COX‐2 in Fracture Callus Chondro‐Osseous Junction Osteoclasts Regulates Chondrocyte Hypertrophy and Callus Vasculogenesis

Cyclooxygenase-2 (COX-2) activity is necessary for bone fracture healing to proceed normally. COX-2 is encoded by Ptgs2 and is expressed by several cell types during fracture healing, suggesting that COX-2 regulates multiple processes to affect fracture healing. Here, the role of COX-2 expression in osteoclasts during mouse femur fracture healing was examined. Mice lacking COX-2 (Ptgs2-cKO^Lyz2) in osteoclasts and other myeloid cells were made using a floxed COX-2 gene (Ptgs2^tm1Hahe) and cre recombinase expressed from the Lyz2^tm1(cre)If° allele. Fracture healing was assessed by radiology, histology, immunohistochemistry, and mRNA quantification. Targeted loss of COX-2 in osteoclasts was confirmed by immunohistochemistry and led to significant reductions in callus osteoclasts. Comparisons between Ptgs2-cKO^Lyz2 and control mice found significant reductions in callus chondrogenesis and bone formation in the Ptgs2-cKO^Lyz2 mice. The reductions were accompanied by delayed callus vascularization and reduced MMP-13 expression. Immunohistochemistry showed that osteoclasts along the callus chondro-osseous junction normally express COX-2. In Ptgs2-cKO^Lyz2 mice, COX-2 expression was reduced in osteoclasts at the chondro-osseous junction and coincided with reduced MMP-13 expression at the chondro-osseous junction. The results indicate that COX-2 expressed by osteoclasts along the chondro-osseous junction promotes vasculogenesis and regulates chondrocyte hypertrophy during endochondral ossification. The results also indicate that osteoclasts at the callus chondro-osseous junction coordinate multiple cellular processes to promote endochondral ossification.