Runx2 deletion in hypertrophic chondrocytes impairs osteoclast mediated bone resorption

Deletion of Runx2 gene in proliferating chondrocytes results in complete failure of endochondral ossification and perinatal lethality. We reported recently that mice with Runx2 deletion specifically in hypertrophic chondrocytes (HCs) using the Col10a1-Cre transgene survive and exhibit enlarged growth plates due to decreased HC apoptosis and cartilage resorption. Bulk of chondrogenesis occurs postnatally, however, the role of Runx2 in HCs during postnatal chondrogenesis is unknown. Despite limb dwarfism, adult homozygous (Runx2^HC/HC) mice showed a significant increase in length of growth plate and articular cartilage. Consistent with doubling of the hypertrophic zone, collagen type X expression was increased in Runx2^HC/HC mice. In sharp contrast, expression of metalloproteinases and aggrecanases were markedly decreased. Impaired cartilage degradation was evident by the retention of significant amount of safranin-O positive cartilage. Histomorphometry and μCT uncovered increased trabecular bone mass with a significant increase in BV/TV ratio, trabecular number, thickness, and a decrease in trabecular space in Runx2^HC/HC mice. To identify if this is due to increased bone synthesis, expression of osteoblast differentiation markers was evaluated and found to be comparable amongst littermates. Histomorphometry confirmed similar number of osteoblasts in the littermates. Furthermore, dynamic bone synthesis showed no differences in mineral apposition or bone formation rates. Surprisingly, three-point-bending test revealed Runx2^HC/HC bones to be structurally less strong. Interestingly, both the number and surface of osteoclasts were markedly reduced in Runx2^HC/HC littermates. Rankl and IL-17a ligands that promote osteoclast differentiation were markedly reduced in Runx2^HC/HC mice. Bone marrow cultures were performed to independently establish Runx2 and hypertrophic chondrocytes role in osteoclast development. The culture from the Runx2^HC/HC mice formed significantly fewer and smaller osteoclasts. The expression of mature osteoclast markers, Ctsk and Mmp9, were significantly reduced in the cultures from Runx2^HC/HC mice. Thus, Runx2 functions extend beyond embryonic development and chondrocyte hypertrophy by regulating cartilage degradation, osteoclast differentiation, and bone resorption during postnatal endochondral ossification.