Induction of dsRNA-activated protein kinase links mitochondrial unfolded protein response to the pathogenesis of intestinal inflammation

Abstract

Objective Inflammatory bowel diseases (IBDs) feature multiple cellular stress responses, including endoplasmic reticulum (ER) unfolded protein responses (UPRs). UPRs represent autoregulatory pathways that adjust organelle capacity to cellular demand. A similar mechanism, mitochondrial UPR (mtUPR), has been described for mitochondria. ER UPR in intestinal epithelial cells (IECs) contributes to the development of intestinal inflammation, and since mitochondrial alterations and dysfunction are implicated in the pathogenesis of IBDs, the authors characterised mtUPR in the context of intestinal inflammation. Methods Truncated ornithine transcarbamylase was used to selectively induce mtUPR in a murine IEC line. Dextran sodium sulphate (DSS) was administered to PKR (double-stranded-RNA-activated protein kinase) knockout mice to induce IEC stress in vivo and to test for their susceptibility to DSS-induced colitis. Expression levels of the mitochondrial chaperone chaperonin 60 (CPN60) and PKR were quantified in IECs from patients with IBDs and from murine models of colitis using immunohistochemistry and Western blot analysis. Results Selective mtUPR induction by truncated ornithine transcarbamylase transfection triggered the phosphorylation of eukaryotic translation initiation factor (eIF) 2α and cJun through the recruitment of PKR. Using pharmacological inhibitors and small inhibitory RNA, the authors identified mtUPR-induced eIF2α phosphorylation and transcription factor activation (cJun/AP1) as being dependent on the activities of the mitochondrial protease ClpP and the cytoplasmic kinase PKR. Pkr−/− mice failed to induce CPN60 in IECs upon DSS treatment at early time points and subsequently showed an almost complete resistance to DSS-induced colitis. Under inflammatory conditions, primary IECs from patients with IBDs and two murine models of colitis exhibited a strong induction of the mtUPR marker protein CPN60 associated with enhanced expression of PKR. Conclusion PKR integrates mtUPR into the disease-relevant ER UPR via eIF2α phosphorylation and AP1 activation. Induction of mtUPR and PKR was observed in IECs from murine models and patients with IBDs. The authors’ results indicate that PKR might link mitochondrial stress to intestinal inflammation.

The kinase activity of PKR represses inflammasome activity

“…It has been suggested that discordant responses that have previously been recorded between the differently targeted PKR transgenic animals [23,24], could be the consequence of residual peptides that may be expressed in each animal. However, the demonstration by He et al [5] of the relative equivalence of the response in BMDM isolated from either mouse, argues against this explanation.…”

Mitochondrial unfolded protein response gene Clpp is required to maintain ovarian follicular reserve during aging, for oocyte competence, and development of pre‐implantation embryos

“…These pathways sense unfolded protein stress in a compartment‐specific manner, and signal to the nucleus for induction of the expression of unfolded protein response (UPR) genes, which are essential for proteostasis (Schinzel & Dillin, 2015). Mitochondrial unfolded protein response (mtUPR) is activated in response to a variety of mitochondrial stress factors including accumulation of unfolded proteins in the mitochondrial matrix (Rath et al., 2012), imbalance between mitochondrial DNA (mtDNA)‐encoded and nuclear‐encoded electron transport chain (ETC) components (Nargund, Pellegrino, Fiorese, Baker & Haynes, 2012; Yoneda et al., 2004), and perturbation of mitochondrial physiology through inhibition of ETC function or accumulation of reactive oxygen species (ROS) (Nargund et al., 2012; Yoneda et al., 2004), and ensures mitochondrial proteostasis by inducing a vigorous transcriptional response that promotes folding, limits import, and reduces translation of mitochondrial proteins (reviewed in Hill & Van Remmen, 2014; Jensen & Jasper, 2014). …”

“…Mitochondrial unfolded protein response and the role of CLPP seem to be conserved in mammals (Benedetti et al., 2006; Zhao et al., 2002), where activation of JNK/c‐JUN pathway leads to the expression of transcription factor C/EBP‐homologous protein (CHOP), which, together with C/EBP, mediates the transcription of mtUPR genes (reviewed in Hill & Van Remmen, 2014). It is important that, in addition to inducing transcription of over 400 genes, mtUPR in yeast, C. elegans , and mammals are associated with phosphorylation of eukaryotic initiation factor 2 alpha (eIF2a) by general control nonderepressible 2 (GCN2), resulting in global suppression of translation while mRNAs that contain upstream open reading frames (uORFs) are preferentially translated (Delaney et al., 2013; Rath et al., 2012). Transcriptional activation of mtUPR genes and translational suppression seem to be mediated by two parallel mechanisms, both requiring CLPP (Aldridge et al., 2007; Benedetti et al., 2006; Haynes et al., 2007; Zhao et al., 2002) and reviewed in (Jensen & Jasper, 2014; Schulz & Haynes, 2015).…”

The mitochondrial unfolded protein response, a conserved stress response pathway with implications in health and disease

“…Similar to the role of GCN-2 in C. elegans , dsRNA-activated protein kinase (PKR), also known as eukaryotic translation initiation factor 2-α kinase 2 (EIF2AK2), mediates phosphorylation of eIF2α, thus attenuating protein translation in the cytosol during UPR mt in mammalian systems (Fig. 2) (Rath et al, 2012). Interestingly, PKR together with ClpP is also required for the activation of c-Jun, the transcriptional regulator of CHOP (Rath et al, 2012).…”

“…2) (Rath et al, 2012). Interestingly, PKR together with ClpP is also required for the activation of c-Jun, the transcriptional regulator of CHOP (Rath et al, 2012).…”