Inactivated Herpes Simplex Virus (HSV-1, MacIntyre)

Results from multiple studies suggest that herpes simplex virus type 1 (HSV-1) infection plays a role in the neurodegenerative process of Alzheimer's disease (AD), but the underlying mechanisms have not been fully elucidated. Here, researchers show that HSV-1 productive infection in cortical neurons leads to the accumulation of DNA damage, including single-strand breaks (SSBs) and double-strand breaks (DSBs). These lesions have been reported to be associated with neuronal loss observed in neurodegenerative diseases. They demonstrate that HSV-1 downregulates the expression levels of Ku80, one of the major components of non-homologous end joining (NHEJ), a major pathway for repairing DSBs. The data also suggest that HSV-1 drives Ku80 for proteasomal degradation and impairs NHEJ activity, leading to DSB accumulation. Because HSV-1 often causes lifelong recurrent infections, it is plausible that accumulated damage, including on DNA, may contribute to virus-induced neurotoxicity and neurodegeneration, further suggesting that HSV-1 is a risk factor for neurodegenerative diseases.

To examine whether the accumulation of neuronal DNA damage is associated with specific steps of the viral life cycle, the researchers analyzed the formation of γH2AX foci in neurons infected with heat- or UV-inactivated viruses (i.e., unable to enter host cells or replicate in their nuclei, respectively) or in the presence of PAA, a specific inhibitor of the viral polymerase and, therefore, of viral DNA synthesis. In this set of experiments, neurons were infected with a lower dose of virus (3 m.o.i.) to highlight the inhibitory activity of PAA. When cells were infected with heat-inactivated HSV-1 (HI), almost no DNA damage was observed at either 8 or 24 h post-infection, since HSV-1 was unable to bind to the neuronal plasma membrane and, therefore, could not infect the cells (Figure 1A). Similar results were obtained with UV-inactivated HSV-1 (UVI), which retained the ability to bind and enter host cells but did not undergo transcription and replication, as also confirmed by the lack of ICP8 immunofluorescence (Figure 1A). In contrast, some HSV-1-induced γH2AX foci were still detected in neurons infected in the presence of PAA (Figure 1B), which blocks viral DNA synthesis but not transcription of early genes, suggesting that it is not essential for viral DNA replication but may partially contribute to the accumulation of γH2AX. Representative images of HSV-1-infected cells (ICP8+) with γH2AX foci are shown in Figure 1C (46% in untreated cells and 12% in PAA-treated cells). These data suggest that the induction of DNA damage requires viral binding and entry into host cells and at least the formation of a previral replication compartment.

Figure 1. HSV-1 replication in host cells is involved in DNA damage induction. (De Chiara G, et al., 2016)