Identification of the skeletal progenitor cells forming osteophytes in osteoarthritis

Abstract

ObjectivesOsteophytes 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.MethodsFluorescent 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.ResultsArticular 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.ConclusionOur 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.

Sites of Cre-recombinase activity in mouse lines targeting skeletal cells

“…(227) The Gdf5-Cre line is commonly used to study the development of the joints (119,228) because it targets all structures of the mature synovial joint, including ligaments, synovial membrane, and articular cartilage. (119)(120)(121)(122)229) During development, Gdf5-Cre LacZ activity was detected firstin themostproximal limb joints (E12.5),reachingdistal joints by E14.5. It was also detected in AF cells of the IVD (P0), but not in epiphyseal chondrocytes.…”

“…(119) Some Gdf5-Cre+ mice also show off-target activity in peripheral blood cells. (119,122) The inducible Gdf5-CreER T2 knock-in line (229) targets virtually all joints at E18.5 after E13.5 induction. Earlier (E10.5) induction targets some epiphyseal chondrocytes, and later induction (E14.5-E15.5) targets an increasing number of articular chondrocytes, with activity restricted to the most superficial articular cartilage layer.…”

Deletion of Hyaluronan-Binding Protein Involved in Hyaluronan Depolymerization (HYBID) Results in Attenuation of Osteoarthritis in Mice

“…Fully-developed osteophyte is composed of cartilage and bone parts, and the cartilage part is formed mainly by proliferation of mesenchymal stem cells stimulated with TGF-β, while bone morphogenic protein 2 may be important for initiation of the bone formation via endochondral ossification 33 . Roelofs et al have recently reported that Pdgfrα+Gdf5-lineage progenitors containing subsets of Prg4-expressing progenitors and Sox9-expressing progenitors, both of which are resident at the junction of periosteum and synovium near the articular cartilage, are activated in OA to form osteophyte cartilage and bone parts, respectively 34 . The molecular mechanisms of reduced osteophyte formation in OA models of Hybid -/mice are not clear at the present time.…”

Growth differentiation factor 5 in cartilage and osteoarthritis: A possible therapeutic candidate

“…91 These platelet-derived growth factor receptor alpha + GDF-5-lineage cells were activated in OA to form both the cartilage and bone of the osteophyte. 91 The expanding mineralizing fibrocartilage has been considered an earlier indicator of OA onset. 92 GDF-5 progenitors participated in the formation of fibrocartilage, which could develop into osteophytes post-injury.…”