Herpes simplex virus type 1 ICP0 regulates expression of immediate-early, early, and late genes in productively infected cells

A viral E3 ubiquitin ligase produced by herpes simplex virus 1 inhibits the NLRP1 inflammasome

Guard proteins initiate defense mechanisms upon sensing pathogen-encoded virulence factors. Successful viral pathogens likely inhibit guard protein activity, but these interactions have been largely undefined. Here, we demonstrate that the human pathogen herpes simplex virus 1 (HSV-1) stimulates and inhibits an antiviral pathway initiated by NLRP1, a guard protein that induces inflammasome formation and pyroptotic cell death when activated. Notably, HSV-1 infection of human keratinocytes promotes posttranslational modifications to NLRP1, consistent with MAPK-dependent NLRP1 activation, but does not result in downstream inflammasome formation. We identify infected cell protein 0 (ICP0) as the critical HSV-1 protein that is necessary and sufficient for inhibition of the NLRP1 pathway. Mechanistically, ICP0's cytoplasmic localization and function as an E3 ubiquitin ligase prevents proteasomal degradation of the auto-inhibitory NT-NLRP1 fragment, thereby preventing inflammasome formation. Further, we demonstrate that inhibiting this inflammasome is important for promoting HSV-1 replication. Thus, we have established a mechanism by which HSV-1 overcomes a guard-mediated antiviral defense strategy in humans.

The precise function of alphaherpesvirus tegument proteins and their interactions during the viral life cycle

Alphaherpesvirus is a widespread pathogen that causes diverse diseases in humans and animals and can severely damage host health. Alphaherpesvirus particles comprise a DNA core, capsid, tegument and envelope; the tegument is located between the nuclear capsid and envelope. According to biochemical and proteomic analyses of alphaherpesvirus particles, the tegument contains at least 24 viral proteins and plays an important role in the alphaherpesvirus life cycle. This article reviews the important role of tegument proteins and their interactions during the viral life cycle to provide a reference and inspiration for understanding alphaherpesvirus infection pathogenesis and identifying new antiviral strategies.

Cytomegalovirus pneumonia with intermittent pulmonary hemorrhage leading to asphyxia death: a case report and literature review

Neonatal pulmonary hemorrhage is a late manifestation of various diseases. Premature delivery and low body weight are frequently observed as high-risk factors, characterized by acute onset, rapid progression, and high mortality rates. Pulmonary hemorrhage caused by cytomegalovirus infection in newborns with normal immune function is a rare occurrence. This case report focuses on a term neonate with normal birth weight who presented solely with nasal obstruction shortly after birth. However, 4 days after birth, the newborn experienced a sudden onset of blood gushing from both the mouth and nasal cavity. The patient was diagnosed with gastrointestinal bleeding, neonatal pneumonia and neonatal lung consolidation. And he was discharged after ten days of symptomatic treatment. However, upon returning home, the patient experienced a sudden onset of bleeding from the mouth and nose, leading to his untimely demise. Subsequent autopsy revealed the presence of pulmonary hemorrhage in newborn, which presented as interstitial pneumonia. The cause of pulmonary hemorrhage is cytomegalovirus infection. This case emphasizes the importance of pediatricians enhancing their skills in differentiating pulmonary hemorrhage, especially from cytomegalovirus pneumonia.

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.

HSV-1 ICP0 Dimer Domain Adopts a Novel β-barrel Fold

Infected cell protein 0 (ICP0) is an immediate-early regulatory protein of herpes simplex virus 1 (HSV-1) that possesses E3 ubiquitin ligase activity. ICP0 transactivates viral genes, in part, through its C-terminal dimer domain (residues 555-767). Deletion of this dimer domain results in reduced viral gene expression, lytic infection, and reactivation from latency. Since ICP0's dimer domain is associated with its transactivation activity and efficient viral replication, we wanted to determine the structure of this specific domain. The C-terminus of ICP0 was purified from bacteria and analyzed by X-ray crystallography to solve its structure. Each subunit or monomer in the ICP0 dimer is composed of nine β-strands and two α-helices. Interestingly, two adjacent β-strands from one monomer "reach" into the adjacent subunit during dimer formation, generating two β-barrel-like structures. Additionally, crystallographic analyses indicate a tetramer structure is formed from two β-strands of each dimer, creating a "stacking" of the β-barrels. The structural protein database searches indicate the fold or structure adopted by the ICP0 dimer is novel. The dimer is held together by an extensive network of hydrogen bonds. Computational analyses reveal that ICP0 can either form a dimer or bind to SUMO1 via its C-terminal SUMO-interacting motifs but not both. Understanding the structure of the dimer domain will provide insights into the activities of ICP0 and, ultimately, the HSV-1 life cycle.

Cellular Processes Induced by HSV-1 Infections in Vestibular Neuritis

Herpesvirus is a prevalent pathogen that primarily infects human epithelial cells and has the ability to reside in neurons. In the field of otolaryngology, herpesvirus infection primarily leads to hearing loss and vestibular neuritis and is considered the primary hypothesis regarding the pathogenesis of vestibular neuritis. In this review, we provide a summary of the effects of the herpes virus on cellular processes in both host cells and immune cells, with a focus on HSV-1 as illustrative examples.

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.

HSV-1 exploits host heterochromatin for nuclear egress

Herpes simplex virus (HSV-1) progeny form in the nucleus and exit to successfully infect other cells. Newly formed capsids navigate complex chromatin architecture to reach the inner nuclear membrane (INM) and egress. Here, we demonstrate by transmission electron microscopy (TEM) that HSV-1 capsids traverse heterochromatin associated with trimethylation on histone H3 lysine 27 (H3K27me3) and the histone variant macroH2A1. Through chromatin profiling during infection, we revealed global redistribution of these marks whereby massive host genomic regions bound by macroH2A1 and H3K27me3 correlate with decreased host transcription in active compartments. We found that the loss of these markers resulted in significantly lower viral titers but did not impact viral genome or protein accumulation. Strikingly, we discovered that loss of macroH2A1 or H3K27me3 resulted in nuclear trapping of capsids. Finally, by live-capsid tracking, we quantified this decreased capsid movement. Thus, our work demonstrates that HSV-1 takes advantage of the dynamic nature of host heterochromatin formation during infection for efficient nuclear egress.

Herpes Simplex Virus 1 MicroRNAs: An Update

BACKGROUND

Herpes simplex virus 1 (HSV-1), an important human pathogen, is capable of latent infection in neurons and productive (lytic) infection in other tissue cells. Once infected with HSV-1, the immune system of the organism cannot eliminate the virus and carries it lifelong. HSV-1 possesses approximately 150 kb of double-stranded linear genomic DNA and can encode at least 70 proteins and 37 mature microRNAs (miRNAs) derived from 18 precursor miRNAs (pre-miRNAs).

SUMMARY

These HSV-1-encoded miRNAs are widely involved in multiple processes in the life cycle of the virus and the host cell, including viral latent and lytic infection, as well as host cell immune signaling, proliferation, and apoptosis.

KEY MESSAGE

In this review, we focused primarily on recent advances in HSV-1-encoded miRNA expression, function, and mechanism, which may provide new research ideas and feasible research methods systemically and comprehensively.

Herpes Simplex Virus 1 (HSV-1) Infected Cell Protein 0 (ICP0) Targets of Ubiquitination during Productive Infection of Primary Adult Sensory Neurons

Herpes simplex virus 1 (HSV-1) enters sensory neurons with the potential for productive or latent infection. For either outcome, HSV-1 must curtail the intrinsic immune response, regulate viral gene expression, and remove host proteins that could restrict viral processes. Infected cell protein 0 (ICP0), a virus-encoded E3 ubiquitin ligase, supports these processes by mediating the transfer of ubiquitin to target proteins to change their location, alter their function, or induce their degradation. To identify ubiquitination targets of ICP0 during productive infection in sensory neurons, we immunoprecipitated ubiquitinated proteins from primary adult sensory neurons infected with HSV-1 KOS (wild-type), HSV-1 n212 (expressing truncated, defective ICP0), and uninfected controls using anti-ubiquitin antibody FK2 (recognizing K29, K48, K63 and monoubiquitinated proteins), followed by LC-MS/MS and comparative analyses. We identified 40 unique proteins ubiquitinated by ICP0 and 17 ubiquitinated by both ICP0 and host mechanisms, of which High Mobility Group Protein I/Y (HMG I/Y) and TAR DNA Binding Protein 43 (TDP43) were selected for further analysis. We show that ICP0 ubiquitinates HMG I/Y and TDP43, altering protein expression at specific time points during productive HSV-1 infection, demonstrating that ICP0 manipulates the sensory neuronal environment in a time-dependent manner to regulate infection outcome in neurons.

