Role for UL12.5 and nucleic acid sensing in Phase I HSV reactivation

Herpes simplex virus (HSV) establishes a lifelong latent infection of neurons, periodically reactivating to produce infectious virus. Reactivation can cause keratitis and lesions on the body surface. Furthermore, substantial data link HSV to the progression of Alzheimer's disease, highlighting the need to develop therapeutics to prevent HSV reactivation. However, little is known about the mechanisms regulating the induction of viral gene expression following a reactivation stimulus. Previous studies from our lab and others have shown that HSV reactivation is biphasic, with an initial synchronous burst of viral gene expression, referred to as Phase I or animation because it is unique to the reactivation process. Viral gene expression during Phase I/animation is independent of viral DNA replication and requires the host cell stress protein, DLK. When initially discovered, Phase I/animation was also thought not to require HSV lytic proteins. However, our lab has found that at least one viral protein, UL12.5, is required to promote expression of additional viral genes during Phase I, suggesting that the process of animation is more complex than previously thought. However, unlike the events during HSV lytic replication, the timing of viral gene expression and promoters used during Phase I/animation have not been elucidated. Therefore, the goals of this project are to map HSV lytic gene expression during animation and determine the contributions of DLK and UL12.5 to ultimately promoting Phase I gene expression. The only known function of UL12.5 is to induce mitochondrial nucleic acid release and subsequent activation of innate immune sensing pathways involving STING and/or MAVS. Intriguingly, we have found that direct activation of STING can induce HSV to reactivate from a latent infection. Therefore, based on our preliminary data, we hypothesize that UL12.5 is expressed early during animation to hijack innate sensing pathways and promote Phase I viral lytic gene expression. In aim 1), we will map at high resolution the lytic transcriptional units expressed during animation and the contribution of the only known host (DLK) and viral (UL12.5) factors important for Phase I gene expression. In aim 2), we will test the hypothesis that HSV hijacks innate sensing pathways for Phase I gene expression. This proposal is significant and innovative because we will create the first high-resolution map of HSV lytic gene expression following a reactivation stimulus, facilitating future investigation into the role of viral genes and host proteins in promoting exit from latency. In addition, we will determine the contribution of the first viral protein identified that is required for HSV Phase I gene expression. Inhibiting UL12.5 expression or its downstream mechanism of action could prevent entry into reactivation and, therefore, limit potential detrimental effects of UL12.5 or additional viral proteins that could ultimately promote neuronal degeneration.