Abstract
To relieve endoplasmic reticulum (ER) stress, IRE1 splices XBP1 messenger RNA (mRNA) or engages regulated IRE1-dependent decay (RIDD) of other mRNAs. Upon XBP1 deficiency, IRE1 switches to perform RIDD. We examined IRE1 in XBP1-deficient B cells and discovered that IRE1 undergoes phosphorylation at S729. We generated an anti-phospho-S729 antibody to investigate such phosphorylation. Compared with pharmacological ER stress inducers or Toll-like receptor ligands, the bacterial subtilase cytotoxin has an unusual capability in causing rapid and strong phosphorylation at S729 and triggering B cells to express spliced XBP1. To assess the function of S729 in IRE1, we generated S729A knock-in mice and found S729 is critically important for lipopolysaccharide-stimulated plasmablasts to respond to additional ER stress and for antibody production in response to immunization. We further crossed mice carrying an S729A mutation or ΔIRE1 (missing the kinase domain) with B cell-specific XBP1-deficient mice to trigger RIDD and discovered a critical role for S729 in regulating RIDD in B cells.
Highlights
The endoplasmic reticulum (ER) is responsible for the folding and assembly of ∼30% of proteins encoded by our genome
IRE1 responded to XBP1 deficiency by undergoing phosphorylation at S729 To investigate the role of IRE1 in XBP1-deficient B cells, we stimulated naive B cells purified from the spleens of XBP1WT and XBP1KO mice with LPS and CpG-1826
The lack of XBP1 in B cells did not affect the production of membrane-bound IgM or κ light chains, it led to significantly suppressed expression of secretory IgM (sIgM), which was attributed to increased expression levels of IRE1 and regulated IRE1dependent decay (RIDD) (Fig. 1 A; Benhamron et al, 2014)
Summary
The ER is responsible for the folding and assembly of ∼30% of proteins encoded by our genome. The ER harbors complex, yet elegant, mechanisms to control protein folding and assembly and to dispose of terminally misfolded proteins. To respond to ER stress, the ER is equipped with transmembrane sensors IRE1, PERK, and ATF6, representing the three major arms of the unfolded protein response (UPR), which help cells relieve the stress and restore homeostasis (Ron and Walter, 2007; Walter and Ron, 2011). In the case of persistent and irreversible stress, the ER can dictate cell death. Aberrant regulation of the UPR is implicated in many diseases (Lin et al, 2008; Hetz et al, 2013; Hetz and Mollereau, 2014; Bettigole and Glimcher, 2015; Chevet et al, 2015; Grootjans et al, 2016)
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