Near-native state imaging by cryo-soft-X-ray tomography reveals remodelling of multiple cellular organelles during HSV-1 infection

Herpes simplex virus-1 (HSV-1) is a large, enveloped DNA virus and its assembly in the cell is a complex multi-step process during which viral particles interact with numerous cellular compartments such as the nucleus and organelles of the secretory pathway. Transmission electron microscopy and fluorescence microscopy are commonly used to study HSV-1 infection. However, 2D imaging limits our understanding of the 3D geometric changes to cellular compartments that accompany infection and sample processing can introduce morphological artefacts that complicate interpretation. In this study, we used soft X-ray tomography to observe differences in whole-cell architecture between HSV-1 infected and uninfected cells. To protect the near-native structure of cellular compartments we used a non-disruptive sample preparation technique involving rapid cryopreservation, and a fluorescent reporter virus was used to facilitate correlation of structural changes with the stage of infection in individual cells. We observed viral capsids and assembly intermediates interacting with nuclear and cytoplasmic membranes. Additionally, we observed differences in the morphology of specific organelles between uninfected and infected cells. The local concentration of cytoplasmic vesicles at the juxtanuclear compartment increased and their mean width decreased as infection proceeded, and lipid droplets transiently increased in size. Furthermore, mitochondria in infected cells were elongated and highly branched, suggesting that HSV-1 infection alters the dynamics of mitochondrial fission/fusion. Our results demonstrate that high-resolution 3D images of cellular compartments can be captured in a near-native state using soft X-ray tomography and have revealed that infection causes striking changes to the morphology of intracellular organelles.

Interactome and Ubiquitinome Analyses Identify Functional Targets of Herpes Simplex Virus 1 Infected Cell Protein 0

Herpes simplex virus 1 (HSV-1) can productively infect multiple cell types and establish latent infection in neurons. Infected cell protein 0 (ICP0) is an HSV-1 E3 ubiquitin ligase crucial for productive infection and reactivation from latency. However, our knowledge about its targets especially in neuronal cells is limited. We confirmed that, like in non-neuronal cells, ICP0-null virus exhibited major replication defects in primary mouse neurons and Neuro-2a cells. We identified many ICP0-interacting proteins in Neuro-2a cells, 293T cells, and human foreskin fibroblasts by mass spectrometry-based interactome analysis. Co-immunoprecipitation assays validated ICP0 interactions with acyl-coenzyme A thioesterase 8 (ACOT8), complement C1q binding protein (C1QBP), ovarian tumour domain-containing protein 4 (OTUD4), sorting nexin 9 (SNX9), and vimentin (VIM) in both Neuro-2a and 293T cells. Overexpression and knockdown experiments showed that SNX9 restricted replication of an ICP0-null but not wild-type virus in Neuro-2a cells. Ubiquitinome analysis by immunoprecipitating the trypsin-digested ubiquitin reminant followed by mass spectrometry identified numerous candidate ubiquitination substrates of ICP0 in infected Neuro-2a cells, among which OTUD4 and VIM were novel substrates confirmed to be ubiquitinated by transfected ICP0 in Neuro-2a cells despite no evidence of their degradation by ICP0. Expression of OTUD4 was induced independently of ICP0 during HSV-1 infection. Overexpressed OTUD4 enhanced type I interferon expression during infection with the ICP0-null but not wild-type virus. In summary, by combining two proteomic approaches followed by confirmatory and functional experiments, we identified and validated multiple novel targets of ICP0 and revealed potential restrictive activities of SNX9 and OTUD4 in neuronal cells.

Neuronal miR-138 Represses HSV-2 Lytic Infection by Regulating Viral and Host Genes with Mechanistic Differences from HSV-1

HSV-1 and HSV-2 are closely related viruses with major differences. Both viruses establish latency in neurons from which they reactivate to cause disease.

Advancing Our Understanding of Corneal Herpes Simplex Virus-1 Immune Evasion Mechanisms and Future Therapeutics

Herpes stromal keratitis (HSK) is a disease that commonly affects the cornea and external eye and is caused by Herpes Simplex Virus type 1 (HSV-1). This virus infects approximately 66% of people worldwide; however, only a small portion of these people will develop symptoms in their lifetime. There is no cure or vaccine available for HSV-1; however, there are treatments available that aim to control the inflammation caused by the virus and prevent its recurrence. While these treatments are beneficial to those suffering with HSK, there is a need for more effective treatments to minimise the need for topical steroids, which can have harmful effects, and to prevent bouts of disease reactivation, which can lead to progressive corneal scarring and visual impairment. This review details the current understanding of HSV-1 infection and discusses potential novel treatment options including microRNAs, TLRs, mAbs, and aptamers.

Rational Design of Live-Attenuated Vaccines against Herpes Simplex Viruses

Diseases caused by human herpes simplex virus types 1 and 2 (HSV-1 and HSV-2) affect millions of people worldwide and range from fatal encephalitis in neonates and herpes keratitis to orofacial and genital herpes, among other manifestations. The viruses can be shed efficiently by asymptomatic carriers, causing increased rates of infection. Viral transmission occurs through direct contact of mucosal surfaces followed by initial replication of the incoming virus in skin tissues. Subsequently, the viruses infect sensory neurons in the trigeminal and lumbosacral dorsal root ganglia, where they are primarily maintained in a transcriptionally repressed state termed "latency", which persists for the lifetime of the host. HSV DNA has also been detected in other sympathetic ganglia. Periodically, latent viruses can reactivate, causing ulcerative and often painful lesions primarily at the site of primary infection and proximal sites. In the United States, recurrent genital herpes alone accounts for more than a billion dollars in direct medical costs per year, while there are much higher costs associated with the socio-economic aspects of diseased patients, such as loss of productivity due to mental anguish. Currently, there are no effective FDA-approved vaccines for either prophylactic or therapeutic treatment of human herpes simplex infections, while several recent clinical trials have failed to achieve their endpoint goals. Historically, live-attenuated vaccines have successfully combated viral diseases, including polio, influenza, measles, and smallpox. Vaccines aimed to protect against the devastation of smallpox led to the most significant achievement in medical history: the eradication of human disease by vaccination. Recently, novel approaches toward developing safe and effective live-attenuated vaccines have demonstrated high efficacy in various preclinical models of herpetic disease. This next generation of live-attenuated vaccines has been tailored to minimize vaccine-associated side effects and promote effective and long-lasting immune responses. The ultimate goal is to prevent or reduce primary infections (prophylactic vaccines) or reduce the frequency and severity of disease associated with reactivation events (therapeutic vaccines). These vaccines' "rational" design is based on our current understanding of the immunopathogenesis of herpesviral infections that guide the development of vaccines that generate robust and protective immune responses. This review covers recent advances in the development of herpes simplex vaccines and the current state of ongoing clinical trials in pursuit of an effective vaccine against herpes simplex virus infections and associated diseases.

Viral Respiratory Pathogens and Lung Injury

Several viruses target the human respiratory tract, causing different clinical manifestations spanning from mild upper airway involvement to life-threatening acute respiratory distress syndrome (ARDS). As dramatically evident in the ongoing SARS-CoV-2 pandemic, the clinical picture is not always easily predictable due to the combined effect of direct viral and indirect patient-specific immune-mediated damage. In this review, we discuss the main RNA (orthomyxoviruses, paramyxoviruses, and coronaviruses) and DNA (adenoviruses, herpesviruses, and bocaviruses) viruses with respiratory tropism and their mechanisms of direct and indirect cell damage. We analyze the thin line existing between a protective immune response, capable of limiting viral replication, and an unbalanced, dysregulated immune activation often leading to the most severe complication. Our comprehension of the molecular mechanisms involved is increasing and this should pave the way for the development and clinical use of new tailored immune-based antiviral strategies.

Regulation of host and virus genes by neuronal miR-138 favours herpes simplex virus 1 latency

MicroRNA miR-138, which is highly expressed in neurons, represses herpes simplex virus 1 (HSV-1) lytic cycle genes by targeting viral ICP0 messenger RNA, thereby promoting viral latency in mice. We found that overexpressed miR-138 also represses lytic processes independently of ICP0 in murine and human neuronal cells; therefore, we investigated whether miR-138 has targets besides ICP0. Using genome-wide RNA sequencing/photoactivatable ribonucleoside-enhanced crosslinking and immunoprecipitation followed by short interfering RNA knockdown of candidate targets, we identified the host Oct-1 and Foxc1 messenger mRNAs as miR-138's targets, whose gene products are transcription factors important for HSV-1 replication in neuronal cells. OCT-1 has a known role in the initiation of HSV transcription. Overexpression of FOXC1, which was not known to affect HSV-1, promoted HSV-1 replication in murine neurons and ganglia. CRISPR-Cas9 knockout of FOXC1 reduced viral replication, lytic gene expression and miR-138 repression in murine neuronal cells. FOXC1 also collaborated with ICP0 to decrease heterochromatin on viral genes and compensated for the defect of an ICP0-null virus. In summary, miR-138 targets ICP0, Oct-1 and Foxc1 to repress HSV-1 lytic cycle genes and promote epigenetic gene silencing, which together enable favourable conditions for latent infection.

