Elucidation of the ISR in pathogenesis of human diseases

PERK pathway, one of the three branches of the UPR, is also one of the four upstream kinases that activate the ISR. Studying the PERK pathway led us to investigate the ISR. When ER stress is induced, not only the PERK pathway, but also the IRE1 and ATF6 pathways are activated. In order to elucidate the physiological role of the PERK activation independent of stress, we developed a system that selectively activates the PERK pathway using the artificial ligand AP20187 to dimerize Fv2E-PERK, a fusion protein of the Fv2E domain and the kinase domain of PERK. By combining this construct with an organ-specific promoter, we generated transgenic mice capable of activating the PERK pathway at will in various organs. Despite the ISR regulates translation control and cellular signals to maintain proteostasis, we have found that the ISR crosstalked with other intracellular signaling pathways and thereby regulated cellular functions that are not related to proteostasis, such as metabolism through analyses of PERK activation in various organs. For example, the ISR activation regulates the glucose and lipid metabolism in the liver via C/EBP family transcription factors [1], promotes brown adipose energy expenditure through the secretion of FGF21 in skeletal muscle [2], and suppresses appetite through the secretion of GDF15 in adipocytes [3]. On the other hand, we demonstrated that long-term or excessive activation of the ISR in skeletal muscle leads to sarcopenia-like muscle atrophy, indicating the contribution of the ISR in pathological conditions [4]. As the importance of the ISR in both physiological and pathological conditions is becoming more evident, we aim to explore novel avenues to unravel the intricate molecular mechanisms underlying the ISR.

　　　　　　　　　　　1.    Cell Metab 7, 520-532 (2008)

　　　　　　　　　　　2.    FASEB J 30, 798-812 (2016)

　　　　　　　　　　　3.    iScience 24, 103448 (2021)

　　　　　　　　　　　4.    PLoS One 12, e0179955 (2017)

Elucidation of the ISR in pathogenesis of human diseases

It is known that in addition to PERK, four kinases, namely GCN2, PKR, and HRI, are capable of sensing different stresses and phosphorylating eIF2α to activate the ISR. To determine whether other kinases exist besides these four, we generated 4KO cells lacking all four known eIF2α kinases using genome editing technique. Using 4KO cells, we demonstrated that eIF2α phosphorylation did not occur in response to any type of stress, providing evidence that only four eIF2α kinases exist [5]. We also created rescue cells (Rescue cells) that overexpress only one of the four known eIF2α kinases in the 4KO cells. Rescue cells allow us to identify which kinase is responsible for stress sensing and can be utilized for drug discovery purposes [5]. Using analysis with rescue cells, we were able to identify which kinases candidate compounds for cancer and viral infections activate to stimulate the ISR [6] [7]. Since the ISR pathway is known to play a crucial role in the pathogenesis of various diseases, our goal is to develop novel therapeutic strategies that target this pathway and identify new therapeutics.  

　　　　　　　　　　　  5.    Sci Rep 6, 32886 (2016)

　　　　　　　　　  　　6.    Blood Adv 3, 4215-4227 (2019)

                             　　7.   Blood Adv 4, 1845-1858 (2020)