West Nile Virus Differentially Modulates the Unfolded Protein Response To Facilitate Replication and Immune Evasion

Abstract

For intracellular survival it is imperative that viruses have the capacity to manipulate various cellular responses, including metabolic and biosynthetic pathways. The unfolded protein response (UPR) is induced by various external and internal stimuli, including the accumulation of misfolded proteins in the endoplasmic reticulum (ER). Our previous studies have indicated that the replication and assembly of the flavivirus West Nile virus strain Kunjin virus (WNV KUN ) is intimately associated with the ER. Thus, we sought to determine whether the UPR was induced during WNV KUN infection. WNV KUN induces UPR signaling during replication, which is coordinated with peak replication. Interestingly, signaling is biased toward the ATF6/IRE-1 arm of the response, with high levels of Xbp-1 activation but negligible eukaryotic translation initiation factor 2␣ phosphorylation and downstream transcription. We show that the PERK-mediated response may partially regulate replication, since external UPR stimulation had a limiting effect on early replication events and cells deficient for PERK demonstrated increased replication and virus release. Significantly, we show that the WNV KUN hydrophobic nonstructural proteins NS4A and NS4B are potent inducers of the UPR, which displayed a high correlation in inhibiting Jak-STAT signaling in response to alpha interferon (IFN-␣). Sequential removal of the transmembrane domains of NS4A showed that reducing hydrophobicity decreased UPR signaling and restored IFN-␣-mediated activation. Overall, these results suggest that WNV KUN can stimulate the UPR to facilitate replication and that the induction of a general ER stress response, regulated by hydrophobic WNV KUN proteins, can potentiate the inhibition of the antiviral signaling pathway.

Chikungunya virus non-structural protein 2-mediated host shut-off disables the unfolded protein response

“…In addition to PERK, eIF2a can be phosphorylated by a number of other kinases, including PKR, which recognizes (viral) dsRNA (Harding et al, 2002). To allow the translation of viral proteins, most viruses prevent the phosphorylation of eIF2a (Ambrose & Mackenzie, 2011;Burnett et al, 2012;Groskreutz et al, 2010). Although inhibition of eIF2a phosphorylation via CHIKV nsP4 has been reported (Rathore et al, 2013), we and others have found that alphaviruses are exceptional in allowing the phosphorylation of eIF2a during productive infection (Gorchakov et al, 2004;Nivitchanyong et al, 2009;Ventoso et al, 2006;White et al, 2011).…”

“…The IRE1a-XBP1 arm of the UPR has been shown to be activated during infection with other viruses (Ambrose & Mackenzie, 2011;Barry et al, 2010;Isler et al, 2005). To investigate whether CHIKV infection induced XBP1 mRNA splicing, Vero cells were mock infected, infected with CHIKV or treated with TM in a time-course experiment.…”

“…The induction of the IRE1a and/or ATF-6 arms of the UPR may help to maintain ER homeostasis by increasing protein-folding capacity, thereby facilitating viral glycoprotein maturation and host-cell survival. In contrast, most viruses block activation of the PERK pathway and the downstream phosphorylation of eIF2a, thereby avoiding translational inhibition and the subsequent induction of apoptosis via CHOP (Ambrose & Mackenzie, 2011;Burnett et al, 2012;Groskreutz et al, 2010).…”

“…For instance, human cytomegalovirus and Semliki Forest virus (SFV) induce XBP1 mRNA splicing (Barry et al, 2010;Isler et al, 2005), whereas hepatitis C virus and herpes simplex virus induce ATF-6 cleavage (Burnett et al, 2012;Tardif et al, 2002). West Nile virus infection initiates both XBP1 mRNA splicing and ATF-6 cleavage (Ambrose & Mackenzie, 2011). The induction of the IRE1a and/or ATF-6 arms of the UPR may help to maintain ER homeostasis by increasing protein-folding capacity, thereby facilitating viral glycoprotein maturation and host-cell survival.…”

Lipid Involvement in Viral Infections: Present and Future Perspectives for the Design of Antiviral Strategies

“…In addition, membrane remodelling can also improve viral multiplication by hiding viral components from the innate immune system [1, 12,86]. In flaviviruses (DENV and WNV) the evasion of interferon response has been shown to depend on the expression of hydrophobic viral proteins involved in membrane rearrangements [87][88][89]; in particular, the cholesterol content of these membranes is important to down regulate the interferon-stimulated antiviral signalling response to infection [90]. Related to this, antiviral interferon response also involves down regulation of sterol biosynthesis [91].…”