ATRX limits the accessibility of histone H3-occupied HSV genomes during lytic infection

Histones are rapidly loaded on the HSV genome upon entry into the nucleus of human fibroblasts, but the effects of histone loading on viral replication have not been fully defined. We showed recently that ATRX is dispensable for de novo deposition of H3 to HSV genomes after nuclear entry but restricted infection through maintenance of viral heterochromatin. To further investigate the roles that ATRX and other histone H3 chaperones play in restriction of HSV, we infected human fibroblasts that were systematically depleted of nuclear H3 chaperones. We found that the ATRX/DAXX complex is unique among nuclear H3 chaperones in its capacity to restrict ICP0-null HSV infection. Only depletion of ATRX significantly alleviated restriction of viral replication. Interestingly, no individual nuclear H3 chaperone was required for deposition of H3 onto input viral genomes, suggesting that during lytic infection, H3 deposition may occur through multiple pathways. ChIP-seq for total histone H3 in control and ATRX-KO cells infected with ICP0-null HSV showed that HSV DNA is loaded with high levels of histones across the entire viral genome. Despite high levels of H3, ATAC-seq analysis revealed that HSV DNA is highly accessible, especially in regions of high GC content, and is not organized largely into ordered nucleosomes during lytic infection. ATRX reduced accessibility of viral DNA to the activity of a TN5 transposase and enhanced accumulation of viral DNA fragment sizes associated with nucleosome-like structures. Together, these findings support a model in which ATRX restricts viral infection by altering the structure of histone H3-loaded viral chromatin that reduces viral DNA accessibility for transcription. High GC rich regions of the HSV genome, especially the S component inverted repeats of the HSV-1 genome, show increased accessibility, which may lead to increased ability to transcribe the IE genes encoded in these regions during initiation of infection.

Comparative proteomics identifies Schlafen 5 (SLFN5) as a herpes simplex virus restriction factor that suppresses viral transcription

Intrinsic antiviral host factors confer cellular defence by limiting virus replication and are often counteracted by viral countermeasures. We reasoned that host factors that inhibit viral gene expression could be identified by determining proteins bound to viral DNA (vDNA) in the absence of key viral antagonists. Herpes simplex virus 1 (HSV-1) expresses E3 ubiquitin-protein ligase ICP0 (ICP0), which functions as an E3 ubiquitin ligase required to promote infection. Cellular substrates of ICP0 have been discovered as host barriers to infection but the mechanisms for inhibition of viral gene expression are not fully understood. To identify restriction factors antagonized by ICP0, we compared proteomes associated with vDNA during HSV-1 infection with wild-type virus and a mutant lacking functional ICP0 (ΔICP0). We identified the cellular protein Schlafen family member 5 (SLFN5) as an ICP0 target that binds vDNA during HSV-1 ΔICP0 infection. We demonstrated that ICP0 mediates ubiquitination of SLFN5, which leads to its proteasomal degradation. In the absence of ICP0, SLFN5 binds vDNA to repress HSV-1 transcription by limiting accessibility of RNA polymerase II to viral promoters. These results highlight how comparative proteomics of proteins associated with viral genomes can identify host restriction factors and reveal that viral countermeasures can overcome SLFN antiviral activity.

Tripartite Motif 22 (TRIM22) protein restricts herpes simplex virus 1 by epigenetic silencing of viral immediate-early genes

Intrinsic resistance is a crucial line of defense against virus infections, and members of the Tripartite Ring Interaction Motif (TRIM) family of proteins are major players in this system, such as cytoplasmic TRIM5α or nuclear promyelocytic leukemia (PML/TRIM19) protein. Previous reports on the antiviral function of another TRIM protein, TRIM22, emphasized its innate immune role as a Type I and Type II interferon-stimulated gene against RNA viruses. This study shows that TRIM22 has an additional intrinsic role against DNA viruses. Here, we report that TRIM22 is a novel restriction factor of HSV-1 and limits ICP0-null virus replication by increasing histone occupancy and heterochromatin, thereby reducing immediate-early viral gene expression. The corresponding wild-type equivalent of the virus evades the TRIM22-specific restriction by a mechanism independent of ICP0-mediated degradation. We also demonstrate that TRIM22 inhibits other DNA viruses, including representative members of the β- and γ- herpesviruses. Allelic variants in TRIM22 showed different degrees of anti-herpesviral activity; thus, TRIM22 genetic variability may contribute to the varying susceptibility to HSV-1 infection in humans. Collectively, these results argue that TRIM22 is a novel restriction factor and expand the list of restriction factors functioning in the infected cell nucleus to counter DNA virus infection.

The host immune response to herpesviruses includes intrinsic immunity, which is a constitutively active line of defense. Members of the Tripartite Motif (TRIM) superfamily of proteins, such as cytoplasmic TRIM5α and nuclear TRIM19, are examples of such restriction factors against the prototypical α-herpesvirus, herpes simplex virus-1 (HSV-1). Previous reports on the antiviral function of the protein encoded by TRIM22, a gene closely related to the TRIM5 gene, emphasized its antiretroviral role. We show that TRIM22 has an additional role as a restriction factor against herpesviruses. We found that TRIM22 inhibits a mutant form of HSV-1, by promoting chromatin compaction of the viral DNA encoding immediate-early viral genes–this consequently inhibits viral replication and reduces virus yields. Unlike other restriction factors that are degraded by the viral infected cell polypeptide 0 (ICP0), TRIM22 is not degraded by ICP0. We also show that TRIM22 inhibits representative members of the β-herpesvirus (cytomegalovirus) and γ- herpesviruses (Epstein-Barr virus). In addition, different TRIM22 genetic variants show differing levels of HSV-1 inhibition. Together, these results argue for the importance of the TRIM22 gene as a restriction factor against herpesviruses, and offer a novel avenue for further investigation on the role of TRIM genes in host genetic variation in herpesviral susceptibility.

"Non-Essential" Proteins of HSV-1 with Essential Roles In Vivo: A Comprehensive Review

Viruses encode for structural proteins that participate in virion formation and include capsid and envelope proteins. In addition, viruses encode for an array of non-structural accessory proteins important for replication, spread, and immune evasion in the host and are often linked to virus pathogenesis. Most virus accessory proteins are non-essential for growth in cell culture because of the simplicity of the infection barriers or because they have roles only during a state of the infection that does not exist in cell cultures (i.e., tissue-specific functions), or finally because host factors in cell culture can complement their absence. For these reasons, the study of most nonessential viral factors is more complex and requires development of suitable cell culture systems and in vivo models. Approximately half of the proteins encoded by the herpes simplex virus 1 (HSV-1) genome have been classified as non-essential. These proteins have essential roles in vivo in counteracting antiviral responses, facilitating the spread of the virus from the sites of initial infection to the peripheral nervous system, where it establishes lifelong reservoirs, virus pathogenesis, and other regulatory roles during infection. Understanding the functions of the non-essential proteins of herpesviruses is important to understand mechanisms of viral pathogenesis but also to harness properties of these viruses for therapeutic purposes. Here, we have provided a comprehensive summary of the functions of HSV-1 non-essential proteins.

Termination of Transcription of LAT Increases the Amounts of ICP0 mRNA but Does Not Alter the Course of HSV-1 Infection in Latently Infected Murine Ganglia

On entering sensory ganglia, herpes simplex viruses 1 (HSV-1) establishes a latent infection with the synthesis of a latency associated transcript (LAT) or initiates productive infection with expression of a set of immediate early viral proteins. The precise mechanisms how expression of α genes is suppressed during the latency are unknown. One mechanism that has been proposed is illustrated in the case of ICP0, a key immediate early viral regulatory protein. Specifically, the 2 kb LAT intron is complementary to the 3' terminal portion of ICP0 mRNA. To test the hypothesis that accumulation of LAT negatively affects the accumulation of ICP0 mRNA, we inserted a DNA fragment encoding two poly(A) sequences into LAT to early terminate LAT transcript without interrupting the complementary sequence of ICP0 transcript (named as SR1603). Comparisons of the parent (SR1601) and mutant (SR1603) HSV-1 viruses showed the following: Neurons harboring latent SR1603 virus accumulated equivalent amounts of viral DNA but higher amounts of ICP0 mRNA and lower amounts of LAT, when compared to neurons harboring the SR1601 virus. One notable difference between the two viruses is that viral RNA accumulation in explanted ganglia harboring SR1603 virus initiated significantly sooner than that in neurons harboring SR1601 virus, suggesting that ICP0 may act as an activator of viral gene expression in permissive cells. Collectively, these data suggest that increased ICP0 mRNA by suppressed LAT did not affect the establishment of latency in latently infected murine ganglia.

