The role of extracellular ERp57/PDIA3 in healthy and osteoarthritic articular cartilage

• Applicant: Privatdozentin Dr. Rita Dreier
• Subject Area: Orthopaedics, Traumatology, Reconstructive Surgery
• Term: since 2024
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 552519689

Osteoarthritis (OA) is a chronic progressive joint disease that affects more than 300 million people worldwide. The destruction of joint cartilage is caused by a combination of degradation of the extracellular matrix (ECM), reduced ECM protein production and the death of chondrocytes. Both, reduction in protein biosynthesis and induction of apoptosis are features of chondrocytes with endoplasmic reticulum (ER) stress, which we and others have recently shown to play a role in OA. We demonstrated that loss of the protein disulfide isomerase ERp57, which acts as an ER chaperone catalyzing the formation of disulfide bridges of various ECM molecules, induces ER stress in chondrocytes in vitro (C28/I2 cells) and in vivo (cartilage-specific ERp57 KO mice). This was detected by expanded ER cisternae, increased expression of ER stress marker proteins, decreased secretion of ECM proteins, and increased rates of apoptosis. Furthermore, we confirmed that loss of ERp57 in articular cartilage cells leads to an accelerated development of osteoarthritis in knee joints of aging mice. 18-month-old cartilage-specific ERp57 KO animals showed severe OA cartilage degeneration with osteophyte formation. These changes were preceded by ER stress-induced apoptotic cell death. Interestingly, ERp57 was released from chondrocytes into the culture medium together with its substrates. Co-immunoprecipitation experiments revealed fibronectin as a binding partner of ERp57 in cell lysates and in cell culture supernatants of chondrocytes. A detailed analysis of the role of ERp57 in C28/I2 cultures showed that the formation of fibronectin fibrils was severely impaired in ERp57 KO cells. Collectively, our data suggest that ERp57 is required for normal function and survival of articular cartilage cells. It is very likely that these effects are not only due to the intracellular ER chaperone function of ERp57, but that extracellular effects of secreted ERp57 are also involved. We will investigate whether ERp57 acts as an extracellular ECM chaperone on ECM molecules and thus alters the biomechanical properties of the cartilage matrix. However, extracellular ERp57 could also contribute to the activation of surface molecules, e.g. integrins, on chondrocytes via disulfide bond-induced structural changes and thus alter the adhesion of cartilage cells or activate intracellular signaling pathways. The role of extracellular ERp57 has been underestimated and has not yet been investigated in articular cartilage. We expect that the planned experiments will shed light on previously unknown mechanisms of OA pathogenesis and thus provide new targets for OA therapies.

• DFG Programme: Research Grants