Abstract
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.
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Data availability
The bulk and scRNA-seq data are available in the Gene Expression Omnibus under accession codes GSE200283 and GSE200898, respectively.
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Acknowledgements
We thank Junko Sugita, Keiko Kaneko, and Ryoko Honma for their technical assistance. We also extend our gratitude to Dr. Mitsutaka Yakabe (Geriatric Medicine, University of Tokyo) for teaching us the method for the mouse tail suspension model. We also thank Kazumi Abe and Yuuta Kuze (Laboratory of Systems Genomics, Department of Computational Biology and Medical Sciences, University of Tokyo) for their technical advice on scRNA-seq.
Funding
This work was supported by JSPS KAKENHI grants 23H05484, 23K27718, 23K27717, 21K19552, 20H03799, 19H05654, 19H05565, and 18KK0254; the Nakatomi Foundation; and grants from the Japan Orthopaedics and Traumatology Research Foundation and the Hip Joint Foundation of Japan.
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HI and TS designed the research project. HI, YK, JH, JM, NT, and KN performed histological experiments. HI, and RC performed RNA-seq and scRNA-seq on the advice of MS and YS. HI and HO conducted bioinformatics analysis. HI, AT, FY, YO, and TS performed figure editing. HI and TS wrote the manuscript with critical input from HO, YS, RB, and ST.
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All mouse experiments were authorized to be approved by the Animal Care and Use Committee of the University of Tokyo (approval number M-P17-091). All methods were carried out in accordance with the relevant guidelines and regulations. All methods are reported in accordance with the ARRIVE guidelines (https://arriveguidelines.org).
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Ishikura, H., Okada, H., Kin, Y. et al. Loss of mechanical stress induces synovitis, fibrosis and articular cartilage degeneration via distinct synovial cell subsets. Sci Rep (2026). https://doi.org/10.1038/s41598-026-39416-4
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DOI: https://doi.org/10.1038/s41598-026-39416-4