Discovery of Small-Molecule Inhibitors Targeting the E3 Ubiquitin Ligase Activity of the Herpes Simplex Virus 1 ICP0 Protein Using an In Vitro High-Throughput Screening Assay

Herpes simplex virus 1 (HSV-1) has infected more than 80% of the population. Reactivation of the virus causes diseases ranging in severity from benign cold sores to fatal encephalitis. Current treatments involve viral DNA replication inhibitors, but the emergence of drug-resistant mutants is observed frequently, highlighting the need for novel antiviral therapies. Infected cell protein 0 (ICP0) of HSV-1 is encoded by an immediate early gene and plays a fundamental role during infection, because it enables viral gene expression and blocks antiviral responses. One mechanism by which ICP0 functions is through an E3 ubiquitin ligase activity that induces the degradation of targeted proteins. A ΔICP0 virus or mutants with deficiencies in E3 ligase activity cannot counteract beta interferon (IFN-β)-induced restriction of viral infection, are highly immunogenic, are avirulent, and fail to spread. Thus, small molecules interfering with essential and conserved ICP0 functions are expected to compromise HSV-1 infection. We have developed a high-throughput screening assay, based on the autoubiquitination properties of ICP0, to identify small-molecule inhibitors of ICP0 E3 ubiquitin ligase activity. Through a pilot screening procedure, we identified nine compounds that displayed dose-dependent inhibitory effects on ICP0 but not on Mdm2, a control E3 ubiquitin ligase. Following validation, one compound displayed ICP0-dependent inhibition of HSV-1 infection. This compound appeared to bind ICP0 in a cellular thermal shift assay, it blocked ICP0 self-elimination, and it blocked wild-type but not ICP0-null virus gene expression. This scaffold displays specificity and could be used to develop optimized ICP0 E3 ligase inhibitors.IMPORTANCE Since acyclovir and its derivatives were launched for herpesviruses control almost four decades ago, the search for novel antivirals has waned. However, as human life expectancy has increased, so has the number of immunocompromised individuals who receive prolonged treatment for HSV recurrences. This has led to an increase in unresponsive patients due to acquired viral drug resistance. Thus, novel treatments need to be explored. Here we explored the HSV-1 ICP0 E3 ligase as a potential antiviral target because (i) ICP0 is expressed before virus replication, (ii) it is essential for infection in vivo, (iii) it is required for efficient reactivation of the virus from latency, (iv) inhibition of its E3 ligase activity would sustain host immune responses, and (v) it is shared by other herpesviruses. We report a compound that inhibits HSV-1 infection in an ICP0-dependent manner by inhibiting ICP0 E3 ligase activity.

Role for a Filamentous Nuclear Assembly of IFI16, DNA, and Host Factors in Restriction of Herpesviral Infection

Mammalian cells exhibit numerous strategies to recognize and contain viral infections. The best-characterized antiviral responses are those that are induced within the cytosol by receptors that activate interferon responses or shut down translation. Antiviral responses also occur in the nucleus, yet these intranuclear innate immune responses are poorly defined at the receptor-proximal level. In this study, we explored the ability of cells to restrict infection by assembling viral DNA into transcriptionally silent heterochromatin within the nucleus. We found that the IFI16 restriction factor forms filaments on DNA within infected cells. These filaments recruit antiviral restriction factors to prevent viral replication in various cell types. Mechanistically, IFI16 filaments inhibit the recruitment of RNA polymerase II to viral genes. We propose that IFI16 filaments with associated restriction factors constitute a “restrictosome” structure that can signal to other parts of the nucleus where foreign DNA is located that it should be silenced.

Several host cell nuclear factors are known to restrict herpes simplex virus 1 (HSV-1) replication, but their mechanisms of action remain to be defined. Interferon-inducible protein 16 (IFI16) and the nuclear domain 10-associated proteins, such as promyelocytic leukemia (PML) protein, localize to input viral genomes, but they are also capable of restricting progeny viral transcription. In this study, we used structured illumination microscopy to show that after HSV DNA replication, IFI16 forms nuclear filamentous structures on DNA within a subset of nuclear replication compartments in HSV-1 ICP0-null mutant virus-infected human cells. The ability to form filaments in different cell types correlates with the efficiency of restriction, and the kinetics of filament formation and epigenetic changes are similar. Thus, both are consistent with the filamentous structures being involved in epigenetic silencing of viral progeny DNA. IFI16 filaments recruit other restriction factors, including PML, Sp100, and ATRX, to aid in the restriction. Although the filaments are only in a subset of the replication compartments, IFI16 reduces the levels of elongation-competent RNA polymerase II (Pol II) in all replication compartments. Therefore, we propose that IFI16 filaments with associated restriction factors that form in replication compartments constitute a “restrictosome” structure that signals in cis and trans to silence the progeny viral DNA throughout the infected cell nucleus. The IFI16 filamentous structure may constitute the first known nuclear supramolecular organizing center for signaling in the cell nucleus.

ATRX promotes maintenance of herpes simplex virus heterochromatin during chromatin stress

The mechanisms by which mammalian cells recognize and epigenetically restrict viral DNA are not well defined. We used herpes simplex virus with bioorthogonally labeled genomes to detect host factors recruited to viral DNA shortly after its nuclear entry and found that the cellular IFI16, PML, and ATRX proteins colocalized with viral DNA by 15 min post infection. HSV-1 infection of ATRX-depleted fibroblasts resulted in elevated viral mRNA and accelerated viral DNA accumulation. Despite the early association of ATRX with vDNA, we found that initial viral heterochromatin formation is ATRX-independent. However, viral heterochromatin stability required ATRX from 4 to 8 hr post infection. Inhibition of transcription blocked viral chromatin loss in ATRX-knockout cells; thus, ATRX is uniquely required for heterochromatin maintenance during chromatin stress. These results argue that the initial formation and the subsequent maintenance of viral heterochromatin are separable mechanisms, a concept that likely extrapolates to host cell chromatin and viral latency.

Cells carefully package their DNA, tightly wrapping the long, stringy molecule around spool-like groups of proteins called histones. However, the genes that are draped around histones are effectively silenced, because they are ‘hidden’ from the molecular actors that read the genetic information to create proteins. A cell can control which of its genes are active by using proteins to move histones on or off specific portions of DNA. For example, a protein known as ATRX associates with a partner to load histones onto precise DNA regions and switch them off.

Wrapping DNA around histones can also be a defense mechanism against viruses, which are tiny cellular parasites that hijack the molecular machinery of a cell to create more of themselves. For instance, the herpes simplex virus, which causes cold sores and genital herpes, injects its DNA into a cell where it is used as a template to create new viral particles. By packaging the DNA of the virus around histones, the cell ensures that this foreign genetic information cannot be used to make more invaders. However, the details of this process remain unknown. In particular, it is still unclear what happens immediately after the virus penetrates the nucleus, the compartment that shelters the DNA of the cell.

Here, Cabral et al. explored this question by dissecting the role of ATRX in silencing the genetic information of the herpes simplex virus. The viral DNA was labeled while inside the virus itself, and then tracked using microscopy imaging techniques as it made its way into the cell and inside the nucleus. This revealed that, almost immediately after the viral DNA had entered the nucleus, ATRX came in contact with the foreign molecule. One possibility was that ATRX would be responsible for loading certain forms of histones onto the viral DNA. However, after Cabral et al. deleted ATRX from the cell, histones were still present on the genetic information of the virus, but this association was less stable. This indicated that ATRX was only required to keep histones latched onto the viral DNA, but not to load the proteins in the first place.

Overall, these results show that using histones to silence viral DNA in done in several steps: first, the foreign genetic material needs to be recognized, then histones have to be attached, and finally molecular actors should be recruited to keep histones onto the DNA.

Knowing how cells ward off the herpes simplex virus could help us find ways to ‘boost’ this defense mechanism. Armed with this knowledge, we could also begin to understand why certain people are more likely to be infected by this virus.

Bowman‒Birk Inhibitor Suppresses Herpes Simplex Virus Type 2 Infection of Human Cervical Epithelial Cells

The Bowman‒Birk inhibitor (BBI), a protease inhibitor derived from soybeans, has been extensively studied in anti-tumor and anti-inflammation research. We recently reported that BBI has an anti-HIV-1 property in primary human macrophages. Because HSV-2 infection plays a role in facilitating HIV-1 sexual transmission, we thus examined whether BBI has the ability to inhibit HSV-2 infection. We demonstrated that BBI could potently inhibit HSV-2 replication in human cervical epithelial cells (End1/E6E7). This BBI-mediated HSV-2 inhibition was partially through blocking HSV-2-mediated activation of NF-κB and p38 MAPK pathways. In addition, BBI could activate the JAK/STAT pathway and enhance the expression of several antiviral interferon-stimulated genes (ISGs). Furthermore, BBI treatment of End1/E6E7 cells upregulated the expression of tight junction proteins and reduced HSV-2-mediated cellular ubiquitinated proteins’ degradation through suppressing the ubiquitin‒proteasome system. These observations indicate that BBI may have therapeutic potential for the prevention and treatment of HSV-2 infections.

