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

ABSTRACT 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.