Role of Runx genes in chondrocyte differentiation

Runx2/Cbfa1 plays a central role in skeletal development as demonstrated by the absence of osteoblasts/bone in mice with inactivated Runx2/Cbfa1 alleles. To further investigate the role of Runx2 in cartilage differentiation and to assess the potential of Runx2 to induce bone formation, we cloned chicken Runx2 and overexpressed it in chick embryos using a retroviral system. Infected chick wings showed multiple phenotypes consisting of (1) joint fusions, (2) expansion of carpal elements, and (3) shortening of skeletal elements. In contrast, bone formation was not affected. To investigate the function of Runx2/Cbfa1 during cartilage development, we have generated transgenic mice that express a dominant negative form of Runx2 in cartilage. The selective inactivation of Runx2 in chondrocytes results in a severe shortening of the limbs due to a disturbance in chondrocyte differentiation, vascular invasion, osteoclast differentiation, and periosteal bone formation. Analysis of the growth plates in transgenic mice and in chick limbs shows that Runx2 is a positive regulator of chondrocyte differentiation and vascular invasion. The results further indicate that Runx2 promotes chondrogenesis either by maintaining or by initiating early chondrocyte differentiation. Furthermore, Runx2 is essential but not sufficient to induce osteoblast differentiation. To analyze the role of runx genes in skeletal development, we performed in situ hybridization with Runx2- and Runx3-specific probes. Both genes were coexpressed in cartilaginous condensations, indicating a cooperative role in the regulation of early chondrocyte differentiation and thus explaining the expansion/maintenance of cartilage in the carpus and joints of infected chick limbs.     

Col10a1 gene expression and chondrocyte hypertrophy during skeletal development and disease

The type X collagen gene, COLIOA1, is specifically expressed by hypertrophic chondrocytes during endochondral ossification. Endochondral ossification is a well-coordinated process that involves a cartilage intermediate and leads to formation of most of the skeleton in vertebrates during skeletogenesis. Chondrocyte hypertrophy is a critical stage of endochondral ossification linking both bone and cartilage development. Given its specific association with chondrocyte hypertrophy, type X collagen plays essential roles in endochondral ossification. It was previously shown that transgenic mice with mutant type X collagen develop variable skeleton-hematopoietic abnormalities indicating defective endochondral ossification, while mutations and abnormal expression of human COLIOA1 cause abnormal chondrocyte hypertrophy that has been seen in many skeletal disorders, including skeletal chondrodysplasia and osteoarthritis. In this review, we summarized the skeletal chondrodysplasia with COLIOA1 gene mutation that shows growth plate defect. We also reviewed recent studies that correlate the type X collagen gene expression and chondrocyte hypertrophy with osteoarthritis. Due to its significant clinical relevance, the type X collagen gene regulation has been extensively studied over the past two decades. Here, we focus on recent progress characterizing the cis-enhancer elements and their binding factors that together confer hypertrophic chondroeyte-specific murine type X collagen gene （CollOal） expression. Based on literature review and our own studies, we surmise that there are multiple factors that contribute to hypertrophic chondrocyte-specific CoHOal expression. These factors include both transactivators （such as Runx2, MEF2C etc.） and repressors （such as AP1, NFATcl, Sox9 etc.）, while other co-factors or epigenetic control of CollOal expression may not be excluded.