiate its gene expression. The activation of the neuronal c-Jun N-terminal kinase (JNK) cell stress pathway is central to initiating biphasic reactivation in response to multiple stimuli. However, how host factors work with JNK to stimulate the initial wave of gene expression (known as Phase I) or the progression to full Phase II reactivation remains unclear. Here, we found that c-Jun, the primary target downstream of neuronal JNK cell stress signaling, functions during reactivation but not during the JNK-mediated initiation of Phase I gene expression. Instead, c-Jun was required to transition from Phase I to full HSV-1 reactivation and was detected in viral replication compartments of reactivating neurons. Interestingly, we also identified a role for both c-Jun and enhanced neuronal stress during initial neuronal infection in promoting a more reactivation-competent form of HSV-1 latency. Therefore, c-Jun functions at multiple stages during the HSV latent infection of neurons to promote reactivation but not during the initial JNK-dependent Phase I. Importantly, by demonstrating that initial infection conditions can contribute to later reactivation abilities, this study highlights the potential for latently infected neurons to maintain a molecular scar of previous exposure to neuronal stressors.IMPORTANCEThe molecular mechanisms that regulate the reactivation of herpes simplex virus-1 (HSV-1) from latent infection are unknown. The host transcription and pioneer factor c-Jun is the main target of the JNK cell stress pathway that is known to be important in exit of HSV from latency. Surprisingly, we found that c-Jun does not act with JNK during exit from latency but instead promotes the transition to full reactivation. Moreover, c-Jun and enhanced neuronal stress during initial neuronal infection promoted a more reactivation-competent form of HSV-1 latency. c-Jun, therefore, functions at multiple stages during HSV-1 latent infection of neurons to promote reactivation. Importantly, this study contributes to a growing body of evidence that de novo HSV-1 infection conditions can modulate latent infection and impact future reactivation events, raising important questions on the clinical impact of stress during initial HSV-1 acquisition on future reactivation events and consequences.

Intimate Relationship Between Stress and Human Alpha‑Herpes Virus 1 (HSV‑1) Reactivation from Latency

Purpose of Review

Numerous studies concluded stress (acute, episodic acute, or chronic) increases the incidence of human alpha-herpes virus 1 (HSV-1) reactivation from latency in neurons. This review will summarize how stress stimulates viral gene expression, replication, and reactivation from latency.

Recent Findings

Stress (capital S) stress-mediated activation of the glucocorticoid receptor (GR) accelerates reactivation from latency, whereas a corticosteroid-specific antagonist impairs viral replication and reactivation from latency. GR and specific stress-induced cellular transcription factors also stimulate viral promoters that drive expression of key viral transcriptional regulators: infected cell protein 0 (ICP0), ICP4, ICP27 and viral tegument protein (VP16). Hence, GR is predicted to initially stimulate viral gene expression. GR-mediated immune-inhibitory functions are also predicted to enhance viral replication and viral spread.

Summary

Identifying cellular factors and viral regulatory proteins that trigger reactivation from latency in neurons may provide new therapeutic strategies designed to reduce the incidence of reactivation from latency.

c-Jun Signaling During Initial HSV-1 Infection Modulates Latency to Enhance Later Reactivation in addition to Directly Promoting the Progression to Full Reactivation

Herpes simplex virus-1 (HSV-1) establishes a latent infection in peripheral neurons and can periodically reactivate to permit transmission and clinical manifestations. Viral transactivators required for lytic infection are largely absent during latent infection and therefore HSV-1 relies on the co-option of neuronal host signaling pathways to initiate its gene expression. Activation of the neuronal c-Jun N-terminal kinase (JNK) cell stress pathway is central to initiating biphasic reactivation in response to multiple stimuli. However, how host factors work with JNK to stimulate the initial wave of gene expression (known as Phase I) or the progression to full, Phase II reactivation remains unclear. Here, we found that c-Jun, the primary target downstream of neuronal JNK cell stress signaling, functions during reactivation but not during the JNK-mediated initiation of Phase I gene expression. Instead, c-Jun was required for the transition from Phase I to full HSV-1 reactivation and was detected in viral replication compartments of reactivating neurons. Interestingly, we also identified a role for both c-Jun and enhanced neuronal stress during initial neuronal infection in promoting a more reactivation-competent form of HSV-1 latency. Therefore, c-Jun functions at multiple stages during HSV latent infection of neurons to promote reactivation. Importantly, by demonstrating that initial infection conditions can contribute to later reactivation abilities, this study highlights the potential for latently infected neurons to maintain a molecular scar of previous exposure to neuronal stressors.

