Epidermal Growth Factor Receptor-deficient Mice Have Delayed Primary Endochondral Ossification Because of Defective Osteoclast Recruitment

Abstract

The epidermal growth factor receptor (EGFR) and its ligands function in diverse cellular functions including cell proliferation, differentiation, motility, and survival. EGFR signaling is important for the development of many tissues, including skin, lungs, intestines, and the craniofacial skeleton. We have now determined the role of EGFR signaling in endochondral ossification. We analyzed long bone development in EGFR-deficient mice. EGFR deficiency caused delayed primary ossification of the cartilage anlage and delayed osteoclast and osteoblast recruitment. Ossification of the growth plates was also abnormal resulting in an expanded area of growth plate hypertrophic cartilage and few bony trabeculae. The delayed osteoclast recruitment was not because of inadequate expression of matrix metalloproteinases, including matrix metalloproteinase-9, which have previously been shown to be important for osteoclast recruitment. EGFR was expressed by osteoclasts, suggesting that EGFR ligands may act directly to affect the formation and/or function of these cells. EGFR signaling regulated osteoclast formation. Inhibition of EGFR tyrosine kinase activity decreased the generation of osteoclasts from cultured bone marrow cells.Skeletal elements develop by two distinct mechanisms: intramembranous and endochondral ossification (1). Endochondral ossification is a process by which a cartilaginous template is first formed and then replaced by bone. During embryogenesis, condensations of mesenchymal cells form, within which chondrocytes develop, proliferate, and differentiate to form a cartilage template that contains distinct zones of resting, proliferative, and hypertrophic chondrocytes. The proliferation and differentiation of chondrocytes within the cartilage template are spatially ordered, with proliferating cells at the two ends of the template and progressively more mature cells forming hypertrophic cartilage in the middle. Hypertrophic chondrocytes secrete a specialized extracellular matrix (ECM) 1 containing collagen X, which becomes calcified. Endochondral ossification begins with the invasion of the calcified hypertrophic cartilage by blood vessels, accompanied by osteoclasts and osteoblasts (primary ossification center). The function of osteoclasts is to remove the hypertrophic cartilage ECM and that of osteoblasts is to replace it with bone ECM. Longitudinal bone growth is accomplished by the continuing proliferation and maturation of chondrocytes at the ends of the cartilage template (the growth plates) to form more hypertrophic cartilage and its continual removal and replacement by bone (growth plate ossification or formation of primary spongiosa). Normal endochondral bone development requires the exquisite coordination of hypertrophic cartilage formation, vascular invasion, and the development and function of osteoclasts and osteoblasts (2, 3).The epidermal growth factor receptor (EGFR) family of receptor tyrosine kinases includes EGFR/ErbB1, HER2/ErbB2, HER3/ErbB3, and HER4/ErbB4 (4, 5). EGFR binds several ligands ...

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Epidermal growth factor signalling and bone metastasis

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ADAM17 Controls Endochondral Ossification by Regulating Terminal Differentiation of Chondrocytes

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