West Nile Virus Infection Activates the Unfolded Protein Response, Leading to CHOP Induction and Apoptosis

Abstract

West Nile virus (WNV)-mediated neuronal death is a hallmark of WNV meningitis and encephalitis. However, the mechanisms of WNV-induced neuronal damage are not well understood. We investigated WNV neuropathogenesis by using human neuroblastoma cells and primary rat hippocampal neurons. We observed that WNV activates multiple unfolded protein response (UPR) pathways, leading to transcriptional and translational induction of UPR target genes. We evaluated the role of the three major UPR pathways, namely, inositol-requiring enzyme 1-dependent splicing of X box binding protein 1 (XBP1) mRNA, activation of activating transcription factor 6 (ATF6), and protein kinase R-like endoplasmic reticulum (ER) kinase-dependent eukaryotic initiation factor 2alpha (eIF2alpha) phosphorylation, in WNV-infected cells. We show that XBP1 is nonessential or can be replaced by other UPR pathways in WNV replication. ATF6 was rapidly degraded by proteasomes, consistent with induction of ER stress by WNV. We further observed a transient phosphorylation of eIF2alpha and induction of the proapoptotic cyclic AMP response element-binding transcription factor homologous protein (CHOP). WNV-infected cells exhibited a number of apoptotic phenotypes, such as (i) induction of growth arrest and DNA damage-inducible gene 34, (ii) activation of caspase-3, and (iii) cleavage of poly(ADP-ribose) polymerase. The expression of WNV nonstructural proteins alone was sufficient to induce CHOP expression. Importantly, WNV grew to significantly higher viral titers in chop(-)(/)(-) mouse embryonic fibroblasts (MEFs) than in wild-type MEFs, suggesting that CHOP-dependent premature cell death represents a host defense mechanism to limit viral replication that might also be responsible for the widespread neuronal loss observed in WNV-infected neuronal tissue.