Interaction between the cellular E3 ubiquitin ligase SIAH-1 and the viral immediate-early protein ICP0 enables efficient replication of Herpes Simplex Virus type 2 in vivo

Herpes Simplex Virus type 2 (HSV-2) is a neurotropic human pathogen. Upon de novo infection, the viral infected cell protein 0 (ICP0) is immediately expressed and interacts with various cellular components during the viral replication cycle. ICP0 is a multifunctional regulatory protein that has been shown to be important for both efficient viral replication and virus reactivation from latency. In particular, as previously demonstrated in transfected tissue culture models, ICP0 interacts with the cellular E3 ubiquitin ligase SIAH-1, which targets ICP0 for proteasomal degradation. However, the consequence of this virus-host interaction during the establishment of HSV-2 infection in vivo has not yet been elucidated. Here we confirmed that ICP0 of HSV-2 interacts with SIAH-1 via two conserved PxAxVxP amino acid binding motifs. We also demonstrate in vitro that a SIAH-1 binding-deficient HSV-2 strain, constructed by homologous recombination technology, exhibits an attenuated growth curve and impaired DNA and protein synthesis. This attenuated phenotype was also confirmed in an in vivo ocular infection mouse model. Specifically, viral load of the SIAH-1 binding-deficient HSV-2 mutant was significantly reduced in the trigeminal ganglia and brain stem at day 5 and 7 post infection. Our findings indicate that the interplay between ICP0 and SIAH-1 is important for efficient HSV-2 replication in vivo, thereby affecting viral dissemination kinetics in newly infected organisms, and possibly revealing novel targets for antiviral therapy.

Latent versus productive infection: the alpha herpesvirus switch

Alpha herpesviruses are common pathogens of mammals. They establish a productive infection in many cell types, but a life-long latent infection occurs in PNS neurons. A vast majority of the human population has latent HSV-1 infections. Currently, there is no cure to clear latent infections. Even though HSV-1 is among the best studied viral pathogens, regulation of latency and reactivation is not well understood due to several challenges including a lack of animal models that precisely recapitulate latency/reactivation episodes; a difficulty in modeling in vitro latency; and a limited understanding of neuronal biology. In this review, we discuss insights gained from in vitro latency models with a focus on the neuronal and viral factors that determine the mode of infection.

Mechanisms of Host IFI16, PML, and Daxx Protein Restriction of Herpes Simplex Virus 1 Replication

The initial events after DNA virus infection involve a race between epigenetic silencing of the incoming viral DNA by host cell factors and expression of viral genes. Several host gene products, including the nuclear domain 10 (ND10) components PML (promyelocytic leukemia) and Daxx (death domain-associated protein 6), as well as IFI16 (interferon-inducible protein 16), have been shown to restrict herpes simplex virus 1 (HSV-1) replication. Whether IFI16 and ND10 components work together or separately to restrict HSV-1 replication is not known. To determine the combinatorial effects of IFI16 and ND10 proteins on viral infection, we depleted Daxx or PML in primary human foreskin fibroblasts (HFFs) in the presence or absence of IFI16. Daxx or IFI16 depletion resulted in higher ICP0 mutant viral yields, and the effects were additive. Surprisingly, small interfering RNA (siRNA) depletion of PML in the HFF cells led to decreased ICP0-null virus replication, while short hairpin RNA (shRNA) depletion led to increased ICP0-null virus replication, arguing that different PML isoforms or PML-related proteins may have restrictive or proviral functions. In normal human cells, viral DNA replication increases expression of all classes of HSV-1 genes. We observed that IFI16 repressed transcription from both parental and progeny DNA genomes. Taken together, our results show that the mechanisms of action of IFI16 and ND10 proteins are independent, at least in part, and that IFI16 exerts restrictive effects on both input and replicated viral genomes. These results raise the potential for distinct mechanisms of action of IFI16 on parental and progeny viral DNA molecules.IMPORTANCE Many human DNA viruses transcribe their genomes and replicate in the nucleus of a host cell, where they exploit the host cell nuclear machinery for their own replication. Host factors attempt to restrict viral replication by blocking such events, and viruses have evolved mechanisms to neutralize the host restriction factors. In this study, we provide information about the mechanisms of action of three host cell factors that restrict replication of herpes simplex virus (HSV). We found that these factors function independently and that one acts to restrict viral transcription from parental and progeny viral DNA genomes. These results provide new information about how cells counter DNA virus replication in the nucleus and provide possible approaches to enhance the ability of human cells to resist HSV infection.

Comprehensive proteome analysis of lysosomes reveals the diverse function of macrophages in immune responses

Phagocytosis and autophagy in macrophages have been shown to be essential to both innate and adaptive immunity. Lysosomes are the main catabolic subcellular organelles responsible for degradation and recycling of both extracellular and intracellular material, which are the final steps in phagocytosis and autophagy. However, the molecular mechanisms underlying lysosomal functions after infection remain obscure. In this study, we conducted a quantitative proteomics analysis of the changes in constitution and glycosylation of proteins in lysosomes derived from murine RAW 264.7 macrophage cells treated with different types of pathogens comprising examples of bacteria (Listeria monocytogenes, L. m), DNA viruses (herpes simplex virus type-1, HSV-1) and RNA viruses (vesicular stomatitis virus, VSV). In total, 3,704 lysosome-related proteins and 300 potential glycosylation sites on 193 proteins were identified. Comparative analysis showed that the aforementioned pathogens induced distinct alterations in the proteome of the lysosome, which is closely associated with the immune functions of macrophages, such as toll-like receptor activation, inflammation and antigen-presentation. The most significant changes in proteins and fluctuations in glycosylation were also determined. Furthermore, Western blot analysis showed that the changes in expression of these proteins were undetectable at the whole cell level. Thus, our study provides unique insights into the function of lysosomes in macrophage activation and immune responses.

CCCTC-Binding Factor Acts as a Heterochromatin Barrier on Herpes Simplex Viral Latent Chromatin and Contributes to Poised Latent Infection

Herpes simplex virus 1 (HSV-1) establishes latent infection in neurons via a variety of epigenetic mechanisms that silence its genome. The cellular CCCTC-binding factor (CTCF) functions as a mediator of transcriptional control and chromatin organization and has binding sites in the HSV-1 genome. We constructed an HSV-1 deletion mutant that lacked a pair of CTCF-binding sites (CTRL2) within the latency-associated transcript (LAT) coding sequences and found that loss of these CTCF-binding sites did not alter lytic replication or levels of establishment of latent infection, but their deletion reduced the ability of the virus to reactivate from latent infection. We also observed increased heterochromatin modifications on viral chromatin over the LAT promoter and intron. We therefore propose that CTCF binding at the CTRL2 sites acts as a chromatin insulator to keep viral chromatin in a form that is poised for reactivation, a state which we call poised latency.

Herpes simplex virus 1 (HSV-1) is a human pathogen that persists for the lifetime of the host as a result of its ability to establish latent infection within sensory neurons. The mechanism by which HSV-1 transitions from the lytic to latent infection program is largely unknown; however, HSV-1 is able to coopt cellular silencing mechanisms to facilitate the suppression of lytic gene expression. Here, we demonstrate that the cellular CCCTC-binding factor (CTCF)-binding site within the latency associated transcript (LAT) region is critical for the maintenance of a specific local chromatin structure. Additionally, loss of CTCF binding has detrimental effects on the ability to reactivate from latent infection. These results argue that CTCF plays a critical role in epigenetic regulation of viral gene expression to establish and/or maintain a form of latent infection that can reactivate efficiently.

A haploid HSV-1 genome platform for vector development: testing of the tetracycline-responsive switch shows interference by infected cell protein 0

BACKGROUND

Although herpes simplex virus type 1 (HSV-1) has outstanding properties for gene delivery vectors and its genome is available in bacterial artificial chromosomes (BACs) for mutagenesis studies, one impediment is the presence of approximately 15.4 kb of DNA sequences that are duplicated in the HSV-1 genome, complicating vector construction and stability.

METHODS

As a useful platform for building HSV-1 vectors, we have constructed a fully haploid HSV-1 genome BAC by deletion of one of these repeats, confirming that viral propagation in culture is not impaired. We used this ΔIR mutant to subsequently investigate whether the insertion of tetracycline-responsive tetO elements into the ICP34.5-ICP0 gene region can be used to control HSV-1 lytic replication.

RESULTS

The results of the present study show that ΔIR mutants deleted for ICP34.5 are viable for replication but not when the ICP0 promoter is also disrupted, thus indicating that regulation of infected cell protein 0 (ICP0) levels in the absence of ICP34.5 could be a viable means for controlling growth of HSV-1 vectors. Surprisingly, however, the tetO elements inserted into the ICP0 promoter did not confer ligand responsiveness to growth or ICP0 expression. Further analysis by transfection experiments revealed that ICP0 itself interferes with the tetracycline switch and reduces the the inducibility of this system.

CONCLUSIONS

Our new haploid HSV-1 BAC is a useful platform for building multiply deleted HSV-1 vectors. Deletion of the gene for ICP34.5 in this backbone renders viral growth dependent on ICP0, although ICP0 expression could not be regulated by tet-responsive transcriptional regulators.

