Regulated IRE1‐dependent decay participates in curtailing immunoglobulin secretion from plasma cells

Abstract

Inositol-requiring enzyme 1 (IRE1) is a kinase and ribonuclease that executes the splicing of X box binding protein 1 (XBP-1) mRNA in response to the accumulation of unfolded protein in the ER, a signal cascade termed the unfolded protein response. Recently, IRE1 has been implicated in mRNA and miRNA cleavage and degradation, a pathway termed regulated IRE1-dependent decay (RIDD). Deletion of XBP-1 in the liver and pancreas strongly enhances RIDD by upregulating IRE1 protein levels and enhancing its ribonuclease activity. Because XBP-1 is essential for generating plasma cells with developed secretory capacity, we sought to evaluate the contribution of RIDD to this regulation. Mice were conditionally deleted for XBP-1 and/or IRE1 in their B-cell lineage. Similarly to the liver, deletion of XBP-1 induces IRE1 expression in LPS-treated B cells. In vitro, IRE1 cleaves the mRNA of secretory μ chains, which explains the reduction in secretory μ mRNA and its synthesis in XBP-1 KO plasma cells. In accordance, the IgM response is partially restored in XBP-1/IRE1 double KO mice relative to XBP-1 KO mice. Interestingly, the IgG1 response is reduced to a similar level in XBP-1 KO, IRE1 KO, and their double knockout animals. Our data demonstrate a specific contribution by RIDD in curtailing immunoglobulin synthesis and secretion.Keywords: ER stress r IRE1 r plasma cells r RIDD r UPR r XBP-1 See accompanying Commentary by van Anken et al.Additional supporting information may be found in the online version of this article at the publisher's web-site IntroductionThe unfolded protein response (UPR) is an adaptive signaling pathway elicited in response to perturbations in ER homeostasis, conditions referred to as ER stress. In the canonical mammalian UPR, three ER stress sensors operate in parallel: inositol-requiring enzyme 1 (IRE1), PKR-like ER kinase, and activating transcripCorrespondence: Dr. Boaz Tirosh e-mail: boazt@ekmd.huji.ac.il tion factor 6 (reviewed in [1]). Upon activation, each sensor arm executes a transcriptional program that is interconnected with the other UPR pathways for amplification and negative feedbacks. Thus, the outcome of the UPR is dependent on the relative contribution of the three arms and their threshold for activation. Much of the molecular details of UPR signaling were brought about using artificial induction with chemicals that robustly perturb protein folding in the ER, such as tunicamycin, thapsigargin, or DTT. Understanding of the UPR in a physiological setting, however, has been more challenging to elucidate.C 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim www.eji-journal.eu 868Sandrine Benhamron et al. Eur. J. Immunol. 2014. 44: 867-876 The first example for a physiological role of the UPR was described in the terminal differentiation of mature B cells into plasma cells (PCs), the latter are responsible for secretion of antibodies [2]. Mature B cells contain very little cytoplasm and intracellular membranes. When triggered by antigen in the context of accessory signals such as toll-lik...

A RIDDle solved: Why an intact IRE1α/XBP‐1 signaling relay is key for humoral immune responses

“…In line with this finding, no putative "splice site" was found in the IgG1 sequence, but this feature still awaits biochemical validation [31]. Importantly, neither IgM nor IgG titers were normal when either XBP-1 or IRE1α alone or in combination were compromised [31].…”

“…Namely, XBP-1 ablation in B cells led to a sizeable increase in IRE1α levels, as has been described in the liver [35]. The authors explain this phenomenon as follows: in wild-type cells the protein product of the unspliced XBP-1 mRNA negatively regulates IRE1α expression, such that when XBP-1 is ablated IRE1α becomes derepressed [31]. Though the molecular basis for this feedback loop awaits clarification, it is conceivable that increased IRE1α levels would prey more on μs mRNA and further deteriorate the antibody output.…”

“…Thus, while RIDD certainly explains in part why xbp1 −/− B cells secrete antibody so poorly, the original interpretation from the pre-RIDD era is still valid: an intact IRE1α-XBP-1 signaling array is key for B cells to reach their full secretory potential, because it drives expansion of the secretory apparatus, particularly the ER. Accordingly, electron micrographs of activated B cells displayed less developed ER when IRE1 endonuclease function or XBP-1 was abrogated as compared with wild-type B cells [31]. As a nice detail, the authors found that in the absence of XBP-1, the ER lumen appeared relatively bright, i.e.…”

“…As mentioned above, ablation of XBP-1 severely compromises plasma cell differentiation [14,20,32]. In this issue, Benhamron et al dissect the extent to which the xbp1 −/− phenotype can be ascribed to the lack of XBP-1, or conversely, to the potential increase in RIDD activity by IRE1α, when the latter has no XBP-1 to chew on, so to speak [31].…”

“…As mentioned above, ablation of XBP-1 severely compromises plasma cell differentiation [14,20,32]. In this issue, Benhamron et al dissect the extent to which the xbp1 −/− phenotype can be ascribed to the lack of XBP-1, or conversely, to the potential increase in RIDD activity by IRE1α, when the latter has no XBP-1 to chew on, so to speak [31].The authors provide evidence that, once B lymphocytes are activated to differentiate into plasma cells not only are the levels of the secretory Ig-μ chain (μs) proteins reduced in the absence of XBP-1, but also those of the corresponding mRNA compared with levels in wild-type mice [31]. In vitro splicing assays suggest that this phenomenon is due to IRE1α nibbling on μs mRNAs when its favorite substrate, XBP-1 mRNA, is not around (Fig.…”