A cultured affair: HSV latency and reactivation in neurons

Abstract

After replicating in surface epithelia, herpes simplex virus type-1 (HSV-1) enters the axonal terminals of peripheral neurons. The viral genome translocates to the nucleus, where it establishes a specialized infection known as latency, re-emerging periodically to seed new infections. Studies using cultured neuron models that faithfully recapitulate the molecular hallmarks of latency and reactivation defined in live animal models have provided fresh insight into the control of latency and connections to neuronal physiology. With this comes a growing appreciation for how the life cycles of HSV-1 and other herpesviruses are governed by key host pathways controlling metabolic homeostasis and cell identity.

Figure 1. The unique polarized morphology of neurons contributes to the establishment and control of HSV-1 latency

(a) During natural infections, HSV-1 enters the nervous system via axon terminals of peripheral neurons innervating the mucosal or corneal epithelial layers and capsids undergo retrograde axonal transport to the cell body where the genome (green circles) is delivered into the nucleus. Directional infection can be mimicked in vitro using chamber devices that allow axons to project through a diffusion barrier, creating physically isolated axonal and cell-body compartments. It is proposed that tegument protein VP16 (blue triangles) dissociates from the capsid almost immediately after release into the cytoplasm and translocate to the nucleus with very low efficiency, due perhaps to the presence of host transcription factor HCF-1 (black dots) in the cytoplasm of both the axons and cell body. VP16 is required for productive replication in neurons and thus the absence of tegument-derived VP16 facilitates establishment of latency. (b) Reactivation stimuli can elicit many changes in the neuron including nuclear accumulation of HCF-1 and VP16, which is synthesized de novo along with other viral regulatory proteins. Stimulation of viral lytic transcription by VP16 leads to viral DNA amplification and synthesis of virion proteins. Capsids are transported in an anterograde fashion to the axonal termini where they mature and are then released, bringing the HSV-1 lifecycle full circle.

Figure 2. Maintenance of HSV-1 latency by different inputs acting at multiple levels

(a) Cell culture experiments have shown that the fundamental unit of HSV-1 latency is the viral genome and its host neuron. Active intracellular signaling is required to maintain latency and suppress lytic gene expression. A key player in this host-mediated control is the cellular kinase mTORC1, whose activity is regulated by neurotrophic factors such as NGF and fundamental indicators of homeostasis, including amino acid sufficiency, oxygen levels and cellular energy reserves. The inset panel shows how mTORC1 is activated by the neurotrophin NGF, which stimulates PI3K-Akt signaling through its high affinity receptor TrkA, and repressed by chemical inhibitors (rapamycin, PP242). A sensor of stress and/or environmental changes, mTORC1 controls the population of mRNAs that are actively translated into proteins, some of which may suppress the lytic cycle by sustaining the repressive chromatin state of the viral genome or by sequestering key host factors in the cytoplasm. Virus-encoded microRNAs help to protect the neuron from apoptosis and dampen the effects of inappropriate lytic mRNA transcription. (b) Other cell types found in proximity to the neuron are known or suspected to influence latency or act as sensors that provide a reactivation stimulus. HSV-specific CD8+ T cells present in the ganglia of latently infected animals and humans suppress reactivation through the secretion of IFN-γ and other non-cytolytic factors. Epithelial cells are well positioned to sample the external environment and act as a source of neurotrophins and other signals. Involvement of satellite glia and other support cells in the regulation of latency has not been demonstrated, however their physical proximity to neurons makes this a strong possibility. Working together, this complex network of interactions defines the natural phenomenon referred to as biological latency.