β-Catenin, a Transcription Factor Activated by Canonical Wnt Signaling, Is Expressed in Sensory Neurons of Calves Latently Infected with Bovine Herpesvirus 1

Like many α-herpesvirinae subfamily members, bovine herpes virus 1 (BoHV-1) expresses an abundant transcript in latently infected sensory neurons: the latency-related (LR)-RNA. LR-RNA encodes a protein (ORF2) that inhibits apoptosis, interacts with Notch family members, interferes with Notch mediated transcription, and stimulates neurite formation in cells expressing Notch. A LR mutant virus containing stop codons at the amino-terminus of ORF2 does not reactivate from latency or replicate efficiently in certain tissues indicating LR gene products are important. In this study, β-catenin, a transcription factor activated by the canonical Wnt signaling pathway was frequently detected in ORF2+ trigeminal ganglionic neurons of latently infected, but not mock-infected calves. Conversely, the lytic cycle regulatory protein (bICP0) was not frequently detected in β-catenin+ neurons in latently infected calves. During dexamethasone-induced reactivation from latency, mRNA expression levels of two Wnt antagonists, dickkopf1 (DKK1) and secreted frizzled protein 2 (SFRP2), were induced in bovine TG, which correlated with reduced β-catenin protein expression in TG neurons six hours after dexamethasone treatment. ORF2 and a coactivator of β-catenin, mastermind like protein 1 (MAML1), stabilized β-catenin protein levels and stimulated β-catenin dependent transcription in mouse neuroblastoma cells more effectively than MAML1 or ORF2 alone. Neuroblastoma cells expressing ORF2, MAML1, and β-catenin were highly resistant to cell death following serum withdrawal, whereas most cells transfected with only one of these genes died. The Wnt signaling pathway interferes with neuro-degeneration but promotes neuronal differentiation suggesting that stabilization of β-catenin expression by ORF2 promotes neuronal survival and differentiation. IMPORTANCE Bovine herpesvirus 1 (BoHV-1) is an important pathogen of cattle and like many α-herpesvirinae subfamily members establish latency in sensory neurons. Life-long latency and the ability to reactivate from latency are crucial for virus transmission. Maintaining the survival and normal functions of terminally differentiated neurons is crucial for life-long latency. Our studies revealed BoHV-1 gene products expressed during latency stabilize expression of the transcription factors, β-catenin, and perhaps its cofactor mastermind like 1 (MAML1). In contrast to latency, β-catenin expression in sensory neurons is not detectable following treatment of latently infected calves with the synthetic corticosteroid dexamethasone to initiate reactivation from latency. A viral protein (ORF2) expressed in a subset of latently infected neurons stabilized β-catenin and MAML1 in transfected cells. ORF2, β-catenin, and MAML1 also enhanced cell survival when growth factors were withdrawn suggesting these genes enhance survival of latently infected neurons.