Following bovine herpesvirus 1 (BoHV-1) acute infection of ocular, oral or nasal cavities, sensory neurons within trigeminal ganglia are an important site for latency. Stress, as mimicked by the synthetic corticosteroid dexamethasone, consistently induces reactivation from latency. Expression of two key viral transcriptional regulatory proteins, infected cell protein 0 (bICP0) and bICP4, are regulated by sequences within the immediate early promoter (IEtu1). A separate early promoter also drives bICP0 expression, presumably to ensure sufficient levels of this important transcriptional regulatory protein. Productive infection and bICP0 early promoter activity are cooperatively transactivated by Krüppel like factor 4 (KLF4) and a Type I nuclear hormone receptor (NHR), androgen receptor, glucocorticoid receptor, or progesterone receptor. The bICP0 early promoter contains 3 separate transcriptional enhancers that mediate cooperative transactivation. In contrast to the IEtu1 promoter, the bICP0 early promoter lacks consensus Type I NHR binding sites. Consequently, we hypothesized KLF4 and Sp1 binding sites are essential for Type I NHR and KLF4 to transactivate the bICP0 promoter. Mutating KLF4 and Sp1 binding sites in each enhancer domain significantly reduced transactivation by KLF4 and a Type I NHR. Chromatin immunoprecipitation (ChIP) studies demonstrated occupancy of bICP0 early promoter sequences by KLF4 and Type I NHR is significantly reduced when KLF4 and/or Sp1 binding sites were mutated. These studies suggest cooperative transactivation of the bICP0 E promoter by Type I NHRs and a stress induced pioneer transcription factor (KLF4) promote viral replication and spread in neurons or non-neural cells in reproductive tissue.
IMPORTANCE
Understanding how stressful stimuli and changes in cellular milieu mediate viral replication and gene expression in the natural host is important for developing therapeutic strategies that impair virus transmission and disease. For example, bovine herpesvirus 1 (BoHV-1) reactivation from latency is consistently induced by the synthetic corticosteroid dexamethasone, which mimics the effects of stress. Furthermore, BoHV-1 infection increases the incidence of abortion in pregnant cows suggesting sex hormones stimulate viral growth in certain tissue. Previous studies revealed Type I nuclear hormone receptors (androgen, glucocorticoid, or progesterone) and the pioneer transcription factor, Krüppel like factor 4 (KLF4), cooperatively transactivate the BoHV-1 infected cell protein 0 (bICP0) early promoter. Transactivation was mediated by Sp1 and/or KLF4 consensus binding sites within the 3 transcriptional enhancers. These studies underscore the complexity by which BoHV-1 exploits Type I NHR fluctuations to enhance viral gene expression, replication, and transmission in the natural host.
Infected cell protein 0 encoded by bovine herpesvirus 1 can activate caspase 3 when overexpressed in transfected cells
