The role of tetraspanin Cd9 in pathogenesis of osteoarthritis

Abstract

Purpose: Osteoarthritis (OA) is a chronic and highly prevalent joint disease, and it is whole-joint disorder that is characterized by progressive degeneration of articular cartilage, synovial hyperplasia and angiogenesis as a result of excessive mechanical load or inflammation in various joint tissues. Therefore, understanding the molecular mechanisms that maintain whole-joint health has been required. Recently, it is reported that tetraspanin CD9 expression in OA synovium is upregulated. However, the role of CD9 in OA pathogenesis is still unclear. We hypothesize that CD9 may play an important role in OA pathogenesis. In order to propose a new concept of OA pathogenesis, we examined histopathological changes in knee joints using experimentally OA-induced models in CD9 KO mice. The purpose of this study is to examine the role of tetraspanine CD9 in OA pathogenesis. Methods: To reveal whether CD9 KO mice exhibit onset of OA-like changes during aging, we examined histology of knee joint using 1M-, 3M-, 6M-, and 12M-old mice. We also examined the expression of cartilage-related genes in articular chondrocytes from 1M-old mice. Next, to examine OA pathogenesis in CD9 KO mice, we generated two arthritis models, an antigen-induced-arthritis (AIA) model and a surgical model in knee joint. Inflammatory arthritis was induced in knee joint of 10-week-old wild-type (WT) and CD9 KO mice by intra-articular injection of methylated bovine serum albumin (mBSA) in mice preimmunized with mBSA and adjuvant. 7 days after intra-articular injection, the knee joints were harvested and processed. The AIA model in WT and CD9 KO mice were evaluated using inflammatory score consisted of 5 parameters which are synovitis, joint space exudate, soft tissue inflammation, cartilage degradation and bone damage. The surgical OA model was induced by resecting the medial meniscotibial ligament (MMTL) of 12-week-old WT and CD9KO mice. Pathological OA-like changes in medial femoral condyle (MFC) and medial tibial plateau (MTP) were evaluated 12 weeks after resecting MMTL on safranin-O-stained sections using Osteoarthritis Research Society International (OARSI) score. Results: The knee joints of 1M-, 3M- 6M-, and 12M-old CD9 KO mice were not showed early onset OA-like changes in their knee joints compared with WT mice and their development appeared normal. The expressions of cartilage-related genes in chondrocytes from CD9 KO were not significantly difference compared with chondrocytes from WT. In AIA model, inflammation in CD9 KO mice was suppressed and inflammation score of them was significantly lower in all 5 parameters and total score. In surgical model, no significant change was observed in the OARSI score of MFC between the WT and CD9 KO mice. However, we detected significantly suppressed OA-like pathological changes in the MTP in the CD9 KO mice compared with those in the WT mice. Conclusions: Inflammation and OA-like pathological changes are suppressed in tetraspanin CD9 KO mice compared with those in WT mice. Our results indicate that CD9 accelerates the development of OA. Recently, it was reported that exosomes, a new paracrine factor for cell-cell communication were decreased in CD9 KO mice because exosomes consist of several members of the tetraspanin family including CD9. Our studies might have the potential to open new insights on OA mechanisms.