Flaviviral regulation of the unfolded protein response: can stress be beneficial?

The Regulation of Integrated Stress Response Signaling Pathway on Viral Infection and Viral Antagonism

The integrated stress response (ISR) is an adaptational signaling pathway induced in response to different stimuli, such as accumulation of unfolded and misfolded proteins, hypoxia, amino acid deprivation, viral infection, and ultraviolet light. It has been known that viral infection can activate the ISR, but the role of the ISR during viral infection is still unclear. In some cases, the ISR is a protective mechanism of host cells against viral infection, while viruses may hijack the ISR for facilitating their replication. This review highlighted recent advances on the induction of the ISR upon viral infection and the downstream responses, such as autophagy, apoptosis, formation of stress granules, and innate immunity response. We then discussed the molecular mechanism of the ISR regulating viral replication and how viruses antagonize this cellular stress response resulting from the ISR.

CHOP Pro-Apoptotic Transcriptional Program in Response to ER Stress Is Hacked by Zika Virus

Zika virus (ZIKV) is an emerging mosquito-borne flavivirus considered as a threat to human health due to large epidemics and serious clinical outcomes such as microcephaly in new-borns. Like all flaviviruses, ZIKV relies on the cellular machinery to complete its viral cycle, with the endoplasmic reticulum (ER) being the critical site of viral replication factories. The sudden high protein load in the ER induces an ER stress to which the cell responds with an appropriate unfolded protein response (UPR) in an attempt to restore its disturbed homeostasis. When the restoration fails, the cell signalling leads to a programmed cell death by apoptosis with the upregulation of the UPR-induced C/EBP homologous protein (CHOP) which acts as the main trigger for this fatal outcome. Our previous studies have shown the ability of ZIKV to manipulate various cellular responses in order to optimize virus production. ZIKV is able to delay apoptosis to its benefit and although ER stress is induced, the UPR is not complete. Here we discovered that ZIKV impairs the expression of CHOP/DDIT3, the main factor responsible of ER-stress driven apoptosis. Surprisingly, the mechanism does not take place at the transcriptional level but at the translational level.

The unfolded protein response induced by Tembusu virus infection

Background

Tembusu virus (TMUV), classified in the genus Flavivirus, causes reduced egg production and neurological problems in poultry. Flavivirus replication depends on the host endoplasmic reticulum (ER) and induces ER stress that leads to activation of the cellular unfolded protein response (UPR), an important signalling pathway that regulates many biological functions involved in viral pathogenesis and innate immunity. However, the mechanism of TMUV-induced UPR activation remains unclear.

Results

In this study, we systematically investigated the three UPR pathways in TMUV-infected BHK-21 cells. Our results showed that expression of glucose-related protein 78 (GRP78) and GRP94 was upregulated during the course of TMUV infection. We then demonstrated that TMUV activated the PERK pathway in the early stage of infection, resulting in upregulation of ATF4, GADD34 and CHOP, with CHOP induction leading to caspase-3 activation. We also found the IRE1 pathway to be activated, leading to splicing of X box binding protein 1 (XBP1) mRNA and enhanced expression of p58^IPK. Finally, we observed increased expression of ATF6 and activity of ER stress-response elements, suggesting stimulation of the ATF6 pathway. In addition, ATF6 pathway activation correlated with the induction of downstream chaperones calnexin, calreticulin, ERp57 and PDI. UPR activity was also observed by the marked elevation in GRP78 and sXBP1 levels in TMUV-infected DF-1 cells.

Conclusions

This is the first report that TMUV infection-induced ER stress activates three branches of the UPR, and these results lay the foundation for elucidating the pathogenesis of TMUV and understanding the inherent mechanism of TMUV infection as well as the host response.

CSFV Infection Up-Regulates the Unfolded Protein Response to Promote Its Replication

Classical swine fever (CSF) is an OIE-listed, highly contagious animal disease caused by classical swine fever virus (CSFV). The endoplasmic reticulum (ER) is an organelle in which the replication of many RNA viruses takes place. During viral infection, a series of events elicited in cells can destroy the ER homeostasis that cause ER stress and induce an unfolded protein response (UPR). In this study, we demonstrate that ER stress was induced during CSFV infection as several UPR-responsive elements such as XBP1(s), GRP78 and CHOP were up-regulated. Specifically, CSFV transiently activated IRE1 pathway at the initial stage of infection but rapidly switched off, likely due to the reduction in cytoplasm Ca²⁺ after viral incubation. Additionally, our data show that the ER stress induced by CSFV can promote CSFV production, which the IRE1 pathway play an important role in it. Evidence of ER stress in vivo was also confirmed by the marked elevation of GRP78 in CSFV-infected pig PBMC and tissues. Collectively, these data indicate that the ER stress was induced upon CSFV infection and that the activation of the IRE1 pathway benefits CSFV replication.

Stress responses in flavivirus-infected cells: activation of unfolded protein response and autophagy

The Flavivirus is a genus of RNA viruses that includes multiple long known human, animal, and zoonotic pathogens such as Dengue virus, yellow fever virus, West Nile virus, or Japanese encephalitis virus, as well as other less known viruses that represent potential threats for human and animal health such as Usutu or Zika viruses. Flavivirus replication is based on endoplasmic reticulum-derived structures. Membrane remodeling and accumulation of viral factors induce endoplasmic reticulum stress that results in activation of a cellular signaling response termed unfolded protein response (UPR), which can be modulated by the viruses for their own benefit. Concomitant with the activation of the UPR, an upregulation of the autophagic pathway in cells infected with different flaviviruses has also been described. This review addresses the current knowledge of the relationship between endoplasmic reticulum stress, UPR, and autophagy in flavivirus-infected cells and the growing evidences for an involvement of these cellular pathways in the replication and pathogenesis of these viruses.