Specific Akt Family Members Impair Stress-Mediated Transactivation of Viral Promoters and Enhance Neuronal Differentiation: Important Functions for Maintaining Latency

Abstract

Neurotropic α-herpesvirinae subfamily members, bovine herpesvirus 1 (BoHV-1) and herpes simplex virus type 1 (HSV-1) for example, establish and maintain life-long latent infections in neurons. Following infection of ocular, oral, or nasal cavities, sensory neurons within trigeminal ganglia (TG) are an important site for latency. Certain external stressors can trigger reactivation from latency, in part, because activation of the glucocorticoid receptor (GR) stimulates productive infection and promoters that drive expression of key viral transcriptional regulators. The Akt serine/threonine protein kinase family is linked to maintaining latency. For example, Akt3 is detected in more TG neurons during BoHV-1 latency compared to reactivation and uninfected calves. Furthermore, Akt signaling correlates with maintaining HSV-1 latency in certain neuronal models of latency. Finally, an active Akt protein kinase is crucial for the ability of the HSV-1 latency associated transcript (LAT) to inhibit apoptosis in neuronal cell lines. Consequently, we hypothesized that viral and/or cellular factors impair stress-induced transcription and reduce the incidence of reactivation triggered by low levels of stress. New studies demonstrate Akt1 and Akt2, but not Akt3, significantly reduced GR-mediated transactivation of the BoHV-1 immediate early transcription unit 1 (IEtu1) promoter, HSV-1 infected cell protein 0 (ICP0) promoter, and mouse mammary tumor virus long terminal repeat. Akt3, but not Akt1 or Akt2, significantly enhanced neurite formation in mouse neuroblastoma cells, which correlates with repairing damaged neurons. These studies suggest unique biological properties of the three Akt family members promote the maintenance of latency in differentiated neurons. IMPORTANCE External stressful stimuli are known to increase the incidence of reactivation of alpha-herpesvirinae subfamily members. Activation of the glucocorticoid receptor (GR) by the synthetic corticosteroid dexamethasone stimulates bovine herpesvirus 1 (BoHV-1) and herpes simplex virus 1 (HSV-1) reactivation. Furthermore, GR and dexamethasone stimulate productive infection and promoters that drive expression of viral transcriptional regulators. These observations lead us to predict that stress-induced transcription is impaired by factors abundantly expressed during latency. Interestingly, activation of the Akt family of serine/threonine protein kinases is linked to maintenance of latency. New studies reveal Akt1 and Ak2, but not Akt3, impaired GR and dexamethasone mediated transactivation of the BoHV-1 immediate early transcription unit 1 and HSV-1 ICP0 promoters. Strikingly, Akt3, but not Akt1 or Akt2 stimulated neurite formation in mouse neuroblastoma cells, a requirement for neurogenesis. These studies provide insight into how Akt family members may promote the maintenance of life-long latency.

Stress Induced Transcription Factors Transactivate the Herpes Simplex Virus 1 Infected Cell Protein 27 (ICP27) Transcriptional Enhancer

“…Two consensus GR response elements (GRE) in the IEtu1 promoter are essential for GR-α and DEX-mediated transactivation. Furthermore, the mouse mammary tumor virus LTR is strongly transactivated by DEX and GR-α expression plasmids in Neuro-2A [65], which is consistent with independent studies [69]. Hence, we believe transactivation of the ICP27 enhancer fragments by GR-α and KLF15 does not require DEX in Neuro-2A cells and only has a modest effect in CV-1 cells.…”

“…The effects of retinoic acid (RA) on these treatments were also examined, because reducing FBS and retinoic acid consistently arrests the cell cycle and establishes a neuronal-like phenotype in Neuro-2A cells, as denoted by neurite formation [59,62]. Consistent with our previous studies [63][64][65][66], incubating Neuro-2A cells in 2% stripped FBS for 48 h stimulated neurite formation (Figure 4A), which was enhanced by RA treatment. Although Neurobasal™ and a B-27 supplement are generally used for primary rodent neurons, its effects were examined in Neuro-2A cells.…”

Single‐cell transcriptomics identifies Gadd45b as a regulator of herpesvirus‐reactivating neurons

“…However, in vitro infection models using cultured primary neurons isolated from dissociated ganglia of rodents (Wilcox & Johnson, 1988; Camarena et al , 2010; Cliffe et al , 2015) recapitulate many aspects of in vivo animal models and provide a powerful opportunity to dissect the molecular circuitry that impacts virus‐neuron interactions and latency. Importantly, studies from our group and others have discovered that continuous neuronal host‐cell signalling through the AKT‐mTORC1 axis is required to maintain HSV‐1 latency (Camarena et al , 2010; Kobayashi et al , 2012; Hu et al , 2019; Zhao et al , 2020). Using this in vitro infection model, we recently showed that maintenance of HSV‐1 latency in primary neurons involves a convergence of the extrinsic TrkA receptor–mediated neurotrophin signalling with a secondary intrinsic signal generated via the endogenous nuclear DNA damage response (DDR) that acts together to maintain AKT activation (Hu et al , 2019).…”

Inhibition of Stress-Induced Viral Promoters by a Bovine Herpesvirus 1 Non-Coding RNA and the Cellular Transcription Factor, β-Catenin

“…While it was clear that wt β-catenin did not significantly reduce IEtu1 promoter activity, we suggest that when the β-catenin destruction complex is not active, β-catenin directly or indirectly can impair certain promoters stimulated by stress. A recent study demonstrated that the serine/threonine protein kinase (Akt) impairs stress-induced transcription [ 55 ]. Since the Akt pathways and Wnt/β-catenin signaling pathways form a feed-forward signaling loop [ 56 , 57 , 58 ], we predict that the ability of β-catenin (S33Y) to impair stress-induced transcription is dependent on the Akt signaling pathway.…”