Evolution and function of the epithelial cell-specific ER stress sensor IRE1β

Barrier epithelial cells lining the mucosal surfaces of the gastrointestinal and respiratory tracts interface directly with the environment. As such, these tissues are continuously challenged to maintain a healthy equilibrium between immunity and tolerance against environmental toxins, food components, and microbes. An extracellular mucus barrier, produced and secreted by the underlying epithelium plays a central role in this host defense response. Several dedicated molecules with a unique tissue-specific expression in mucosal epithelia govern mucosal homeostasis. Here, we review the biology of Inositol-requiring enzyme 1β (IRE1β), an ER-resident endonuclease and paralogue of the most evolutionarily conserved ER stress sensor IRE1α. IRE1β arose through gene duplication in early vertebrates and adopted functions unique from IRE1α which appear to underlie the basic development and physiology of mucosal tissues

ER stress in antigen‐presenting cells promotes NKT cell activation through endogenous neutral lipids

CD1d-restricted invariant natural killer T (iNKT) cells constitute a common glycolipid-reactive innate-like T-cell subset with a broad impact on innate and adaptive immunity. While several microbial glycolipids are known to activate iNKT cells, the cellular mechanisms leading to endogenous CD1d-dependent glycolipid responses remain largely unclear. Here, we show that endoplasmic reticulum (ER) stress in APCs is a potent inducer of CD1d-dependent iNKT cell autoreactivity. This pathway relies on the presence of two transducers of the unfolded protein response: inositol-requiring enzyme-1a (IRE1α) and protein kinase R-like ER kinase (PERK). Surprisingly, the neutral but not the polar lipids generated within APCs undergoing ER stress are capable of activating iNKT cells. These data reveal that ER stress is an important mechanism to elicit endogenous CD1d-restricted iNKT cell responses through induction of distinct classes of neutral lipids

IRE1β negatively regulates IRE1α signaling in response to endoplasmic reticulum stress

IRE1β is an ER stress sensor uniquely expressed in epithelial cells lining mucosal surfaces. Here, we show that intestinal epithelial cells expressing IRE1β have an attenuated unfolded protein response to ER stress. When modeled in HEK293 cells and with purified protein, IRE1β diminishes expression and inhibits signaling by the closely related stress sensor IRE1α. IRE1β can assemble with and inhibit IRE1α to suppress stress-induced XBP1 splicing, a key mediator of the unfolded protein response. In comparison to IRE1α, IRE1β has relatively weak XBP1 splicing activity, largely explained by a nonconserved amino acid in the kinase domain active site that impairs its phosphorylation and restricts oligomerization. This enables IRE1β to act as a dominant-negative suppressor of IRE1α and affect how barrier epithelial cells manage the response to stress at the host–environment interface

The ins and outs of IRE1β signaling : the role of IRE1β in mucosal homeostasis

Over the last few decades, our understanding of how cells cope with fluctuations in protein folding demand has increased exponentially. Lower eukaryotic organisms such as yeast rely on a single endoplasmic reticulum (ER) transmembrane protein inositol-requiring enzyme (IRE)1, which initiates a so-called unfolded protein response (UPR) upon accumulation of unfolded proteins in the ER. In metazoans, two additional pathways have developed to assist in restoring homeostasis: PKR-like ER kinase (PERK) and activating transcription factor (ATF)6. Strikingly, vertebrates also express two distinct IRE1 genes. The activation sequence of the ubiquitous paralogue IRE1α has been well-characterized. Upon accumulation of unfolded proteins, IRE1α chains associate and their kinase domains mediate trans-autophosphorylation, which induces full activation of the IRE1α endonuclease domain. The endonuclease domain catalyzes both the unconventional splicing of the transcript encoding the UPR transcription factor X-box binding protein (XBP)1, and degradation of other mRNA species. The second IRE1 paralogue, IRE1β, displays a highly restricted expression pattern and is detected only in the epithelia lining the airways and gastrointestinal tract. Its physiological role and its contribution to XBP1 splicing and mRNA degradation has not been fully determined as of yet. In this thesis, IRE1β-mediated signaling was investigated on several fronts, to further our understanding on why this second IRE1 paralogue was retained by the vertebrate lineage specifically in the mucosal niche. First, we utilized inducible co-expression models and size exclusion chromatography to determine that IRE1β occupies IRE1α oligomers and as such dampens IRE1α-mediated XBP1 splicing. Second, the IRE1β interactome was explored in detail using mass spectrometry and this uncovered a novel regulator of IRE1β-mediated signaling, Anterior Gradient Protein Homolog (AGR)2. AGR2 is an important maturation factor of the intestinal mucin MUC2, and the AGR2-IRE1β axis appears to represent a feedback mechanism that guards the mucin folding capacity of intestinal goblet cells. Finally, we attempted to confirm the published role of IRE1β in asthma. However, a causative role for IRE1β could not be confirmed in the physiologically relevant house dust mite model of asthma. Taken together, although IRE1β’s role in airway goblet cells remains somewhat obscure, our results in intestine contribute to solidifying IRE1β’s position as an important mediator of goblet cell homeostasis