Objectives Osteophytes are highly prevalent in osteoarthritis (OA) and are associated with pain and functional disability. These pathological outgrowths of cartilage and bone typically form at the junction of articular cartilage, periosteum and synovium. The aim of this study was to identify the cells forming osteophytes in OA.
Methods Fluorescent genetic cell-labelling and tracing mouse models were induced with tamoxifen to switch on reporter expression, as appropriate, followed by surgery to induce destabilisation of the medial meniscus. Contributions of fluorescently labelled cells to osteophytes after 2 or 8 weeks, and their molecular identity, were analysed by histology, immunofluorescence staining and RNA in situ hybridisation. Pdgfrα-H2BGFP mice and Pdgfrα-CreER mice crossed with multicolour Confetti reporter mice were used for identification and clonal tracing of mesenchymal progenitors. Mice carrying Col2-CreER, Nes-CreER, LepR-Cre, Grem1-CreER, Gdf5-Cre, Sox9-CreER or Prg4-CreER were crossed with tdTomato reporter mice to lineage-trace chondrocytes and stem/progenitor cell subpopulations.
Results Articular chondrocytes, or skeletal stem cells identified by Nes, LepR or Grem1 expression, did not give rise to osteophytes. Instead, osteophytes derived from Pdgfrα-expressing stem/progenitor cells in periosteum and synovium that are descendants from the Gdf5-expressing embryonic joint interzone. Further, we show that Sox9-expressing progenitors in periosteum supplied hybrid skeletal cells to the early osteophyte, while Prg4-expressing progenitors from synovial lining contributed to cartilage capping the osteophyte, but not to bone.
Conclusion Our findings reveal distinct periosteal and synovial skeletal progenitors that cooperate to form osteophytes in OA. These cell populations could be targeted in disease modification for treatment of OA.
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AJR and KK contributed equally.
Contributors AJRo: Conceptualisation, experimental design, data acquisition, analysis and interpretation, writing of the manuscript; KK: Experimental design, data acquisition, analysis and interpretation, contributed to writing of the manuscript; AJRa and FLC: Data acquisition, analysis and interpretation, contributed to writing of the manuscript. MSa and SK: Data acquisition, analysis and interpretation; JSm, MSe, LR and CBL: Data acquisition and analysis; HW, RG: Data acquisition; CK and SMF: Experimental design and data acquisition; FVM, TP and JFB: Conceptualisation, experimental design, data interpretation; JSh: Conceptualisation, experimental design, data acquisition, analysis and interpretation; JGC: Conceptualisation, experimental design, data acquisition, analysis and interpretation, contributed to writing of the manuscript; CDB: Conceptualisation, experimental design, data analysis and interpretation, writing of the manuscript.
Funding CDB, AJRo, KK, AJRa, FLC and HW were supported by funding from Versus Arthritis, formerly Arthritis Research UK (20775, 21156, 20050, 19429), and the Medical Research Council (MR/L020211/1). TP, JSh, MSa and RG were supported by funding from the Bundesministerium für Bildung und Forschung (BMBF) Overload-PrevOP consortium (01EC1408F) and the Innovative Medizinische Forschung (IMF) Programme of the University Hospital Münster (Project I-SH121608). JGC, STK, JSm, MAS and FVM were supported by funding from the National Insitute of Arthritis and Musculoskeletal and Skin Diseases (NIAMS: R01AR069700). JFB, LR and CBL were supported by the National Health and Medical Research Council (NHMRC: APP1063133), and part funding was provided to JFB by the Victorian Government‘s Operational Infrastructure Support Programme to the Murdoch Children’s Research Institute. CK and SMF were supported by funding from the Wellcome Trust (203151/Z/16/Z), Horizon2020 (ERC-2014-CoG-648765), Cancer Research UK (C61367/A26670) and NHS Blood and Transplant.
Competing interests None declared.
Patient and public involvement Patients and/or the public were not involved in the design, or conduct, or reporting, or dissemination plans of this research.
Patient consent for publication Not required.
Ethics approval All animal experimental protocols were approved by the UK Home Office and the Animal Welfare and Ethical Review Committees of the University of Aberdeen and University of Cambridge, the University of Southern California Institutional Animal Care and Use Committee, the Murdoch Children’s Research Institute Animal Ethics Committee, or the Animal Use Committee for University Hospital Münster.
Provenance and peer review Not commissioned; externally peer reviewed.
Data availability statement All data supporting the findings of this study are available within the Article and its online supplemental information files, or are available from the corresponding authors on reasonable request.