Deletion of the Transcriptional Coactivator HCF-1 In Vivo Impairs the Removal of Repressive Heterochromatin from Latent HSV Genomes and Suppresses the Initiation of Viral Reactivation

Mechanisms of HSV gene regulation during latency and reactivation

Herpes simplex virus 1 and 2 (HSV-1 and HSV-2) are prevalent human pathogens associated with many diseases. After productive (lytic) infection in peripheral tissues, HSV establishes lifelong latent infection in neurons of the peripheral nervous system. Periodic reactivation from latency, triggered by certain stimuli, can resume the lytic cycle. Lytic infection, latent infection and reactivation follow distinct viral gene expression patterns. The switch between the different infection programs is controlled by complicated regulatory mechanisms involving numerous viral and host molecules. Recent studies integrating cutting-edge technologies including neuronal culture techniques have greatly improved our understanding of the molecular details of latency and reactivation but many questions remain. This review summarizes the current knowledge about how HSV gene expression is regulated during latency and reactivation and discusses the important questions remaining to be addressed in future.

The opportunities and challenges of epigenetic approaches to manage herpes simplex infections

INTRODUCTION

Despite the existence of antivirals that potently and efficiently inhibit the replication of herpes simplex virus 1 and 2 (HSV-1, -2), their ability to establish and maintain, and reactivate from, latency has precluded the development of curative therapies. Several groups are exploring the opportunities of targeting epigenetic regulation to permanently silence latent HSV genomes or induce their simultaneous reactivation in the presence of antivirals to flush the latent reservoirs, as has been explored for HIV.

AREAS COVERED

This review covers the basic principles of epigenetic regulation with an emphasis on those mechanisms relevant to the regulation of herpes simplex viruses, as well as the current knowledge on the regulation of lytic infections and the establishment and maintenance of, and reactivation from, latency, with an emphasis on epigenetic regulation. The differences with the epigenetic regulation of viral and cellular gene expression are highlighted as are the effects of known epigenetic regulators on herpes simplex viruses. The major limitations of current models to the development of novel antiviral strategies targeting latency are highlighted.

EXPERT OPINION

We provide an update on the epigenetic regulation during lytic and latent HSV-1 infection, highlighting the commonalities and differences with cellular gene expression and the potential of epigenetic drugs as antivirals, including the opportunities, challenges, and potential future directions.

A fur plucking model to study herpes simplex virus reactivation and recurrent disease

Herpes simplex viruses (HSV-1 and HSV-2) most commonly cause ulcerative epithelial lesions (cold sores and genital herpes). Importantly, HSV establishes life-long persistent (latent) infection in peripheral neurons. Reactivation from latency produces recurrent epithelial lesions, which constitute the greatest burden of HSV disease in people. The mechanisms that regulate latency and reactivation remain incompletely understood, in part due to limitations in the animal models available for studying HSV reactivation. We have developed a simple and tractable model to induce HSV-1 and HSV-2 reactivation from latency to cause recurrent skin disease. We infected C57BL/6 mice with HSV-1 (strains NS, F, SC16, 17syn+) or HSV-2 (strain 333) on flank skin depilated by manual plucking. After at least 35 days post-infection (dpi), we replucked the fur from the infected flank and observed recurrent lesions in the same dermatome as the primary infection. We detected HSV DNA in dermatome skin through 4 days post-replucking and observed viral antigen and reporter signal in skin lesions by histology, consistent with viral replication following reactivation. In addition to C57BL/6 mice, we were able to produce reactivation in Balb/c and SKH-1 mice. We found that shaving the ipsilateral flank or plucking the contralateral flank did not induce recurrent skin lesions, suggesting that fur plucking is a specific stimulus that induces HSV reactivation. Furthermore, we were able to induce multiple rounds of plucking-induced recurrent disease, providing a model to investigate the lifelong nature of HSV infection. This new model provides a tractable system for studying pathogenic mechanisms of and therapeutic interventions against HSV reactivation and recurrent disease.

IMPORTANCE

Herpes simplex viruses (HSV-1 and HSV-2) have infected over half of the US adult population to cause a lifelong, persistent infection; however, our understanding of the mechanisms that govern HSV reactivation and recurrent disease is incomplete. This is in part due to limitations in the animal models used to study recurrent disease, which are laborious and inefficient in mice. To address this technical gap, we developed a mouse model in which fur plucking after flank skin infection is sufficient to induce episodes of HSV reactivation and recurrent disease. Our work provides a model for the field to investigate the pathogenic mechanisms of HSV and immune responses during recurrent disease and provides an opportunity to investigate the neurobiology of HSV infection.

Neuronal miR-9 promotes HSV-1 epigenetic silencing and latency by repressing Oct-1 and Onecut family genes

Herpes simplex virus 1 (HSV-1) latent infection entails repression of viral lytic genes in neurons. By functional screening using luciferase-expressing HSV-1, we identify ten neuron-specific microRNAs potentially repressing HSV-1 neuronal replication. Transfection of miR-9, the most active candidate from the screen, decreases HSV-1 replication and gene expression in Neuro-2a cells. Ectopic expression of miR-9 from lentivirus or recombinant HSV-1 suppresses HSV-1 replication in male primary mouse neurons in culture and mouse trigeminal ganglia in vivo, and reactivation from latency in the primary neurons. Target prediction and validation identify transcription factors Oct-1, a known co-activator of HSV transcription, and all three Onecut family members as miR-9 targets. Knockdown of ONECUT2 decreases HSV-1 yields in Neuro-2a cells. Overexpression of each ONECUT protein increases HSV-1 replication in Neuro-2a cells, human induced pluripotent stem cell-derived neurons, and primary mouse neurons, and accelerates reactivation from latency in the mouse neurons. Mutagenesis, ChIP-seq, RNA-seq, ChIP-qPCR and ATAC-seq results suggest that ONECUT2 can nonspecifically bind to viral genes via its CUT domain, globally stimulate viral gene transcription, reduce viral heterochromatin and enhance the accessibility of viral chromatin. Thus, neuronal miR-9 promotes viral epigenetic silencing and latency by targeting multiple host transcription factors important for lytic gene activation.

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.