The secretory capacity of a cell is constantly challenged by physiological demands and pathological perturbations. To adjust and match the protein-folding capacity of the endoplasmic reticulum (ER) to changing secretory needs, cells employ a dynamic intracellular signaling pathway known as the unfolded protein response (UPR). Homeostatic activation of the UPR enforces adaptive programs that modulate and augment key aspects of the entire secretory pathway, whereas maladaptive UPR outputs trigger apoptosis. Here, we discuss recent advances into how the UPR integrates information about the intensity and duration of ER stress stimuli in order to control cell fate. These findings are timely and significant because they inform an evolving mechanistic understanding of a wide variety of human diseases, including diabetes mellitus, neurodegeneration, and cancer, thus opening up the potential for new therapeutic modalities to treat these diverse diseases.

Numerous genetic and environmental insults impede the ability of cells to properly fold and posttranslationally modify secretory and transmembrane proteins in the endoplasmic reticulum (ER), leading to a buildup of misfolded proteins in this organelle—a condition called ER stress. ER-stressed cells must rapidly restore protein-folding capacity to match protein-folding demand if they are to survive. In the presence of high levels of misfolded proteins in the ER, an intracellular signaling pathway called the unfolded protein response (UPR) induces a set of transcriptional and translational events that restore ER homeostasis. However, if ER stress persists chronically at high levels, a terminal UPR program ensures that cells commit to self-destruction. Chronic ER stress and defects in UPR signaling are emerging as key contributors to a growing list of human diseases, including diabetes, neurodegeneration, and cancer. Hence, there is much interest in targeting components of the UPR as a therapeutic strategy to combat these ER stress–associated pathologies.

SUMMARY During endoplasmic reticulum (ER) stress, homeostatic signaling through the unfolded protein response (UPR) augments ER protein-folding capacity. If homeostasis is not restored, the UPR triggers apoptosis. We found that the ER transmembrane kinase/endoribonuclease (RNase) IRE1α is a key component of this apoptotic switch ER stress induces IRE1α kinase autophosphorylation, activating the RNase to splice XBP1 mRNA and produce the homeostatic transcription factor, XBP1s. Under ER stress—or forced autophosphorylation—IRE1α’s RNase also causes endonucleolytic decay of many ER-localized mRNAs, including those encoding chaperones, as early events culminating in apoptosis. Using chemical-genetics, we show that kinase inhibitors bypass autophosphorylation to activate the RNase by an alternate mode that enforces XBP1 splicing, and averts mRNA decay and apoptosis. Alternate RNase activation by kinase-inhibited IRE1α can be reconstituted in vitro. We propose that divergent cell fates during ER stress hinge on a balance between IRE1α RNase outputs that can be tilted with kinase inhibitors to favor survival.

SUMMARY When unfolded proteins accumulate to irremediably high levels within the endoplasmic reticulum (ER), intracellular signaling pathways called the unfolded protein response (UPR) become hyperactivated to cause programmed cell death. We discovered that thioredoxin-interacting protein (TXNIP) is a critical node in this “Terminal UPR.” TXNIP becomes rapidly induced by IRE1α, an ER bifunctional kinase/endoribonuclease (RNase). Hyperactivated IRE1α increases TXNIP mRNA stability by reducing levels of a TXNIP destabilizing micro-RNA, miR-17. In turn, elevated TXNIP protein activates the NLRP3 inflammasome, causing Caspase-1 cleavage and interleukin 1β (IL-1β) secretion. Txnip gene deletion reduces pancreatic β-cell death during ER stress, and suppresses diabetes caused by proinsulin misfolding in the Akita mouse. Finally, small molecule IRE1α RNase inhibitors suppress TXNIP production to block IL-1β secretion. In summary, the IRE1α-TXNIP pathway is used in the terminal UPR to promote sterile inflammation and programmed cell death, and may be targeted to develop effective treatments for cell degenerative diseases.

Summary Protein misfolding stimulates a signaling pathway involving noncoding RNAs to promote cell death. The endoplasmic reticulum (ER) is the primary organelle for folding and maturation of secretory and transmembrane proteins. Inability to meet protein-folding demand leads to “ER stress,” and activates IRE1α, an ER transmembrane kinase-endoribonuclease (RNase). IRE1α promotes adaptation through splicing Xbp1 mRNA or apoptosis through incompletely understood mechanisms. Here we found that sustained IRE1α RNase activation caused rapid decay of select microRNAs (miRs -17, -34a, -96, -125b) that normally repress translation of Caspase-2 mRNA, and thus sharply elevates protein levels of this initiator protease of the mitochondrial apoptotic pathway. In cell-free systems, recombinant IRE1α endonucleolytically cleaved microRNA precursors at sites distinct from DICER. Thus, IRE1α regulates translation of a proapoptotic protein through terminating microRNA biogenesis, and noncoding RNAs are part of the ER stress response.

Unfolded proteins in the endoplasmic reticulum (ER) activate the ER transmembrane sensor Ire1 to trigger the unfolded protein response (UPR), a homeostatic signaling pathway that adjusts ER protein folding capacity according to need. Ire1 is a bifunctional enzyme, containing cytoplasmic kinase and RNase domains whose roles in signal transduction downstream of Ire1 are understood in some detail. By contrast, the question of how its ER-luminal domain (LD) senses unfolded proteins has remained an enigma. The 3.0-Å crystal structure and consequent structure-guided functional analyses of the conserved core region of the LD (cLD) leads us to a proposal for the mechanism of response. cLD exhibits a unique protein fold and is sufficient to control Ire1 activation by unfolded proteins. Dimerization of cLD monomers across a large interface creates a shared central groove formed by ␣-helices that are situated on a ␤-sheet floor. This groove is reminiscent of the peptide binding domains of major histocompatibility complexes (MHCs) in its gross architecture. Conserved amino acid side chains in Ire1 that face into the groove are shown to be important for UPR activation in that their mutation reduces the response. Mutational analyses suggest that further interaction between cLD dimers is required to form higher-order oligomers necessary for UPR activation. We propose that cLD directly binds unfolded proteins, which changes the quaternary association of the monomers in the membrane plane. The changes in the ER lumen in turn position Ire1 kinase domains in the cytoplasm optimally for autophosphorylation to initiate the UPR.Ire1 ͉ unfolded protein response ͉ MHC ͉ protein folding ͉ secretory pathway

The unfolded protein response (UPR) is an intracellular signaling pathway that is activated by the accumulation of unfolded proteins in the endoplasmic reticulum (ER). UPR activation triggers an extensive transcriptional response, which adjusts the ER protein folding capacity according to need. As such, the UPR constitutes a paradigm of an intracellular control mechanism that adjusts organelle abundance in response to environmental or developmental clues. The pathway involves activation of ER unfolded protein sensors that operate in parallel circuitries to transmit information across the ER membrane, activating a set of downstream transcription factors by mechanisms that are unusual yet rudimentarily conserved in all eukaryotes. Recent results shed light on the mechanisms by which unfolded proteins are sensed in the ER and by which the unfolded protein signals are relayed and integrated to reestablish homeostasis in the cell's protein folding capacity or-if this cannot be achieved-commit cells to apoptosis.

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.