Herpes Simplex Virus Reactivation Induced Ex Vivo Involves a DLK-Dependent but Histone Demethylase-Independent Wave of Lytic Gene Expression

Herpes Simplex virus-1 (HSV-1) maintains a lifelong latent infection in neurons and periodically reactivates, resulting in the production of infectious virus. The exact cellular pathways that induce reactivation are not understood. In primary neuronal models of HSV latency, the cellular protein Dual Leucine Zipper kinase (DLK) has been found to initiate a wave of viral gene expression known as Phase I. Phase I occurs independently of both viral DNA replication and the activities of histone demethylase enzymes required to remove repressive heterochromatin modifications associated with the viral genome. Here we investigated whether Phase-I like gene expression occurs in ganglia reactivated from infected mice. Using the combined trigger of explant-induced axotomy and inhibition of PI3K signaling, we found that HSV lytic gene expression was induced rapidly from both sensory and sympathetic neurons. Ex vivo reactivation involved a wave of viral late gene expression that occurred independently of viral genome synthesis and histone demethylase activity, and preceded the detection of infectious virus. Importantly, we found that DLK was required for the initial induction of lytic gene expression. These data confirm the essential role of DLK in inducing HSV-1 gene expression from the heterochromatin associated genome and further demonstrate that HSV-1 gene expression during reactivation occurs via mechanisms that are distinct from lytic replication.