Control of viral latency in neurons by axonal mTOR signaling and the 4E-BP translation repressor

Abstract

Latent herpes simplex virus-1 (HSV1) genomes in peripheral nerve ganglia periodically reactivate, initiating a gene expression program required for productive replication. Whether molecular cues detected by axons can be relayed to cell bodies and harnessed to regulate latent genome expression in neuronal nuclei is unknown. Using a neuron culture model, we found that inhibiting mTOR, depleting its regulatory subunit raptor, or inducing hypoxia all trigger reactivation. While persistent mTORC1 activation suppressed reactivation, a mutant 4E-BP (eIF4E-binding protein) translational repressor unresponsive to mTORC1 stimulated reactivation. Finally, inhibiting mTOR in axons induced reactivation. Thus, local changes in axonal mTOR signaling that control translation regulate latent HSV1 genomes in a spatially segregated compartment.Supplemental material is available for this article.Received March 1, 2012; revised version accepted June 6, 2012.The ability to respond to physiological cues in the host neuron and switch between two distinct developmental programs, latency and productive viral replication, is a defining feature of herpes simplex virus (HSV) biology and pathogenesis (Roizman et al. 2007). During latency, the viral genome is maintained as an episome within the neuronal nucleus, and genes required for viral replication are epigenetically silenced (Knipe and Cliffe 2008;Bloom et al. 2010). Thus, viral proteins are not detected, and productive replication is suppressed. In response to incompletely understood physiological cues or changes in immune status, the virus emerges from latency or ''reactivates,'' initiating a complex gene expression program that results in viral replication and culminates in infectious virus production (Knipe and Cliffe 2008). Studies using small animal models have been instrumental in defining viral gene products involved in reactivation and the role of host innate and acquired immune systems in controlling latency (Wagner and Bloom 1997;Cunningham et al. 2006;Knickelbein et al. 2008;Thompson et al. 2009). While it was first hypothesized in the early 20th century that the virus colonizes peripheral neurons and establishes lifelong latent infections capable of triggering recurrent disease (Cushing 1905;Goodpasture 1929), the molecular signals regulating HSV latency specifically within host neurons, how reactivation is triggered in response to varied environmental stimuli, and how signal transmission occurs from the periphery to repress latent genomes hidden in neuronal nuclei are questions that remain a mystery.Recently, phosphatidylinositol 3-kinase (PI3-K)/Akt signaling, mediated by nerve growth factor (NGF) binding to the TrkA receptor tyrosine kinase (RTK), was shown to be required to maintain latency and suppress HSV productive (lytic) growth in a primary neuron cell culture model system that faithfully exhibits key hallmarks of latency as defined in animal models (Camarena et al. 2010;Kim et al. 2012;Kobayashi et al. 2012). Even transient interruption of this signaling cascade wa...

Restarting Lytic Gene Transcription at the Onset of Herpes Simplex Virus Reactivation

“…This discovery came from in vitro studies using primary neurons isolated from peripheral ganglia of prenatal rats or postnatal mice (18,19). In these models, reactivation can be deliberately induced by interruption of signaling by neurotrophic factors, such as nerve growth factor (NGF), through inhibition of phosphoinositide 3-kinase (PI3K), AKT, mammalian target of rapamycin complex 1 (mTORC1), and/or cap-dependent protein synthesis (20,21).…”

Latence et réactivation du virus de l’herpès simplex de type 1 (HSV-1)

“…Au contraire, notre perception actuelle indique que l'établissement, le maintien et la réactivation de la latence résultent de l'inté-gration d'un certain nombre de facteurs, favorables ou défavorables, informant à tout moment le génome tels que la privation en facteur de croissance des cellules nerveuses (NGF), l'ajout de tricostatine A (TSA), de forskoline, de capsaïcine, de dexaméthasone, etc. En utilisant des inhibiteurs spécifiques des diffé-rentes étapes de la voie de signalisation du NGF, l'équipe de Ian Mohr [39,40] a montré que pour maintenir la latence dans des cultures de neurones périphériques, le NGF doit agir en permanence sur le récep-teur à activité tyrosine kinase TrkA, gardant ainsi active la voie de signalisation de la phosphoinositide 3-kinase (PI3K). L'activation de la PI3K induit le recrutement de la kinase PDK1 (phosphoinositidedependent kinase 1) à la membrane plasmique où elle phosphoryle la protéine kinase B (PKB, ou AKT).…”

Viruses in human central nervous system — invasion, transmission, and latency: a literature review

“…Предполагают, что латентность HSV1 поддерживается вследствие влияния вируса на свя-зывание ФРН с TrKA рецептором и активации фосфо-инозитол 3 киназа -(PI3-K)-Akt сигнального пути [74,75]. Основной мишенью PI3K-Akt сигнала в аксоне является mTORC1 [76], регулирующего трансляци-онную репрессию elF4E-связывающего белка (4E-BP), влияющего на установление латентности вирусного генома. Метаболические изменения: дефицит ФРН, ингибирование активности mTOR, гипоксия наруша-ют контроль латентности и промотируют реактива-цию.…”

“…Очевидно, многие физиологические сигналы, берущие начало на периферии, интегрированы с локальной системой mTOR в аксоне и связанной с ним телом клетки. Локальный мониторинг в аксонах и на расстоянии до тела клетки дает сигналы в тело клетки для инициации защитных механизмов [76,77]. В последнее время все большее внимание уделяют генетической предрасположенности к заражению вирусами и клиническому течению заболевания.…”