Loss of mechanical stress induces synovitis, fibrosis and articular cartilage degeneration via distinct synovial cell subsets.

Joint function is impaired by disuse, as well as overuse. However, the underlying mechanisms remain unclear. Here, we elucidate the mechanisms of synovial and cartilage changes using a minimized mechanical stress (MMS) mouse model by combining knee joint immobilization and unloading. In this model, synovitis appeared by day 3, followed by subsequent fibrosis leading to joint contracture within two weeks. In contrast, articular cartilage degeneration developed gradually after the synovial alterations. Notably, synovial changes were attenuated by discontinuation of joint immobilization, while cartilage changes improved after discontinuation of joint immobilization and loading. Bulk RNA sequencing (RNA-seq) analyses supported the transcriptomic alterations for synovitis, fibrosis, and cartilage degeneration, and identified ten cytokines associated with cartilage changes. Single-cell RNA-seq (scRNA-seq) further identified distinct subsets in the MMS synovium: Lrrc15(+) myofibroblasts and Mmp9(+) macrophages, expressing many of these cytokines. Histological examination showed that MMS initially induced macrophage proliferation, while macrophage depletion by intra-articular administration of clodronate liposomes inhibited MMS-induced synovitis, fibrosis and cartilage degeneration, accompanied by a marked reduction in the MMS-distinct subsets. Our findings identified MMS-induced alterations in synovial cells and their roles in joint phenotype, suggesting that joint motion and mechanical loading contribute to the regulation of joint homeostasis.