De Novo Synthesis of VP16 Coordinates the Exit from HSV Latency In Vivo

Abstract

The mechanism controlling the exit from herpes simplex virus latency (HSV) is of central importance to recurrent disease and transmission of infection, yet interactions between host and viral functions that govern this process remain unclear. The cascade of HSV gene transcription is initiated by the multifunctional virion protein VP16, which is expressed late in the viral replication cycle. Currently, it is widely accepted that VP16 transactivating function is not involved in the exit from latency. Utilizing the mouse ocular model of HSV pathogenesis together with genetically engineered viral mutants and assays to quantify latency and the exit from latency at the single neuron level, we show that in vivo (i) the VP16 promoter confers distinct regulation critical for viral replication in the trigeminal ganglion (TG) during the acute phase of infection and (ii) the transactivation function of VP16 (VP16TF) is uniquely required for the exit from latency. TG neurons latently infected with the VP16TF mutant in1814 do not express detectable viral proteins following stress, whereas viruses with mutations in the other major viral transcription regulators ICP0 and ICP4 do exit the latent state. Analysis of a VP16 promoter/reporter mutant in the background of in1814 demonstrates that the VP16 promoter is activated in latently infected neurons following stress in the absence of other viral proteins. These findings support the novel hypothesis that de novo expression of VP16 regulates entry into the lytic program in neurons at all phases of the viral life cycle. HSV reactivation from latency conforms to a model in which stochastic derepression of the VP16 promoter and expression of VP16 initiates entry into the lytic cycle.

Analysis of latent viral gene expression in natural and experimental latency models of human cytomegalovirus and its correlation with histone modifications at a latent promoter

“…2b, lane 12) -concomitant with the known reactivation of lytic gene expression in this experimental model of HCMV latency (Reeves et al, 2005a). Detectable UL82 RNA expression did not precede reactivation of HCMV major IE gene expression and thus, at least in this experimental model of HCMV latency, there is no evidence for pp71 acting as a 'latency breaker', as proposed for VP16 (Thompson et al, 2009). …”

“…Finally, we asked whether expression of the viral tegument protein pp71, a potent transactivator of the MIEP during lytic infection (Liu & Stinski, 1992;Spaete & Mocarski, 1985), could promote reactivation of major IE gene expression analogous to that proposed for VP16 during herpes simplex virus type 1 reactivation (Thompson & Sawtell, 2006;Thompson et al, 2009). However, no pp71 expression was detected in latently infected cells immediately prior to maturation and reactivation (Fig.…”

HSV-1 gene expression from reactivated ganglia is disordered and concurrent with suppression of latency-associated transcript and miRNAs

“…The fundamental question raised by this report is whether disordered gene expression is characteristic of neurons deprived of regulatory NGF harboring latent virus, a general characteristic of neurons triggered to reactivate virus, or a general characteristic of neurons infected with HSV-1. Reports of aberrant HSV gene expression in neurons based on studies in vivo and in vitro abound (22)(23)(24)(25). If this aberrant gene expression is a general characteristic of neurons, the model in which retention of VP16 in the cytoplasm results in silencing of α genes, and hence the expression of all viral genes, becomes untenable.…”

Control of viral latency in neurons by axonal mTOR signaling and the 4E-BP translation repressor

“…In response to incompletely understood physiological cues or changes in immune status, the virus emerges from latency or ''reactivates,'' initiating a complex gene expression program that results in viral replication and culminates in infectious virus production (Knipe and Cliffe 2008). Studies using small animal models have been instrumental in defining viral gene products involved in reactivation and the role of host innate and acquired immune systems in controlling latency (Wagner and Bloom 1997;Cunningham et al 2006;Knickelbein et al 2008; Thompson et al 2009). While it was first hypothesized in the early 20th century that the virus colonizes peripheral neurons and establishes lifelong latent infections capable of triggering recurrent disease (Cushing 1905;Goodpasture 1929), the molecular signals regulating HSV latency specifically within host neurons, how reactivation is triggered in response to varied environmental stimuli, and how signal transmission occurs from the periphery to repress latent genomes hidden in neuronal nuclei are questions that remain a mystery.…”