During cellular stress the protein folding capacity of the ER is diminished. In order to maintain homeostasis and ensure proper protein folding cells activate the unfolded protein response (UPR), a signaling network consisting of sensors and effectors to enhance the chaperone activity of the cell, increase degradation of accumulated proteins, and/or trigger apoptosis. Inositol-requiring enzyme 1 (IRE1), an ER-transmembrane protein, is an essential component of the UPR pathway important for sensing and responding to ER stress. IRE1 contains an ER luminal stress-sensing domain and a cytoplasmic facing RNase domain. Upon activation by the presence of misfolded substrates IRE1 proteins oligomerize and activate their RNase activity. The main function of IRE1’s RNase activity is the unconventional splicing of the transcription factor XBP1 mRNA. Splicing of XBP1 allows for the translation of functional transcription factor and the upregulation of target genes including ER chaperones and ER-associated degradation components. IRE1 also acts on targets through Regulated IRE1-Dependent Decay (RIDD) to decrease mRNA translation and to alleviate the load on protein folding machinery. RIDD is also important for UPR-induced apoptosis by alleviating the inhibition of the proapoptotic Caspase-2.
IRE1 antibodies, including the phospho-specific IRE1 antibody, are important tools to monitor UPR activation and allow the investigation of UPR in diverse cellular processes. Stahl et al. identified a viral strategy to suppress the UPR in response to infection (1). Through western blotting with a phospho-specific IRE1 antibody their research demonstrated the viral M50 protein binds to and downregulates IRE1 to enhance viral protein folding. While IRE1 clustering is an important step in the activation of UPR, the Arai group at the University of Tokyo demonstrated an alternative mechanism for IRE1 activation (2). Using the phospho-specific IRE1 antibody they showed perturbation of cellular lipids leads to the phosphorylation of IRE1 and activation of the UPR without IRE1 clustering. This suggests unique mechanisms of UPR activation depending on the stimulus. Hoozemans et al. showed active IRE1 is a pathological marker in Alzheimer’s disease (3). Examination of hippocampal neurons through immunohistochemistry with the phospho-specific IRE1 antibody showed elevated activity of the UPR pathway in Alzheimer’s disease. The Defrance group sought to induce cell death of multiple myeloma cells by manipulating UPR sensor proteins (4). They used siRNA targeting UPR sensors and monitored their levels through western blotting with antibodies including the IRE1 antibody. Their experiments showed blocking UPR leads to autophagy-dependent cell death of multiple myeloma cells and may lead to the discovery of future therapies.
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