Selective activation of unfolded protein response (UPR) by herpes simplex virus type 1 (HSV-1) in permissive and non permissive cells

Abstract

The unfolded protein response (UPR) is induced by a variety of external and internal stimuli, including accumulation of misfolded proteins in the endoplasmic reticulum (ER). Viruses such as Herpes Simplex Virus type 1 (HSV-1) induce host cells to produce viral proteins many of which undergo glycosylation and other modifications in the ER, causing stress to the ER and consequently UPR activation. I have tested the hypothesis that HSV-1 has evolved strategies to regulate the UPR in order to suppress aspects of the UPR that might interfere with viral replication and to promote pathways that aid its own survival and replication. The purpose of this study was to test the hypothesis that HSV-1 selectively modulates the three pathways (PERK, ATF6, and IRE-1) of the UPR in epithelial and neuronal cells and to examine the similarities and the differences between these two types of cells in their responses to ER stress. Vero and ONS-76 cells were used as models of epithelial and neuronal cells respectively and qRT PCR technique was used for analyzing RNA levels of transcripts of spliced Xbp1, HERP, CHOP and BIP, selected target genes for three pathways of the UPR. HSV-1 DNA synthesis and infectious virus production in infected cells showed that compared to the permissive Vero cells, ONS-76 cells seemed to be semi-permissive to HSV-1 infection with limited viral DNA synthesis and infectious virus production. The kinetics of transcript and protein synthesis for genes representing immediate early, early and late classes of viral genes was also monitored. Expression of the immediate early gene, ICP0, was similar in both cell types but the expression of the early gene, TK and late genes VP16 and VP 5 was different. My work reveals that HSV-1 infection in cells of epithelial and neuronal origins results in activation of the UPR, but through cell type selective regulation of the three signal transduction pathways of the UPR (PERK, ATF6, and IRE-1). While HSV-1 infection resulted in upregulation of Spliced Xbp1 and its target gene HERP (IRE1 pathway) and downregulation of BIP (ATF6 pathway) in both cell types, CHOP (PERK pathway) was upregulated only in ONS cells. My results suggest that some aspects of the UPR are regulated differently in cells representing the sites for HSV-1 lytic and latent infections. This may indicate the need for increasing the capacity for protein folding and degradation (Xbp1 and ATF6-induced) in both cells but a requirement for suppressing apoptosis (PERK-induced) only in epithelial cells. As well, I show that HSV-1 infection not only selectively activates the UPR pathways in different cell types, but also inactivates some components of the UPR pathways activated by the drug thapsigargin.