FAD-linked presenilin-1 mutants impede translation regulation under ER stress

FAD mutations in presenilin-1 (PS1) cause attenuation of the induction of the endoplasmic reticulum (ER)-resident chaperone GRP78/BiP under ER stress, due to disturbed function of IRE1, the sensor for accumulation of unfolded protein in the ER lumen. PERK, an ER-resident transmembrane protein kinase, is also a sensor for the unfolded protein response (UPR), causing phosphorylation of eukaryotic initiation factor 2alpha (eIF2alpha) to inhibit translation initiation. Here, we report that the FAD mutant PS1 disturbs the UPR by attenuating both the activation of PERK and the phosphorylation of eIF2alpha. Consistent with the results of a disturbed UPR, inhibition of protein synthesis under ER stress was impaired in cells expressing PS1 mutants. These results suggest that mutant PS1 impedes general translational attenuation regulated by PERK and eIF2alpha, resulting in an increased load of newly synthesized proteins into the ER and subsequently increasing vulnerability to ER stress.     

Coronavirus infection modulates the unfolded protein response and mediates sustained translational repression

During coronavirus replication, viral proteins induce the formation of endoplasmic reticulum (ER)-derived double-membrane vesicles for RNA synthesis, and viral structural proteins assemble virions at the ER-Golgi intermediate compartment. We hypothesized that the association and intense utilization of the ER during viral replication would induce the cellular unfolded protein response (UPR), a signal transduction cascade that acts to modulate translation, membrane biosynthesis, and the levels of ER chaperones. Here, we report that infection by the murine coronavirus mouse hepatitis virus (MHV) triggers the proximal UPR transducers, as revealed by monitoring the IRE1-mediated splicing of XBP-1 mRNA and the cleavage of ATF6alpha. However, we detected minimal downstream induction of UPR target genes, including ERdj4, ER degradation-enhancing alpha-mannosidase-like protein, and p58(IPK), or expression of UPR reporter constructs. Translation initiation factor eIF2alpha is highly phosphorylated during MHV infection, and translation of cellular mRNAs is attenuated. Furthermore, we found that the critical homeostasis regulator GADD34, which recruits protein phosphatase 1 to dephosphorylate eIF2alpha during the recovery phase of the UPR, is not expressed during MHV infection. These results suggest that MHV modifies the UPR by impeding the induction of UPR-responsive genes, thereby favoring a sustained shutdown of the synthesis of host cell proteins while the translation of viral proteins escalates. The role of this modified response and its potential relevance to viral mechanisms for the evasion of innate defense signaling pathways during coronavirus replication are discussed.     

Ultraviolet light inhibits translation through activation of the unfolded protein response kinase PERK in the lumen of the endoplasmic reticulum

Exposure to ultraviolet light can cause inflammation, premature skin aging, and cancer. UV irradiation alters the expression of multiple genes that encode functions to repair DNA damage, arrest cell growth, and induce apoptosis. In addition, UV irradiation inhibits protein synthesis, although the mechanism is not known. In this report, we show that UV irradiation induces phosphorylation of eukaryotic translation initiation factor 2 on the alpha-subunit (eIF2alpha) and inhibits protein synthesis in a dosage- and time-dependent manner. The UV-induced phosphorylation of eIF2alpha was prevented by the overexpression of a non-phosphorylatable mutant of eIF2alpha (S51A). PERK is an eIF2alpha protein kinase localized to the endoplasmic reticulum that is activated by the accumulation of unfolded proteins in the endoplasmic reticulum. Expression of trans-dominant-negative mutants of PERK also prevented eIF2alpha phosphorylation upon UV treatment and protected from the associated translation attenuation. The luminal domain of dominant-negative mutant PERK formed heterodimers with endogenous PERK to inhibit the PERK signaling pathway. In contrast, eIF2alpha phosphorylation was not inhibited by overexpression of a trans-dominant-negative mutant kinase, PKR, supporting the theory that UV-induced eIF2alpha phosphorylation is specifically mediated by PERK. These results support a novel mechanism by which UV irradiation regulates translation via an endoplasmic reticulum-stress signaling pathway.