Compartmented neuronal cultures reveal two distinct mechanisms for alpha herpesvirus escape from genome silencing

Alpha herpesvirus genomes encode the capacity to establish quiescent infections (i.e. latency) in the peripheral nervous system for the life of their hosts. Multiple times during latency, viral genomes can reactivate to start a productive infection, enabling spread of progeny virions to other hosts. Replication of alpha herpesviruses is well studied in cultured cells and many aspects of productive replication have been identified. However, many questions remain concerning how a productive or a quiescent infection is established. While infections in vivo often result in latency, infections of dissociated neuronal cultures in vitro result in a productive infection unless lytic viral replication is suppressed by DNA polymerase inhibitors or interferon. Using primary peripheral nervous system neurons cultured in modified Campenot tri-chambers, we previously reported that reactivateable, quiescent infections by pseudorabies virus (PRV) can be established in the absence of any inhibitor. Such infections were established in cell bodies only when physically isolated axons were infected at a very low multiplicity of infection (MOI). In this report, we developed a complementation assay in compartmented neuronal cultures to investigate host and viral factors in cell bodies that prevent establishment of quiescent infection and promote productive replication of axonally delivered genomes (i.e. escape from silencing). Stimulating protein kinase A (PKA) signaling pathways in isolated cell bodies, or superinfecting cell bodies with either UV-inactivated PRV or viral light particles (LP) promoted escape from genome silencing and prevented establishment of quiescent infection but with different molecular mechanisms. Activation of PKA in cell bodies triggers a slow escape from silencing in a cJun N-terminal kinase (JNK) dependent manner. However, escape from silencing is induced rapidly by infection with UVPRV or LP in a PKA- and JNK-independent manner. We suggest that viral tegument proteins delivered to cell bodies engage multiple signaling pathways that block silencing of viral genomes delivered by low MOI axonal infection.

Alpha herpesvirus infections stay life-long in infected human and animal hosts`nervous systems in a silent state ready to reactivate upon various stress signals. Remarkably, infection of epithelial cells with these viruses results in productive infection whereas infection of peripheral nervous system neurons results in non-productive silent infection (i.e. latency) in the natural hosts. More interestingly, infection of dissociated peripheral neurons in culture also results in productive infection unless DNA replication inhibitors are used. To study the molecular mechanisms of escape from latency, we used primary neurons cultured in compartmented tri-chambers. By this way, we recapitulated the natural route of infection by infecting axons with low dose of virus which resulted in a silent infection in a small number of neuronal cell bodies without the use of any inhibitors. Using these cultures, we developed a new complementation assay to investigate the molecular signals leading to escape from latency and establishment of productive infection. We found two different mechanisms to escape from latency: Cellular stress-mediated slow route and viral tegument mediated-fast route. Furthermore, we showed that the stress-mediated pathway requires protein kinase A and c-Jun N-terminal kinase activity while the viral tegument-mediated fast escape does not require these host cell kinase activities. We also concluded that a general response to DNA virus infection or presence of excess herpesviral genomes in the nucleus to saturate silencing complexes is not enough to escape from latency. Induction of a productive infection requires presence of tegument proteins or activation of PKA and JNK pathway.

Viral Ubiquitin Ligase Stimulates Selective Host MicroRNA Expression by Targeting ZEB Transcriptional Repressors

Infection with herpes simplex virus-1 (HSV-1) brings numerous changes in cellular gene expression. Levels of most host mRNAs are reduced, limiting synthesis of host proteins, especially those involved in antiviral defenses. The impact of HSV-1 on host microRNAs (miRNAs), an extensive network of short non-coding RNAs that regulate mRNA stability/translation, remains largely unexplored. Here we show that transcription of the miR-183 cluster (miR-183, miR-96, and miR-182) is selectively induced by HSV-1 during productive infection of primary fibroblasts and neurons. ICP0, a viral E3 ubiquitin ligase expressed as an immediate-early protein, is both necessary and sufficient for this induction. Nuclear exclusion of ICP0 or removal of the RING (really interesting new gene) finger domain that is required for E3 ligase activity prevents induction. ICP0 promotes the degradation of numerous host proteins and for the most part, the downstream consequences are unknown. Induction of the miR-183 cluster can be mimicked by depletion of host transcriptional repressors zinc finger E-box binding homeobox 1 (ZEB1)/-crystallin enhancer binding factor 1 (δEF1) and zinc finger E-box binding homeobox 2 (ZEB2)/Smad-interacting protein 1 (SIP1), which we establish as new substrates for ICP0-mediated degradation. Thus, HSV-1 selectively stimulates expression of the miR-183 cluster by ICP0-mediated degradation of ZEB transcriptional repressors.

Impaired STING Pathway in Human Osteosarcoma U2OS Cells Contributes to the Growth of ICP0-Null Mutant Herpes Simplex Virus

Human herpes simplex virus 1 (HSV-1) is a widespread pathogen, with 80% of the population being latently infected. To successfully evade the host, the virus has evolved strategies to counteract antiviral responses, including the gene-silencing and innate immunity machineries. The immediately early protein of the virus, infected cell protein 0 (ICP0), plays a central role in these processes. ICP0 blocks innate immunity, and one mechanism is by degrading hostile factors with its intrinsic E3 ligase activity. ICP0 also functions as a promiscuous transactivator, and it blocks repressor complexes to enable viral gene transcription. For these reasons, the growth of a ΔICP0 virus is impaired in most cells, except cells of the human osteosarcoma cell line U2OS, and it is only partially impaired in cells of the human osteosarcoma cell line Saos-2. We found that the two human osteosarcoma cell lines that supported the growth of the ΔICP0 virus failed to activate innate immune responses upon treatment with 2'3'-cyclic GAMP (2'3'-cGAMP), the natural agonist of STING (i.e., stimulator of interferon genes) or after infection with the ΔICP0 mutant virus. Innate immune responses were restored in these cells by transient expression of the STING protein but not after overexpression of interferon-inducible protein 16 (IFI16). Restoration of STING expression resulted in suppression of ΔICP0 virus gene expression and a decrease in viral yields. Overexpression of IFI16 also suppressed ΔICP0 virus gene expression, albeit to a lesser extent than STING. These data suggest that the susceptibility of U2OS and Saos-2 cells to the ΔICP0 HSV-1 is in part due to an impaired STING pathway.IMPORTANCE The DNA sensor STING plays pivotal role in controlling HSV-1 infection both in cell culture and in mice. The HSV-1 genome encodes numerous proteins that are dedicated to combat host antiviral responses. The immediate early protein of the virus ICP0 plays major role in this process as it targets hostile host proteins for degradation with its E3 ligase activity, and it disrupts repressor complexes via protein-protein interaction to enable viral gene transcription. Therefore, the ΔICP0 HSV-1 virus is defective for growth in most cells, except the human osteosarcoma cell lines U2OS and Saos-2. We found that both cell lines that support ΔICP0 virus infection have defects in the STING DNA-sensing pathway, which partially accounts for the rescue of the ΔICP0 virus growth. Restoration of STING expression in these cells rescued innate immunity and suppressed ΔICP0 virus infection. This study underscores the importance of STING in the control of HSV-1.

Induction of Multiple miR-200/182 Members in the Brains of Mice Are Associated with Acute Herpes Simplex Virus 1 Encephalitis

Important roles of microRNAs (miRNAs) in regulating the host response during viral infection have begun to be defined. However, little is known about the functional roles of miRNAs within an in vivo acute viral encephalitis model. We therefore identified global changes in miRNA expression during acute herpes simplex virus type 1 (HSV-1) encephalitis (HSVE) in mice. We found that many of the highly upregulated miRNAs (miR-155, miR-146a and miR-15b) detected in HSV-1 infected brain tissue are known regulators of inflammation and innate immunity. We also observed upregulation of 7 members belonging to the related group of miRNAs, the miR-200 family and miR-182 cluster (miR-200/182). Using in situ hybridization, we found that these miRNAs co-localized to regions of the brain with severe HSVE-related pathology and were upregulated in various cell types including neurons. Induction was apparent but not limited to cells in which HSV-1 was detected by immunohistochemistry, suggesting possible roles of these miRNAs in the host response to viral-induced tissue damage. Bioinformatic prediction combined with gene expression profiling revealed that the induced miR-200/182 members could regulate the biosynthesis of heparan sulfate proteoglycans. Using luciferase assays, we found that miR-96, miR-141, miR-183 and miR-200c all potentially targeted the syndecan-2 gene (Sdc2), which codes for a cell surface heparan sulfate proteoglycan involved in HSV-1 cellular attachment and entry.

Mutations Inactivating Herpes Simplex Virus 1 MicroRNA miR-H2 Do Not Detectably Increase ICP0 Gene Expression in Infected Cultured Cells or Mouse Trigeminal Ganglia

Herpes simplex virus 1 (HSV-1) latency entails the repression of productive ("lytic") gene expression. An attractive hypothesis to explain some of this repression involves inhibition of the expression of ICP0, a lytic gene activator, by a viral microRNA, miR-H2, which is completely complementary to ICP0 mRNA. To test this hypothesis, we engineered mutations that disrupt miR-H2 without affecting ICP0 in HSV-1. The mutant virus exhibited drastically reduced expression of miR-H2 but showed wild-type levels of infectious virus production and no increase in ICP0 expression in lytically infected cells, which is consistent with the weak expression of miR-H2 relative to the level of ICP0 mRNA in that setting. Following corneal inoculation of mice, the mutant was not significantly different from wild-type virus in terms of infectious virus production in the trigeminal ganglia during acute infection, mouse mortality, or the rate of reactivation from explanted latently infected ganglia. Critically, the mutant was indistinguishable from wild-type virus for the expression of ICP0 and other lytic genes in acutely and latently infected mouse trigeminal ganglia. The latter result may be related to miR-H2 being less effective in inhibiting ICP0 expression in transfection assays than a host microRNA, miR-138, which has previously been shown to inhibit lytic gene expression in infected ganglia by targeting ICP0 mRNA. Additionally, transfected miR-138 reduced lytic gene expression in infected cells more effectively than miR-H2. While this study provides little support for the hypothesis that miR-H2 promotes latency by inhibiting ICP0 expression, the possibility remains that miR-H2 might target other genes during latency.

