Inhibition of IRE1α-mediated XBP1 mRNA cleavage by XBP1 reveals a novel regulatory process during the unfolded protein response

Abstract

Background: The mammalian endoplasmic reticulum (ER) continuously adapts to the cellular secretory load by the activation of an unfolded protein response (UPR). This stress response results in expansion of the ER, upregulation of proteins involved in protein folding and degradation, and attenuation of protein synthesis. The response is orchestrated by three signalling pathways each activated by a specific signal transducer, either inositol requiring enzyme α (IRE1α), double-stranded RNA-activated protein kinase-like ER kinase (PERK) or activating transcription factor 6 (ATF6). Activation of IRE1α results in its oligomerisation, autophosphorylation and stimulation of its ribonuclease activity. The ribonuclease initiates the splicing of an intron from mRNA encoding the transcription factor, X-box binding protein 1 (XBP1), as well as degradation of specific mRNAs and microRNAs. Methods: To investigate the consequence of expression of exogenous XBP1, we generated a stable cell-line expressing spliced XBP1 mRNA under the control of an inducible promotor. Results: Following induction of expression, high levels of XBP1 protein were detected, which allowed upregulation of target genes in the absence of induction of the UPR. Remarkably under stress conditions, the expression of exogenous XBP1 repressed splicing of endogenous XBP1 mRNA without repressing the activation of PERK. Conclusions: These results illustrate that a feedback mechanism exists to attenuate Ire1α ribonuclease activity in the presence of XBP1.

Dysregulation of the unfolded protein response (UPR) in the dorsolateral prefrontal cortex in elderly patients with schizophrenia

“…We found decreased p-IRE1α and increased sXBP1 protein expression. Activation of IRE1α leads to an unconventional post-transcriptional splicing of uXbp1 mRNA to produce a spliced form of sXbp1 mRNA that is translated as sXBP1 protein 55 , 56 . To determine at what stage the disconnect between IRE1α activation, as measured by p-IRE1α expression, and XBP1 splicing, as measured by sXBP1 protein expression, occurs, we measured mRNA expression of different forms of Xbp1.…”

Redox signaling and unfolded protein response coordinate cell fate decisions under ER stress

“…Activated IRE1α is in oligomerization and is autophosphorylated by itself due to its kinase activity to facilitate performing the RNase activity, then IRE1α specially splices a 26 nucleotides intron of downstream X-box binding protein 1 (XBP1) mRNA. The spliced mRNA is translated into a specific protein, XBP1s, which is a potent transcription factor and subsequently be translocated into the nucleus to initiate transcription of related genes responsible for regulating ER quality control and ERAD pathway [69], [70], [71]. In addition to XBP1, IRE1α also targets other transcripts through a process called regulated IRE1-dependent decay (RIDD), which participates in maintaining ER homeostasis by reducing ER client protein loading through mRNA degradation [72].…”

Regulation of Age-Related Protein Toxicity

“…In neurons, overexpression of Xbp1 can prevent the decline in ability to induce the UPR ER with aging ( Frakes et al, 2020 ). At the same time, enhanced lipid biogenesis increases protein folding and protein degradation in the ER ( Chalmers et al, 2017 ; Frakes and Dillin, 2017 ). However, the ability to induce the UPR ER and its downstream targets decline with increasing age ( Sabath et al, 2020 ; Taylor and Hetz, 2020 ).…”