A peek behind the paper – Lynn Enquist on the lifecycle of alpha herpesvirus

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Take a look behind the scenes of a recent Future Virology article entitled ‘Latent vs productive infections: The alpha herpesvirus switch’ as we ask authors Lynn Enquist, Orkide Koyuncu and Margaret MacGibeny (Princeton University, NJ, USA) about latency, reactivation and the best models.

Can you summarize the life cycle of the alpha herpesvirus?

Alpha herpesviruses infect mucosal epithelial cells (lips, genitals) – these epithelial cells rapidly produce viral proteins and replicate the viral genome leading to the production of thousands of viral progeny. Some of these viral progeny invade the sensory or autonomic nerve endings innervating the mucosa. Viral particles are transported by microtubule-based systems in these long axons to reach neuronal cell body nuclei residing in the peripheral ganglia. During this long-distance transport most of the viral tegument proteins dissociate from the viral nucleocapsids. In part due to this long distance transport, as well as intrinsic neuronal mechanisms to detect foreign DNA, when viral genomes enter neuronal nuclei, the viral DNA is immediately circularized and decorated with nucleosomes with many silencing histone modifications. Such a viral genome can express only a limited number of viral products, cannot replicate its genome and cannot assemble viral progeny. This mode of infection is called latency. Latent viral genomes can be reactivated upon activation of various neuronal stress pathways to initiate productive replication.

What would you say has been the most breakthrough finding in the last 5 years on the topic of alpha herpesvirus latent infection?

Probably the ost significant improvement in the latency field is the molecular understanding of the multistep reactivation program relying on the neuronal stress machinery that is kinetically distinct from the initiation of productive infection. The work from many labs has contributed to the understanding of neuronal pathways required for the step-by-step activation as well as the nucleosome modification of repressed viral genomes.

Read the full review in Future Virology >

What do we still not know about alpha Herpesvirus latent infection and what is the biggest hurdle in the way of these findings?

Major questions concern the temporal dynamics of how genome silencing is established in the ganglia, what controls reactivation cycles, how neurons respond to the silent infection, what happens to latently infected neurons after each reactivation episode, and whether reactivated virus reaches the brain and spinal cord to start another round of  latent infection (or neuroinflammation).

Some of the largest hurdles are lack of an animal model that recapitulates the human infection patterns of the human pathogens HSV-1 and VZV, and systems to establish robust neuronal culture models derived from human tissues. One of the biggest disadvantages of the currently used rodent neuron culture models is that they do not preserve the highly differentiated neuronal and epithelial tissue architecture.

Another issue is that some models rely on establishment of a latent infection by blocking productive infection with antiviral drugs or interferon. One common problem is that different strains of viruses are used by different laboratories, which can complicate interpretation.

What are the best models for studying latency in neurons?

It is difficult to say which one is ‘best’ as they all have limitations and benefits. Widely used models include the in vitro dissociated rodent superior cervical ganglionic neuron culture and the ex vivo mice trigeminal or dorsal root ganglia explant model. These models provided robust systems to study molecular mechanisms of HSV-1 reactivating stimuli. However, neither allows facile analysis of the early events in axons and cell bodies leading to genome silencing, or start of productive infection. Explants of latently infected rodent ganglia have been quite useful. Here, animals are peripherally infected with HSV-1 or HSV-2, and some fraction of the animals establish a latent infection and recover from the acute infection. Reactivation of the latent infection can be triggered by hyperthermia, hormone treatment or nerve injury.

What is the key difference between escape from genome silencing and reactivation of the virus?

The processes differ in several ways. The decision to silence genomes or to initiate a productive infection takes place rapidly after viral genomes are translocated to the neuronal nucleus following the long-distance transport of viral particles in peripheral nervous system axons. Inner tegument proteins are most likely co-transported with the nucleocapsids, but outer tegument proteins are not. These outer tegument proteins include viral transcription activators. It may be that silencing of viral genomes occurs if outer tegument proteins are not present in the nucleus at the same time when genomes enter. This step could happen if very few virus particles invade axons. However, if outer tegument proteins are delivered simultaneously with viral genomes to the nucleus (e.g., when many virus particles invade axons), silencing may be avoided and a productive infection can be established.

By contrast, reactivation of viral genomes can occur only after the silenced infection is established. Latent viral genomes, at this stage, are covered with nucleosomes with repressive histone modifications, and few viral gene products are produced. The latency associated transcript is the most abundant viral gene product in the latently infected neuron. Importantly, virion structural components (e.g., tegument proteins) are not present in latently infected neurons.

Read the full review

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