Herpes simplex virus (HSV) is found in the brain cells of approximately 90% of the world’s population and leads to cold sores, recurrent eye infections, genital lesions, and in rare cases encephalitis. Its closely related virus, VZV, also causes chicken pox and shingles. When a person develops cold sores, the reason is that the neurons in which HSV reside, are under stress.
HSV reactivation from latent neuronal infection requires stimulation of lytic gene expression from promoters associated with tightly-packed DNA known as heterochromatin. Various neuronal stresses trigger reactivation, however, these stimuli remain unknown. Now, a study from researchers at the University of North Carolina has identified the cellular mechanism that allows the virus to reactivate. The team state that they also found how brain cells are duped into allowing bits of virus to escape the very repressive environment in neurons and cause disease. The opensource study is published in the journal Cell Host & Microbe
Previous studies show during lytic replication, over 70 viral gene products are expressed in a cascade-dependent fashion. Recruitment of the cellular transcriptional machinery is dependent on both cellular and HSV transcriptional activators to promote expression of mRNA. This initial phase of viral gene expression appears to represent an event that is distinct from full reactivation, and has been termed phase I or animation. During phase I, the observation that all three classes of viral genes are induced in the absence of viral protein
synthesis suggests that host cell proteins initiate this process. Although cellular proteins, including histone demethylases, have been found to be required for HSV reactivation, as yet no direct link has been identified between a reactivation stimulus and the earliest induction of lytic gene expression. The current study shows a direct link between activation of a neuronal stress response that would permit an increase in viral lytic gene expression from an epigenetically repressed state during phase I of HSV reactivation.
The current study used an experimental assay to force the virus to go latent in mouse primary neurons in a dish and then to become reactivated. This allowed the lab to observe specific cellular protein pathways that could be involved in viral reactivation. Results show that the JNK protein pathway, which includes proteins called DLK and JIP3, was activated just before the virus began to leave neurons. Data findings show that when a chemical inhibitor was added to knock out the JNK pathway, the virus was no longer able to reactivate.
The lab state that the herpes virus can be reactivated even though the viral DNA in neurons is still in a repressed state. They go on to explain that the histones associated with viral DNA doesn’t undergo demethylation, a process that allows tightly packaged DNA to become more open so that gene expression can occur, which was precisely what the virus needed in order to be reactivated.
Results show that the virus has figured out a way to modify its tightly packaged DNA right next to the methyl marks, via phosphorylation of the histone adjacent to the methyl mark. The group explain that the methyl marks act as brakes to refuse gene expression, and this phosphorylation releases the brakes just enough so that a little bit of viral gene expression can occur; this is called a methyl/phospho switch. The researchers conclude that the phosphorylation was also dependent on activation of the JNK pathway and, therefore, the experiments link the stress-activated pathway to the very earliest changes to the viral DNA.
The team surmise that full viral gene expression requires removal of histone methylation, which allows the virus to complete the reactivation process, which leads to full virus formation outside the neuron. The researchers state that the next step is to establish this model of HSV infection and reactivation in human neurons, which has not yet been accomplished. They go on to add that if it can be, and if the JNK pathway is crucial for viral reactivation in humans, then it could be possible to develop treatments for the diseases that are linked to HSV.
Source: University of North Carolina School of Medicine
HSV Reactivating from Neurons. Green fluorescent protein-tagged herpes simplex virus is reactivating from cultures of neurons. Credit: Anna Cliffe, PhD, UNC School of Medicine.