Herpesvirus quiescence in neuronal cells: antiviral conditions not required to establish and maintain HSV-2 quiescence

ICP0 antagonizes ICP4-dependent silencing of the herpes simplex virus ICP0 gene

ICP0 is a regulatory protein that plays a critical role in the replication-latency balance of herpes simplex virus (HSV). Absence of ICP0 renders HSV prone to establish quiescent infections, and thus cellular repressor(s) are believed to silence HSV mRNA synthesis when ICP0 fails to accumulate. To date, an ICP0-antagonized repressor has not been identified that restricts HSV mRNA synthesis by more than 2-fold. We report the unexpected discovery that HSV's major transcriptional regulator, ICP4, meets the criteria of a bona fide ICP0-antagonized repressor of viral mRNA synthesis. Our study began when we noted a repressive activity that restricted ICP0 mRNA synthesis by up to 30-fold in the absence of ICP0. When ICP0 accumulated, the repressor only restricted ICP0 mRNA synthesis by 3-fold. ICP4 proved to be necessary and sufficient to repress ICP0 mRNA synthesis, and did so in an ICP4-binding-site-dependent manner. ICP4 co-immunoprecipitated with FLAG-tagged ICP0; thus, a physical interaction likely explains how ICP0 antagonizes ICP4's capacity to silence the ICP0 gene. These findings suggest that ICP0 mRNA synthesis is differentially regulated in HSV-infected cells by the virus-encoded repressor activity embedded in ICP4, and a virus-encoded antirepressor, ICP0. Bacteriophage λ relies on a similar repression-antirepression regulatory scheme to “decide” whether a given infection will be productive or silent. Therefore, our findings appear to add to the growing list of inexplicable similarities that point to a common evolutionary ancestry between the herpesviruses and tailed bacteriophage.

Efficient quiescent infection of normal human diploid fibroblasts with wild-type herpes simplex virus type 1

Quiescent infection of cultured cells with herpes simplex virus type 1 (HSV-1) provides an important, amenable means of studying the molecular mechanics of a nonproductive state that mimics key aspects of in vivo latency. To date, establishing high-multiplicity nonproductive infection of human cells with wild-type HSV-1 has proven challenging. Here, we describe simple culture conditions that established a cell state in normal human diploid fibroblasts that supported efficient quiescent infection using wild-type virus and exhibited many important properties of the in vivo latent state. Despite the efficient production of immediate early (IE) proteins ICP4 and ICP22, the latter remained unprocessed, and viral late gene products were only transiently and inefficiently produced. This low level of virus activity in cultures was rapidly suppressed as the nonproductive state was established. Entry into quiescence was associated with inefficient production of the viral trans-activating protein ICP0, and the accumulation of enlarged nuclear PML structures normally dispersed during productive infection. Lytic replication was rapidly and efficiently restored by exogenous expression of HSV-1 ICP0. These findings are in agreement with previous models in which quiescence was established with HSV mutants disrupted in their expression of IE gene products that included ICP0 and, importantly, provide a means to study cellular mechanisms that repress wild-type viral functions to prevent productive replication. We discuss this model in relation to existing systems and its potential as a simple tool to study the molecular mechanisms of quiescent infection in human cells using wild-type HSV-1.

Herpesvirus quiescence in neuronal cells IV: virus activation induced by pituitary adenylate cyclase-activating polypeptide (PACAP) involves the protein kinase A pathway

Pituitary adenylate cyclase-activating polypeptide (PACAP) is a naturally occurring peptide found in the central nervous system that plays a role in somatosensory processing and activation of protein kinase A (PKA) and protein kinase C (PKC). Because activation of PKA or PKC results in reactivation of HSV-1 from latently infected embryonic neuronal cells, PACAP was used to evaluate HSV-1 activation from quiescently infected (QIF)-PC12 cells. Our studies demonstrate that physiologically relevant concentrations of PACAP38 and PACAP27 induce HSV-1 activation from QIF-PC12 cell cultures in a dose-dependent fashion. PACAP-induced activation of virus was significantly impaired by the PKA-inhibitor, H-89 (20 microM), whereas treatment with the PKC-inhibitor, GF109203X (1 microM), was without affect. Additionally, direct activation of PKA with cAMP analogs, 8-(4-chlorophenylthio)- and dibutyryl-cAMP, only partially mimicked the effect of PACAP on virus activation. Taken together, PACAP induced HSV-1 activation from QIF-PC12 cells involves the PKA and possibly cAMP-independent pathways. This report is the first to demonstrate that PACAP induces HSV-1 activation from a quiescent state and that this in vitro cell model is useful for studying early inductive events that lead to virus production from quiescence.