IMPORTANCE

Herpes simplex virus 1 (HSV-1), which causes a variety of diseases, can establish lifelong latent infections from which virus can reactivate to cause recurrent disease. Latency is the most biologically interesting and clinically vexing feature of the virus. Ever since miR-H2's discovery as a viral microRNA bearing complete sequence complementarity to the mRNA for the important viral gene activator ICP0, inhibition of ICP0 expression by miR-H2 has been a major hypothesis to help explain the repression of lytic gene expression during latency. However, this hypothesis remained untested in latently infected animals. Using a miR-H2-deficient mutant virus, we found no evidence that miR-H2 represses the expression of ICP0 or other lytic genes in cells or mice infected with HSV-1. Although miR-H2 can repress ICP0 expression in transfection assays, such repression is weak. The results suggest that other mechanisms for miR-H2 activity and for the repression of lytic gene expression during latency deserve investigation.

Relative Contributions of Herpes Simplex Virus 1 ICP0 and vhs to Loss of Cellular IFI16 Vary in Different Human Cell Types

The herpes simplex virus 1 (HSV-1) ICP0 protein is an E3 ubiquitin ligase that promotes the degradation of several host cell proteins. Most studies have found that ICP0 promotes the loss of IFI16 in infected cells, but one study reported that ICP0 was not necessary or sufficient for loss of IFI16 in a tumor-derived cell line. Therefore, in this study, we examined the requirement for ICP0 in promoting the loss of IFI16 in several normal and tumor-derived cell lines. HSV-1 infection resulted in an observable decrease of IFI16 protein levels in normal human foreskin fibroblasts (HFFs), normal oral keratinocytes (NOKs), and HeLa cells but not in U2OS cells. During infection with an ICP0-null virus, we observed a reduced loss of IFI16 in HFFs and NOKs but not in HeLa cells. Ectopic expression of ICP0 from a transfected plasmid was sufficient to promote the loss of IFI16 in HFFs and NOKs. In the absence of ICP0, we observed a delayed reduction of IFI16 protein that correlated with a reduction in the steady-state levels of IFI16 mRNA. In addition, we show that the ICP0-independent loss of IFI16 in HeLa cells is dependent in part on the activity of the viral virion host shutoff (vhs) tegument protein. Together, these results demonstrate that HSV-1 promotes the loss of IFI16 through at least two mechanisms: (i) by ICP0-dependent degradation of IFI16 and (ii) by vhs-dependent turnover of IFI16 mRNA. In addition, this study highlights a potential intrinsic difference between normal and tumor-derived cells for the activities of IFI16 and HSV-1 ICP0.

IMPORTANCE

HSV-1 is a ubiquitous virus that establishes a lifetime persistent infection in humans. The relative success of HSV-1 as a pathogen is, in part, dependent on the expression of viral proteins that counteract host intrinsic defense mechanisms and that modulate immune responses during viral infection. In this study, we examined the relative roles of two viral gene products for the ability to promote loss of the antiviral IFI16 DNA sensor. We demonstrate that the viral immediate early ICP0 protein plays a dominant role in the loss of IFI16 in normal, but not tumor-derived, human cell lines. In contrast, viral vhs-mediated loss of IFI16 by mRNA destabilization is revealed to be dominant in tumor-derived cells in which ICP0 is nonfunctional. Together, these results contribute to our understanding of how HSV-1 modulates IFI16 protein levels and highlight cell-type-dependent differences between normal and tumor-derived cells.

Changes in Corneal Innervation after HSV-1 Latency Established with Different Reactivation Phenotypes

PURPOSE OF THE STUDY

We used a rabbit model infected with high phenotypic reactivators (HPRs) as well as recombinant HSV-1 (herpes simplex virus 1) with deletions to study their effect on corneal innervations after latency was established.

MATERIALS AND METHODS

Corneas from noninfected New Zealand white rabbits were used to obtain the entire map of corneal innervation. Others were inoculated with the HSV-1 strains McKrae, 17Syn+, or recombinant mutants with glycoprotein K (gK) deletion, or with infected early protein 0 (ICP0) deletion. The animals were euthanized at 124 to 125 days postinfection and the corneas were immunostained with a mouse monoclonal anti-βIII tubulin antibody. Images were acquired with a fluorescence microscope and corneal sub-basal nerve density was calculated on the basis of the whole mount images. Differences between the HSV-infected eyes, and comparison with normal control, were analyzed.

RESULTS

In the noninfected rabbit, the stroma was densely innervated in the central area and as a consequence the sub-basal epithelial nerve bundles were shorter, and no vortex was found. The HSV-infected corneas showed nerve damage in both epithelial and stromal nerves. Corneas infected with ICP0 and gK deletion mutants showed mild to moderate damage, while those infected with 17Syn+ and McKrae strains were seriously damaged. In the eyes infected with ICP0 and gK deletion, there were reduced numbers of sub-basal nerve bundles, but most of the corneas retained a normal stromal network. Corneas infected with 17 Syn+ and McKrae displayed destroyed nerve structures and formation of a scar tissue in the central cornea, in which only a few nerve fibers could be detected.

CONCLUSION

HSV-1 primary corneal infection seriously damages the corneal nerves, persisting for more than 4 months. Reduction of axonal transport (by gK deletion) or virus replication (by ICP0 deletion) significantly attenuated the nerve damage induced by the virus.

Transcription Factor NF-κB: An Update on Intervention Strategies

The nuclear factor (NF)-κB family of transcription factors are ubiquitous and pleiotropic molecules that regulate the expression of more than 150 genes involved in a broad range of processes including inflammation, immunity, cell proliferation, differentiation, and survival. The chronic activation or dysregulation of NF-κB signaling is the central cause of pathogenesis in many disease conditions and, therefore, NF-κB is a major focus of therapeutic intervention. Because of this, understanding the relationship between NF-κB and the induction of various downstream signaling molecules is imperative. In this review, we provide an updated synopsis of the role of NF-κB in DNA repair and in various ailments including cardiovascular diseases, HIV infection, asthma, herpes simplex virus infection, chronic obstructive pulmonary disease, and cancer. Furthermore, we also discuss the specific targets for selective inhibitors and future therapeutic strategies.

A Herpesviral Lytic Protein Regulates the Structure of Latent Viral Chromatin

Latent infections by viruses usually involve minimizing viral protein expression so that the host immune system cannot recognize the infected cell through the viral peptides presented on its cell surface. Herpes simplex virus (HSV), for example, is thought to express noncoding RNAs such as latency-associated transcripts (LATs) and microRNAs (miRNAs) as the only abundant viral gene products during latent infection. Here we describe analysis of HSV-1 mutant viruses, providing strong genetic evidence that HSV-infected cell protein 0 (ICP0) is expressed during establishment and/or maintenance of latent infection in murine sensory neurons in vivo Studies of an ICP0 nonsense mutant virus showed that ICP0 promotes heterochromatin and latent and lytic transcription, arguing that ICP0 is expressed and functional. We propose that ICP0 promotes transcription of LATs during establishment or maintenance of HSV latent infection, much as it promotes lytic gene transcription. This report introduces the new concept that a lytic viral protein can be expressed during latent infection and can serve dual roles to regulate viral chromatin to optimize latent infection in addition to its role in epigenetic regulation during lytic infection. An additional implication of the results is that ICP0 might serve as a target for an antiviral therapeutic acting on lytic and latent infections.

IMPORTANCE

Latent infection by viruses usually involves minimizing viral protein synthesis so that the host immune system cannot recognize the infected cells and eliminate them. Herpes simplex virus has been thought to express only noncoding RNAs as abundant gene products during latency. In this study, we found genetic evidence that an HSV lytic protein is functional during latent infection, and this protein may provide a new target for antivirals that target both lytic and latent infections.

Innate defense mechanisms against HSV-1 infection in the target tissues, skin and brain

Herpes simplex virus type 1 (HSV-1) initiates productive infection in mucocutaneous tissues to cause cold sores and establishes latent infection in the trigeminal ganglia. Under certain circumstances, HSV-1 may cause encephalitis. Here, we compared host innate defenses against HSV-1 in the two clinically relevant tissues, skin and brain, using a unique ex vivo system of organ culture. Upon HSV-1 infection and spread, apoptosis induction was observed in the skin, but not in brain tissues. While the two tissues elicited interferon (IFN-β) response upon HSV1 infection, IFN induction was more robust in the skin compared to the brain. Moreover, antiviral response to exogenous IFNβ treatment was much stronger in the skin compared to brain tissues. This observation was not related to the availability of the IFN receptor on cells' surface. Taken together, our study demonstrates differential innate antiviral responses to HSV-1 infection that may be exploited in future development of selective and tissue-specific anti-viral treatments.

