Long bones develop through the strictly regulated process of endochondral ossification within the growth plate, resulting in the replacement of cartilage by bone. Defects in this process result in skeletal abnormalities and can predispose to disease such as osteoarthritis (OA). Studies suggest that activation of the transcription factor peroxisome proliferator activated receptor gamma (PPARγ) is a therapeutic target for OA. In order to devise PPARγ-related therapies in OA and related diseases, it is critical to identify its role in cartilage biology. Therefore, we determined the in vivo role of PPARγ in endochondral ossification and cartilage development using cartilage-specific PPARγ knockout (KO) mice.
Cartilage-specific PPARγ KO mice were generated using LoxP/Cre system. Histomorphometric and immunohistochemical analysis was performed to account for ossification patterns, chondrocyte proliferation, differentiation, hypertrophy, skeletal organization, bone density and calcium deposition. Real-Time PCR and western blotting was performed to determine the expression of key markers involved in endochondral ossification.
PPARγ KO mice exhibited reduced body length, weight, length of long bones, skeletal growth, cellularity, bone density, calcium deposition and trabecular bone thickness, abnormal growth plate organization, loss of columnar organization, shorter hypertrophic zones, and delayed primary and secondary ossification. Immunohistochemistry for Sox9, BrdU, p57, collagen X and PECAM revealed reduction in chondrocyte differentiation and proliferation, and hypertrophy and vascularisation in growth plates of mutant mice. Isolated chondrocytes and cartilage explants from mutant mice showed aberrant expression of ECM markers including aggrecan, collagen II and MMP-13.
PPARγ is required for normal endochondral ossification and cartilage development in vivo.