Allosteric Inhibition of the IRE1α RNase Preserves Cell Viability and Function during Endoplasmic Reticulum Stress

Abstract

SUMMARY Depending on endoplasmic reticulum (ER) stress levels, the ER transmembrane multi-domain protein IRE1α promotes either adaptation or apoptosis. Unfolded ER proteins cause IRE1α lumenal domain homo-oligomerization, inducing trans auto-phosphorylation that further drives homo-oligomerization of its cytosolic kinase/ endoribonuclease (RNase) domains to activate mRNA splicing of adaptive XBP1 transcription factor. However, under high/chronic ER stress, IRE1α surpasses an oligomerization threshold that expands RNase substrate repertoire to many ER-localized mRNAs, leading to apoptosis. To modulate these effects, we developed ATP-competitive IRE1α Kinase Inhibiting RNase Attenuators—KIRAs—that allosterically inhibit IRE1α’s RNase by breaking oligomers. One optimized KIRA, KIRA6, inhibits IRE1α in vivo and promotes cell survival under ER stress. Intravitreally, KIRA6 preserves photoreceptor functional viability in rat models of ER stress-induced retinal degeneration. Systemically, KIRA6 preserves pancreatic β-cells, increases insulin, and reduces hyperglycemia in Akita diabetic mice. Thus, IRE1α powerfully controls cell fate, but can itself be controlled with small molecules to reduce cell degeneration.

Imaging of single cell responses to ER stress indicates that the relative dynamics of IRE1/XBP1 and PERK/ATF4 signalling rather than a switch between signalling branches determine cell survival

“…4,5,7 Pro-survival as well as pro-apoptotic roles have been suggested for IRE1-dependent decay of mRNAs (RIDD). 5,6,18 Our system does not allow direct monitoring of RIDD, however, the data available to date would indicate that RIDD cannot occur independent of XBP1 splicing. 5,7,18 Thus, we assume that the observed attenuation of XBP1-YFP splicing is indicative of a decrease in all IRE1 endonuclease activities including mRNA decay.…”

“…5,6,18 Our system does not allow direct monitoring of RIDD, however, the data available to date would indicate that RIDD cannot occur independent of XBP1 splicing. 5,7,18 Thus, we assume that the observed attenuation of XBP1-YFP splicing is indicative of a decrease in all IRE1 endonuclease activities including mRNA decay. In fact, if RIDD contributes to pro-apoptotic cell decision in the later stages of the UPR, this could explain why in our single cell experiments, the time point the fluorescence intensity of the IRE1 reporter reached a plateau, indicating an attenuation in IRE1 activity, was observed earlier in resistant compared with dying cells.…”

Surviving protein quality control catastrophes – from cells to organisms

“…Because stress responses are essential for cell survival, inhibitors of stress signaling pathways are predicted to be deleterious, a property that could be advantageous for the discovery of cancer drugs. This possible effect on cell survival has motivated the development of inhibitors of PERK (Axten et al, 2012) and IRE1 (Cross et al, 2012;Ghosh et al, 2014), and of the integrated stress-response inhibitor ISRIB, which inhibits the signaling downstream of eIf2α phosphorylation (Sidrauski et al, 2013). Recent and comprehensive reviews on the pharmacological manipulation of stress responses are available elsewhere (Hetz et al, 2013;Maly and Papa, 2014).…”

Role of endoplasmic reticulum stress in disuse osteoporosis

“…Disturbance to normal functions of the ER leads to an evolutionarily conserved stress response, the unfolded protein response, which is primarily aimed at damage compensating but may eventually trigger cell death if the dysfunction is severe or prolonged [2,3].Although the ER stress has been reported to be linked to various diseases such as diabetes [4,5], neurodegenerative diseases [6], and osteogenesis imperfecta [7], the role of the ER stress in the pathogenesis of osteoporosis still remains unclear.…”