Regulation of Herpes Simplex Virus-1 Latent Infection and Reactivation by Facultative Heterochromatin and Neuronal Stress Signaling determined using a Novel Primary Neuronal Model

Herpes simplex virus-1 (HSV-1) establishes a latent infection in peripheral neurons, during which repressed lytic viral gene promoters associate with cellular heterochromatin. Periodically following stress, the viral genome can re-initiate lytic viral gene expression, reactivate, and cause serious clinical disease. The molecular events which mediate the repression and reactivation of lytic viral genes directly from the viral genome remain unresolved, and this study is complicated by heterogeneous ability of an individual neuron to undergo latency or reactivation. We developed a new model in primary neurons to preserve the heterogenous nature of these processes, which we use to explore how 1) neuronal stress signaling and 2) Polycomb silencing contribute to HSV-1 latency establishment, latency maintenance, and reactivation.
Neuronal stress kinases dual leucine zipper kinase (DLK) and c-Jun N-terminal kinase (JNK) mediate initial viral gene expression during reactivation. However, how downstream host factors work with JNK to stimulate reactivation remains unclear. We find that c-Jun, the primary target downstream of physiological neuronal DLK/JNK signaling, functions on the viral genome during reactivation to permit the transition to full HSV-1 reactivation initiated by multiple stressors. We also find that this stress signaling pathway critical for reactivation becomes activated upon de novo neuronal infection and promotes the formation of more reactivation-competent genomes.
We further discover a predominant role for Polycomb repressor complex 1 (PRC1) and its associated histone modification, the mono-ubiquitination of histone H2A (H2AK119ub), for silencing a population of HSV-1 genomes in a manner that is amenable to later reactivation. Our data also suggest that a viral long noncoding RNA may mediate PRC1 recruitment to the HSV-1 genome. We altogether propose that events during initial HSV-1 infection and acquisition modify the type of latency HSV-1 establishes at the level of the viral genome, characterized by how amenable to reactivation it may be. Our work provides compelling evidence for the heterogenous nature of HSV-1 latent infection and reactivation and introduces new methods to study these processes at a higher biological resolution to create more targeted, accurate therapeutic interventions.