IADR Abstract Archives
Runx2 Activity in Hypertrophic Chondrocytes Regulates Resorption of Calcified Cartilage
Objectives: Global deletion of the Runx2-gene results in complete failure of mineralized tissue formation and embryonic lethality. Deletion of the Runx2-gene specifically in the resting chondrocyte blocks chondrocyte differentiation and endochondral ossification. The expression of the Runx2-gene increases from resting to hypertrophic chondrocytes. However, Runx2 role in hypertrophic chondrocytes is unknown.
Methods: Collagen type X-Cre model was used to delete the Runx2-gene in hypertrophic chondrocytes. Molecular and histologic techniques were used to analyze osteogenesis.
Results: Newborn mice lacking the Runx2-gene in hypertrophic chondrocytes (Runx2HC) have a well-formed skeleton but long bones are short with unmineralized extremities. Histologic analysis revealed the length of the hypertrophic zone is double in Runx2HC bones. TUNEL-assay confirmed that decrease apoptosis of Runx2 deficient hypertrophic-chondrocytes leads to the expansion of the hypertrophic zone. Compared to WT littermates, Runx2HC bones contain a large amount of calcified cartilage. RNA analysis showed that poor cartilage resorption in Runx2HC bones is due to a significant decrease in the expression of matrix-metalloproteinases. Interestingly, μCT showed a 3-fold increase in the trabecular bone but the cortical bones were comparable among the littermates. To better understand the poor resorption of calcified cartilage and increased trabecular bone in Runx2HC mice, we evaluated osteoclasts. The number of TRAP-positive osteoclasts was decreased by 30% in Runx2HC bones. Consistent with these results the Rankl/Opg ratio was decreased in mutant bones. Importantly, the expanded zone of hypertrophy and unresorbed calcified cartilage persists in the adult mice. Histologic and mCT analysis of 2.5-month old littermates revealed a locally osteopetrotic phenotype in the mutant bones. Dynamic histomorphometric analysis indicated a 40% decrease in the osteoclasts number but the number osteoblast and bone formation rate are comparable. Our results demonstrate that high trabecular bone mass is due to poor bone resorption and not linked to increased bone synthesis.
Conclusions: Runx2 activity after hypertrophy is essential for degradation and resorption of calcified cartilage.
Methods: Collagen type X-Cre model was used to delete the Runx2-gene in hypertrophic chondrocytes. Molecular and histologic techniques were used to analyze osteogenesis.
Results: Newborn mice lacking the Runx2-gene in hypertrophic chondrocytes (Runx2HC) have a well-formed skeleton but long bones are short with unmineralized extremities. Histologic analysis revealed the length of the hypertrophic zone is double in Runx2HC bones. TUNEL-assay confirmed that decrease apoptosis of Runx2 deficient hypertrophic-chondrocytes leads to the expansion of the hypertrophic zone. Compared to WT littermates, Runx2HC bones contain a large amount of calcified cartilage. RNA analysis showed that poor cartilage resorption in Runx2HC bones is due to a significant decrease in the expression of matrix-metalloproteinases. Interestingly, μCT showed a 3-fold increase in the trabecular bone but the cortical bones were comparable among the littermates. To better understand the poor resorption of calcified cartilage and increased trabecular bone in Runx2HC mice, we evaluated osteoclasts. The number of TRAP-positive osteoclasts was decreased by 30% in Runx2HC bones. Consistent with these results the Rankl/Opg ratio was decreased in mutant bones. Importantly, the expanded zone of hypertrophy and unresorbed calcified cartilage persists in the adult mice. Histologic and mCT analysis of 2.5-month old littermates revealed a locally osteopetrotic phenotype in the mutant bones. Dynamic histomorphometric analysis indicated a 40% decrease in the osteoclasts number but the number osteoblast and bone formation rate are comparable. Our results demonstrate that high trabecular bone mass is due to poor bone resorption and not linked to increased bone synthesis.
Conclusions: Runx2 activity after hypertrophy is essential for degradation and resorption of calcified cartilage.
