The role of neurons in the herpes simplex virus type 1 infection

Abstract

Herpes simplex virus type 1 (HSV-1), a highly pervasive virus among humans, causes a variety of diseases, ranging in severity from cold sores to encephalitis. Initially, virus replicates at the site of inoculation at mucosal surfaces. HSV-1 then enters innervating sensory neuron termini, and travels along the axons to the nuclei in sensory ganglia where viral DNA can latently persist. Once HSV-1 establishes latency in the peripheral nervous system, the virus remains with the individual for life. From here, virus can periodically reactivate throughout the lifetime of an infected individual, causing virus shedding, as well as disease. The main questions addressed in this dissertation are: (1) what makes neurons unique such that they can support a latent HSV-1 infection, and (2) what events take place between HSV-1 and neurons during the establishment of latency? The First interaction between virus and cell is the binding and entry step. HSV-1 entry into neurons was shown to be mediated by viral glycoprotein D. Antibody blocking experiments revealed that HSV-1 uses cellular HveC but not HveA to enter rat sensory neurons. In fact, HveA is not present on sensory neurons. In primary rat fibroblasts, however, as in other cell lines, HSV-1 uses both HveC and HveA to enter cells. In contrast to rat sensory neurons, antibodies to HveC or HveA were not able to block HSV-1 entry into mouse sensory neurons. This may mean that rat and mouse HveC are sufficiently different such that the antibodies are not cross-reactive. It could also mean that HSV-1 uses a different molecule to enter mouse neurons than rat neurons, perhaps another glycoprotein D cellular receptor, 3-O-sulfated heparan sulfate. Perhaps the entry of HSV-1 in rat neurons via HveC may mediate a signal that aids in the establishment of latency. The use of a recombinant HSV-1 virus that expresses the immediate early protein ICP0 fused to GFP demonstrated that ICP0 localized differently in neurons than in nonneuronal cells. In non-neuronal cells, ICP0 normally localizes to discrete, punctate structures in the nucleus called Nuclear Domain 10s (ND10s). In neurons, ICP0 was diffusely located throughout the cell at a low level. This suggested that perhaps ND10s were not present in sensory neurons. By examining the presence of the ND10 proteins promyelocytic leukemia protein (PML), SUMO-1 and Daxx, we showed that sensory neurons did not contain ND10 structures. In contrast, PC 12 cells, a neuron-like cell line that can be differentiated with nerve growth factor, were shown to contain ND10 structures. The absence of ND10 structures in sensory neurons may in part help explain why HSV-1 establishes a latent infection in this cell-type. While ND10 proteins were not normally present in sensory neurons, ND10 proteins were induced by interferon treatment, HSV-1 infection, or heat shock treatment. However, ND10 structures were not inducible. Interferon-induced PML and SUMO-1 were found in the cytoplasm of neurons. This suggests that the putative antiviral action of some of the ND10 proteins is not dependent on ND10 structures, but can be mediated from the cytoplasm. Evidence demonstrated that HSV-1 did not degrade PML in neurons as seen in other cell types. Perhaps the inability of HSV-1 to overcome the antiviral response in neurons is another factor that contributes to the establishment of latency.