Inhibition of HSV-1 Replication by Gene Editing Strategy

HSV-1 induced illness affects greater than 85% of adults worldwide with no permanent curative therapy. We used RNA-guided CRISPR/Cas9 gene editing to specifically target for deletion of DNA sequences of the HSV-1 genome that span the region directing expression of ICP0, a key viral protein that stimulates HSV-1 gene expression and replication. We found that CRISPR/Cas9 introduced InDel mutations into exon 2 of the ICP0 gene profoundly reduced HSV-1 infectivity in permissive human cell culture models and protected permissive cells against HSV-1 infection. CRISPR/Cas9 mediated targeting ICP0 prevented HSV-1-induced disintegration of promonocytic leukemia (PML) nuclear bodies, an intracellular event critical to productive HSV-1 infection that is initiated by interaction of the ICP0 N-terminus with PML. Combined treatment of cells with CRISPR targeting ICP0 plus the immediate early viral proteins, ICP4 or ICP27, completely abrogated HSV-1 infection. We conclude that RNA-guided CRISPR/Cas9 can be used to develop a novel, specific and efficacious therapeutic and prophylactic platform for targeted viral genomic ablation to treat HSV-1 diseases.

Widely Used Herpes Simplex Virus 1 ICP0 Deletion Mutant Strain dl1403 and Its Derivative Viruses Do Not Express Glycoprotein C Due to a Secondary Mutation in the gC Gene

Herpes simplex virus 1 (HSV-1) ICP0 is a multi-functional phosphoprotein expressed with immediate early kinetics. An ICP0 deletion mutant, HSV-1 dl1403, has been widely used to study the roles of ICP0 in the HSV-1 replication cycle including gene expression, latency, entry and assembly. We show that HSV-1 dl1403 virions lack detectable levels of envelope protein gC, and that gC is not synthesized in infected cells. Sequencing of the gC gene from HSV-1 dl1403 revealed a single amino acid deletion that results in a frameshift mutation. The HSV-1 dl1403 gC gene is predicted to encode a polypeptide consisting of the original 62 N-terminal amino acids of the gC protein followed by 112 irrelevant, non-gC residues. The mutation was also present in a rescuant virus and in two dl1403-derived viruses, D8 and FXE, but absent from the parental 17+, suggesting that the mutation was introduced during the construction of the dl1403 virus, and not as a result of passage in culture.

Development of a novel cell-based assay to monitor the transactivation activity of the HSV-1 protein ICP0

The herpes simplex virus type 1 (HSV-1) immediate-early phosphoprotein infected cell protein 0 (ICP0) is a potent transcriptional activator of viral genes and is required for efficient viral replication and reactivation from latency. However, it is largely unknown what role specific cellular factors play in the transactivator function of ICP0. With the long-term goal of identifying these factors, we developed a cell-based assay in a 96-well format to measure this activity of ICP0. We designed a system using a set of HSV-1 GFP reporter viruses in which the expression of GFP is potently induced by ICP0 in cell culture. The initial feasibility of this system was confirmed over a 24-h period by fluorescence microscopy. We adapted this assay to a 96-well plate format, quantifying GFP expression with a fluorescence scanner. Our results indicate that the cell-based assay we developed is a valid and effective method for examining the transactivating activity of ICP0. This assay can be used to identify cellular factors that regulate the transactivating activity of ICP0.

Quantification of the host response proteome after herpes simplex virus type 1 infection

Viruses employ numerous host cell metabolic functions to propagate and manage to evade the host immune system. For herpes simplex virus type 1 (HSV1), a virus that has evolved to efficiently infect humans without seriously harming the host in most cases, the virus-host interaction is specifically interesting. This interaction can be best characterized by studying the proteomic changes that occur in the host during infection. Previous studies have been successful at identifying numerous host proteins that play important roles in HSV infection; however, there is still much that we do not know. This study identifies host metabolic functions and proteins that play roles in HSV infection, using global quantitative stable isotope labeling by amino acids in cell culture (SILAC) proteomic profiling of the host cell combined with LC-MS/MS. We showed differential proteins during early, mid and late infection, using both cytosolic and nuclear fractions. We identified hundreds of differentially regulated proteins involved in fundamental cellular functions, including gene expression, DNA replication, inflammatory response, cell movement, cell death, and RNA post-transcriptional modification. Novel differentially regulated proteins in HSV infections include some previously identified in other virus systems, as well as fusion protein, involved in malignant liposarcoma (FUS) and hypoxia up-regulated 1 protein precursor (HYOU1), which have not been identified previously in any virus infection.

cGAS-mediated stabilization of IFI16 promotes innate signaling during herpes simplex virus infection

Interferon γ-inducible protein 16 (IFI16) and cGMP-AMP synthase (cGAS) have both been proposed to detect herpesviral DNA directly in herpes simplex virus (HSV)-infected cells and initiate interferon regulatory factor-3 signaling, but it has been unclear how two DNA sensors could both be required for this response. We therefore investigated their relative roles in human foreskin fibroblasts (HFFs) infected with HSV or transfected with plasmid DNA. siRNA depletion studies showed that both are required for the production of IFN in infected HFFs. We found that cGAS shows low production of cGMP-AMP in infected cells, but instead cGAS is partially nuclear in normal human fibroblasts and keratinocytes, interacts with IFI16 in fibroblasts, and promotes the stability of IFI16. IFI16 is associated with viral DNA and targets to viral genome complexes, consistent with it interacting directly with viral DNA. Our results demonstrate that IFI16 and cGAS cooperate in a novel way to sense nuclear herpesviral DNA and initiate innate signaling.

A neuron-specific host microRNA targets herpes simplex virus-1 ICP0 expression and promotes latency

After infecting peripheral sites, herpes simplex virus (HSV) invades the nervous system and initiates latent infection in sensory neurons. Establishment and maintenance of HSV latency require host survival, and entail repression of productive cycle ("lytic") viral gene expression. We find that a neuron-specific microRNA, miR-138, represses expression of ICP0, a viral transactivator of lytic gene expression. A mutant HSV-1 (M138) with disrupted miR-138 target sites in ICP0 mRNA exhibits enhanced expression of ICP0 and other lytic proteins in infected neuronal cells in culture. Following corneal inoculation, M138-infected mice have higher levels of ICP0 and lytic transcripts in trigeminal ganglia during establishment of latency, and exhibit increased mortality and encephalitis symptoms. After full establishment of latency, the fraction of trigeminal ganglia harboring detectable lytic transcripts is greater in M138-infected mice. Thus, miR-138 is a neuronal factor that represses HSV-1 lytic gene expression, promoting host survival and viral latency.

Characterization of a proteolytically stable D-peptide that suppresses herpes simplex virus 1 infection: implications for the development of entry-based antiviral therapy

Uncontrolled herpes simplex virus 1 (HSV-1) infection can advance to serious conditions, including corneal blindness or fatal encephalitis. Here, we describe a highly potent anti-HSV-1 peptide (DG2) that inhibits HSV-1 entry into host cells and blocks all aspects of infection. Importantly, DG2 is highly resistant to proteases and shows minimal toxicity, paving the way for prophylactic or therapeutic application of the peptide in vivo.

Fungal metabolite myriocin promotes human herpes simplex virus-2 infection

AIMS

Myriocin is a fungal metabolite with antiviral activity, including influenza, hepatitis B, and hepatitis C viruses. We investigated whether myriocin has activity against human HSV-2, one of the most prevalent pathogens of sexually transmitted disease.

MAIN METHODS

Cell culture systems were used to evaluate myriocin effect on HSV-2 infection. Plaque forming assay and immunoblotting studies were used to determine virus production and viral protein expression, respectively.

KEY FINDINGS

Myriocin showed no cytotoxic effect at up to 5 μM. Myriocin treatment did not inhibit HSV-2 infection. Instead, the treatment resulted in accelerated replication of HSV-2 and increased titers of infectious virion. The effect was detected at concentrations as low as 3 nM and plateaued at approximately 30 nM. Myriocin at 30 nM increased HSV-2 production by approximately 1.7 logs. Myriocin also promoted HSV-1 infection but required higher concentrations. A time course study revealed that myriocin promoted HSV-2 infection by acceleration of virus replication. Unlike trichostatin A that promotes HSV-2 infection and histone modifications, myriocin treatment did not alter histone modifications. Myriocin is a well characterized inhibitor of sphingolipid biosynthesis pathway. Structurally different inhibitors of the pathway showed no effect on HSV-2 infection. Exogenous sphingolipids did not reverse the effect of myriocin on HSV-2 infection either.

SIGNIFICANCE

We found that myriocin promotes HSV-2 replication at nanomolar concentrations with yet unknown mechanisms. Further studies may uncover novel mechanisms regulating HSV replication and targets of myriocin action. This may have potential application in enhancing efficacy of oncolytic HSV for cancer therapy and other diseases.