Impaired glucocorticoid receptor function attenuates herpes simplex virus 1 production during explant-induced reactivation from latency in female mice

IMPORTANCE

A correlation exists between stress and increased episodes of human alpha-herpes virus 1 reactivation from latency. Stress increases corticosteroid levels; consequently, the glucocorticoid receptor (GR) is activated. Recent studies concluded that a GR agonist, but not an antagonist, accelerates productive infection and reactivation from latency. Furthermore, GR and certain stress-induced transcription factors cooperatively transactivate promoters that drive the expression of infected cell protein 0 (ICP0), ICP4, and VP16. This study revealed female mice expressing a GR containing a serine to alanine mutation at position 229 (GRS229A) shed significantly lower levels of infectious virus during explant-induced reactivation compared to male GRS229A or wild-type parental C57BL/6 mice. Furthermore, female GRS229A mice contained fewer VP16 + TG neurons compared to male GRS229A mice or wild-type mice during the early stages of explant-induced reactivation from latency. Collectively, these studies revealed that GR transcriptional activity has female-specific effects, whereas male mice can compensate for the loss of GR transcriptional activation.

Slug, a Stress-Induced Transcription Factor, Stimulates Herpes Simplex Virus 1 Replication and Transactivates a cis-Regulatory Module within the VP16 Promoter

Stress-mediated activation of the glucocorticoid receptor (GR) and specific stress-induced transcription factors stimulate herpes simplex virus 1 (HSV-1) productive infection, explant-induced reactivation, and immediate early (IE) promoters that drive expression of infected cell protein 0 (ICP0), ICP4, and ICP27. Several published studies concluded the virion tegument protein VP16, ICP0, and/or ICP4 drives early steps of reactivation from latency. Notably, VP16 protein expression was induced in trigeminal ganglionic neurons of Swiss Webster or C57BL/6J mice during early stages of stress-induced reactivation. If VP16 mediates reactivation, we hypothesized stress-induced cellular transcription factors would stimulate its expression. To address this hypothesis, we tested whether stress-induced transcription factors transactivate a VP16 cis-regulatory module (CRM) located upstream of the VP16 TATA box (-249 to -30). Initial studies revealed the VP16 CRM cis-activated a minimal promoter more efficiently in mouse neuroblastoma cells (Neuro-2A) than mouse fibroblasts (NIH-3T3). GR and Slug, a stress-induced transcription factor that binds enhancer boxes (E-boxes), were the only stress-induced transcription factors examined that transactivated the VP16 CRM construct. GR- and Slug-mediated transactivation was reduced to basal levels when the E-box, two 1/2 GR response elements (GREs), or NF-κB binding site was mutated. Previous studies revealed GR and Slug cooperatively transactivated the ICP4 CRM, but not ICP0 or ICP27. Silencing of Slug expression in Neuro-2A cells significantly reduced viral replication, indicating Slug-mediated transactivation of ICP4 and VP16 CRM activity correlates with enhanced viral replication and reactivation from latency. IMPORTANCE Herpes simplex virus 1 (HSV-1) establishes lifelong latency in several types of neurons. Periodically cellular stressors trigger reactivation from latency. Viral regulatory proteins are not abundantly expressed during latency, indicating cellular transcription factors mediate early stages of reactivation. Notably, the glucocorticoid receptor (GR) and certain stress-induced transcription factors transactivate cis-regulatory modules (CRMs) essential for expression of infected cell protein 0 (ICP0) and ICP4, key viral transcriptional regulatory proteins linked to triggering reactivation from latency. Virion protein 16 (VP16) specifically transactivates IE promoter and was also reported to mediate early stages of reactivation from latency. GR and Slug, a stress-induced enhancer box (E-box) binding protein, transactivate a minimal promoter downstream of VP16 CRM, and these transcription factors occupy VP16 CRM sequences in transfected cells. Notably, Slug stimulates viral replication in mouse neuroblastoma cells suggesting Slug, by virtue of transactivating VP16 and ICP4 CRM sequences, can trigger reactivation in certain neurons.