Cartilage-specific deletion of prar-gamma in mice results in early endochondral ossification defects and accelerated aging-dependent development of osteoarthritis

Abstract

Purpose: Purpose: Long bones develop through a strict coordinated process of endochondral ossification within the growth plate resulting in the replacement of cartilage by bone and defect in this coordinated process may result in skeletal abnormalities such as dwarfism, kyposis and also age-related defects such as osteoarthritis (OA) and osteoporosis. PPARg, a transcription factor, plays a key role in lipid homeostasis but its in vivo role in cartilage/bone development is unknown. Therefore, we determined the specific in vivo role of PPARg in endochondral bone ossification, cartilage/bone development and in OA using cartilagespecific PPARg knockout (KO) mice. Methods: Cartilage-specific PPARg KO mice were generated using LoxP/Cre system. Skeletal and histological staining of long bones was performed to determine skeletal changes, growth plate organization, bone density, calcium deposition and joint phenotypic changes during aging. Immunohistochemistry for Collagen X, BrdU, p57, Sox9, PECAM and MMP-13 was performed. RT-PCR on isolated chondrocytes was performed to determine the expression of ECM markers. Results: PPARg KO mice at birth showed reduced length, weight, skeletal growth and length of long bones compared to wild-type (WT) mice. Histomorphometric analysis of embryonic and adult mutant mice demonstrate reduced long bone growth, calcium deposition, bone density, and vascularity as well as delayed primary and secondary ossification. Mutant growth plates were disorganized with abnormal chondrocyte shape, proliferation, and differentiation, reduced cellularity, loss of columnar organization, and shorter hypertrophic zones. Isolated chondrocytes and cartilage explants from mutant mice further show decreased expression of VEGF-A, extracellular matrix (ECM) production products, aggrecan and collagen II, and increased expression of the catabolic enzyme, MMP-13. Furthermore, aged PPARg KO mice exhibit an accelerated OA-like phenotype associated with enhanced cartilage degradation, synovial inflammation, and increased expression of MMP- 13, accompanied by increased staining for MMP-generated aggrecan and collagen II neoepitopes. Conclusions: For the first time, we demonstrate that loss of PPARg in the cartilage results in endochondral ossification defects and subsequently accelerated OA in mice. PPARg is essential for normal development of the cartilage.