Characterization of a Nerve Growth Factor-inducible Cellular Activity That Enhances Herpes Simplex Virus Type 1 Gene Expression and Replication of an ICP0 Null Mutant in Cells of Neural Lineage

Herpes simplex virus type 1 (HSV-1) ICP0 is required for efficient viral gene expression during lytic infection, especially at low multiplicities. A series of cellular activities that can substitute for ICP0 has been identified, suggesting that when the activity of ICP0 is limiting, these activities can substitute for ICP0 to activate viral gene expression. The cellular activities may be especially important during reactivation of HSV from neuronal latency when viral gene expression is initiated in the absence of prior viral protein synthesis. Consistent with this hypothesis, we have identified an inducible activity in cells of neural lineage (PC12) that can complement the low-multiplicity growth phenotype of an ICP0 null mutant, n212. Pretreatment of PC12 cells with nerve growth factor (NGF) or fibroblast growth factor (FGF) prior to infection produced a 10- to 20-fold increase in the 24-h yield of n212 but only a 2- to 4-fold increase in the yield of wild-type virus relative to mock treatment. Slot blot analysis of nuclear DNA isolated from infected cells treated or mock treated with NGF indicated that NGF treatment does not significantly affect viral entry. The NGF-induced activity in PC12 cells was expressed transiently, with peak complementing activity observed when cells were treated with NGF 12 h prior to infection. Addition of NGF 3 h after infection had little effect on virus yield. The NGF-induced cellular activity was inhibited by pretreatment of PC12 cells with kinase inhibitors that have high specificity for kinases involved in NGF/FGF-dependent signal transduction. RNase protection assays demonstrated that the NGF-inducible PC12 cell activity, like that of ICP0, functions to increase the level of viral mRNA during low-multiplicity infection. These results suggest that activation of viral transcription by ICP0 and transcriptional activation of cellular genes by NGF and FGF utilize common signal transduction pathways in PC12 cells.