EDEM1 targets misfolded HLA-B27 dimers for endoplasmic reticulum associated degradation

HLA-B27 is strongly associated with a group of inflammatory arthritic conditions known as the spondyloarthropathies, with Ankylosing Spondyltis (AS) representing the prototypical presentation (1, 2). Up to 90–95% of patients with AS express HLA-B27 but despite this genetic association being known for almost four decades, the disease mechanism remains poorly understood. Genome-wide genetic screens have identified a number of other genes contributing to AS, but none have yet fully explained the disease process (3). One current theory linking HLA-B27 to AS involves the predisposition of HLA-B27 heavy chain to misfold and form non-native heavy chain dimeric structures (4). HLA-B27 dimers have been correlated with disease in the rat model for SpA (5). Dimers may interact aberrantly with immune receptors at the cell surface, or act within the environment of the endoplasmic reticulum (ER) to induce cellular stress responses, potentially leading to the onset of pro-inflammatory responses (6, 7). MHC class I molecules present unique protein folding challenges to the ER quality control machinery and HLA-B27 is particularly prone to misfolding forming disulfided bonded dimers (8). Misfolding proteins within the ER can induce the Unfolded Protein Response (UPR), which is a cellular stress response initiating transcriptional changes whose function is to restore ER homeostasis (9, 10). There are three main effector molecules of the UPR, the ATF6 transcription factor and the two kinases IRE1 and PERK, which reside within the ER and all are maintained in an inactive state by the ER resident chaperone Immunoglobulin Binding Protein (BiP) (10–12). One of the most potent transcriptional changes induced by the UPR follows the oligomerisation and autophosphorylation of IRE1. Phosphorylated IRE1 can splice the cytosolically located X-box binding protein 1 (XBP-1) mRNA by excising a 26 nucleotide intronic sequence, generating the XBP-1 spliced(s) transcription factor (13). XBP-1s can activate chaperones to enhance both cellular folding capacity and/or proteins involved in degradation of misfolding substrates (14). ER degradation-enhancing α-mannosidase-like protein 1 (EDEM1) was originally cloned as an ER stress induced gene (15), which is targeted by XBP-1s (14). EDEM1 overexpression studies have demonstrated a role in the degradation of misfolded proteins. EDEM1 is thought to handle misfolded proteins in concert with the ER chaperone calnexin and the UPR induced XBP-1 target E3 ubiquitin ligase HRD-1-SEL1 complex (16–18). Together with the E2-ubiquitin conjugating enzyme UBE2J1 they were shown to participate in the degradation of misfolded MHC class I heavy chains at steady state (19). Very recently, a role for EDEM1 in the MHC class I pathway has also been demonstrated (20), although its interaction with HLA-B27 was not investigated. In this current study we demonstrate that EDEM1 participates in the degradation of HLA-B27 dimers under ER stress conditions. This degradation pathway requires XBP-1 and HRD1. Significantly, our data highlight the ER stress induced ERAD pathway as a potential route for therapeutic intervention in HLA-B27 associated diseases.