4.4. ER Stress
The multiple stages of CoV replication cycle are closely associated with cellular membrane compartments, especially the endoplasmic reticulum (ER). The shape and functions of ER can be influenced by different physiological states and environmental conditions. When protein synthesis amounts surpass the folding capacity, the accumulation of a large amount of unfolded proteins in the ER leads to ER stress. Consequently, cells manifest a corresponding biological reaction that is widely known as the unfolded protein response (UPR) [233]. Once the UPR is induced, it alleviates the problems by host protein translation inhibition (by the transducer PKR-like ER protein kinase (PERK)-induced phosphorylation of eIF2a), stimulating protein folding. If homeostasis cannot be re-established, apoptosis eventually is triggered. Indeed, the activation of UPR regulates a wide variety of signaling pathways, such as apoptosis, autophagy, mitogen-activated protein (MAP) kinase activation, and innate immune response [234].
Furthermore, α-CoV and β-CoV may induce ER stress in the infected cells [235]. PEDV ORF3, as the only accessory protein encoded by PEDV, is thought to be related to virus production and virulence of PEDV [68]. A series of studies suggest that ORF3 plays multiple roles, in addition to acting as an ion channel during PEDV replication. Recent studies show that PEDV ORF3 consists of four transmembrane domains (TMDs) and localizes in the cytoplasm in the aggregation manner [236]. ORF3 is a transmembrane protein, and the confocal microscopy analysis indicates that the aggregated ORF3 localizes in the ER to induce the ER stress associated with either apoptosis or autophagy. However, PEDV ORF3 induces the autophagy via driving conversion of LC3-I to LC3-II, but not influencing the apoptosis. ORF3-induced autophagy is dependent on ER stress response. PEDV ORF3 triggers ER stress response via the up-regulation of GRP78 protein expression and the activation of the PERK-eIF2α signaling pathway. Moreover, ORF3 protein is identified as an IFN antagonist to block IFN response by an unknown mechanism in PEDV-infected cells [65]. The functions of PEDV ORF3 should be further exploited.