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Objective:  Protein kinase-like endoplasmic reticulum kinase (PERK) and protein kinase R (PKR) are implicated in endoplasmic reticulum stress-induced arthritis and pro-inflammatory cytokine-mediated cartilage degradation  in vitro , respectively. We determined whether knockout of the cellular inhibitor of PERK and PKR, P58 IPK  causes joint degeneration  in vivo  and whether these molecules are activated in human osteoarthritis (OA).   Materials and Methods:  Sections of knee joints from P58 IPK -null and wild-type mice aged 12–13 and 23–25 months were stained with toluidine blue and scored for degeneration using the osteoarthritis research society international (OARSI) system. Bone changes were assessed by radiology and high-resolution micro-computed tomography of hind limbs. Sections from the medial tibial plateaus of two human knees, removed in total knee replacement surgery for OA, were immunolabelled for phosphorylated PERK and PKR and P58 IPK .   Results:  Knockout mice exhibited narrower tibiae ( p  = 0.0031) and smaller epiphyses in tibiae ( p  = 0.0004) and femora ( p  = 0.0214). Older knockout mice had reduced total volume inside the femoral periosteal envelope ( p  = 0.023), reduced tibial ( p  = 0.03), and femoral ( p  = 0.0012) bone volumes (BV) and reduced femoral BV fraction ( p  = 0.025). Compared with wild-types, younger P58 IPK -null mice had increased OARSI scores in medial femoral condyles ( p  = 0.035). Thirty four percent of null mice displayed severe joint degeneration with complete articular cartilage loss from the medial compartment and heterotopic chondro-osseous tissue in the medial joint capsule. Phosphorylated PERK and PKR were localized throughout human osteoarthritic tibial plateaus but, in particular, in areas exhibiting the most degeneration. There was limited expression of P58 IPK .   Conclusion:  This study is the first to reveal a critical role for P58 IPK  in maintaining joint integrity  in vivo , implicating the PKR and PERK stress signaling pathways in bony changes underlying the pathogenesis of joint degeneration.

Deletion of P58IPK, the Cellular Inhibitor of the Protein Kinases PKR and PERK, Causes Bone Changes and Joint Degeneration in Mice