UNLABELLED

Bovine alphaherpesvirus 1 (BoHV-1) continues to be a significant bovine pathogen. Acute infections culminate in lifelong latent infections in sensory neurons of trigeminal ganglia (TG) and the central nervous system. The synthetic corticosteroid dexamethasone consistently initiates BoHV-1 reactivation from latency in calves or rabbits. The immediate early transcription unit 1 (IEtu1) promoter contains two glucocorticoid receptor (GR) response elements and drives the expression of infected cell protein 0 (bICP0) and bICP4. GR, Krüppel-like Factor 15 (KLF15), and dexamethasone treatment cooperatively transactivate the IEtu1 promoter. Conversely, the bICP0 early (E) promoter is cooperatively transactivated by two pioneer transcription factors, GR and KLF4, that are capable of binding silent heterochromatin and activating transcription. Consequently, we hypothesized that GR activates IEtu1 and bICP0 E promoters, which trigger productive infection and reactivation from latency. Notably, BoHV-1 does not replicate in GR null monkey kidney cells (COS-7), unless a GR expression plasmid is transfected with the BoHV-1 genome. Furthermore, GR occupancy of IEtu1 and bICP0 E promoters correlates with viral replication in COS-7 cells. In TG of latently infected calves, IEtu1 and bICP0 E promoters were occupied by a heterochromatin marker H3K9me3 (histone 3 trimethylation at lysine 9), but not GR. Following dexamethasone treatment of latently infected calves for 3 hours, IEtu1 and bICP0 E promoters were occupied by GR and histone 3 acetylated at lysine 9, which correlates with active transcription. Collectively, these studies provide new insights into the mechanism by which stress triggers bICP0 and bICP4 protein expression during reactivation from latency.

IMPORTANCE

Bovine alphaherpesvirus 1 (BoHV-1), a significant pathogen, establishes life-long latency in certain neurons. Dexamethasone, a synthetic corticosteroid, consistently induces BoHV-1 reactivation from latency. Glucocorticoid receptor (GR) and dexamethasone transactivate the immediate early transcription unit 1 (IEtu1) promoter, which drives expression of infected cell protein 0 (bICP0) and bICP4. GR also transactivates the bICP0 early (E) promoter via a ligand-independent manner. Notably, GR and DEX induced BoHV-1 replication in non-permissive COS-7 cells. Furthermore, GR and a histone 3 marker, H3K9 acetylation, are associated with active chromatin and occupy the IEtu1 and bICP0 E promoters when latently infected calves are treated with dexamethasone for 3 hours. Conversely, a heterochromatin marker, histone 3 trimethylated at lysine 9, but not GR, occupied these viral promoters during latency. These studies revealed that GR and dexamethasone play crucial roles in chromatin remodeling of IEtu1 and bICP0 E promoters, which correlate with viral replication and reactivation from latency.