Chondrocyte-Specific Regulatory Activity of Runx2 Is Essential for Survival and Skeletal Development

Abstract

Coordinated activities of multiple mesenchymal cell types contribute to the development of the mammalian skeleton formed through endochondral ossification. Synthesis of a cartilage template by chondrocytes is an obligatory step for the generation of skeletal elements during endochondral ossification. Gene ablation studies have established that Runx2 is an essential transcription factor for bone formation and the differentiation of skeletal cells. However, global gene deletion has failed to discern the tissue- and cell type-specific roles of Runx2. We generated floxed mice to elucidate the Runx2 regulatory control distinctive to cartilage tissue during bone development. Exon 8 of the Runx2 gene was selectively deleted in developing chondrocytes by utilizing Col2a-Cre mice. Cell- and tissue-specific gene recombination was confirmed by β-gal activity in R26R mice. The chondrocyte-specific loss of Runx2 caused failure of endochondral ossification, impaired craniofacial development, dwarfism, and perinatal lethality. Radiographic imaging and histochemical approaches were used to characterize the skeletal phenotype. We conclude that regulatory control of Runx2 in chondrocytes is essential for endochondral ossification, and it is independent of the role of Runx2 in osteoblasts.

Conditioned Medium from Bone Marrow Mesenchymal Stem Cells Transiently Retards Osteoblast Differentiation by Downregulating Runx2

“…During osteoblast maturation, it is usually upregulated at first. Runx-2 deficiency or mutation causes severe bone abnormalities [Pratap et al, 2003;Chen et al, 2011]. In the present study, both the mRNA and protein level of Runx-2 were decreased by MSC-CM on day 3.…”

Runx2 activity in committed osteoblasts is not essential for embryonic skeletogenesis

“…In contrast, the C-terminus of Runx2 is distinct, and has been shown to participate in protein–protein interactions that synergistically induce the expression of osteoblast-specific genes (4). To understand the unique functions of Runx2, we generated Runx2-floxed mice by insertion of loxP sites around exon 8 of the Runx2 gene, which encodes over one-third of the Runx2 protein, including the C-terminus (5). Here, we employed mice expressing the 2.3kbCol1a1-Cre transgene to conditionally delete Runx2 C-terminus after osteoblast commitment.…”

“…Osteoblast-specific Runx2 knockout mice were established by employing a previously described Runx2-floxed line (5). Runx2-floxed mice were bred with 2.3kbCol1a1-Cre transgenic mice (11).…”

“…Mice with global deletion of Runx2 exhibit a complete absence of skeletal development, while those with chondrocyte-specific deletion lack endochondral ossification (5,6). Homozygous mutants in both models exhibit perinatal lethality.…”

“…We generated Runx2-floxed mice by inserting loxP sites around exon 8 of the Runx2 gene, which encodes the biologically active C-terminus of the Runx2 protein. We have previously shown that Cre-mediated recombination results in a truncated, non-functional Runx2 protein (5). Surprisingly, deletion of Runx2 from committed osteoblasts did not cause perinatal lethality, and mutant mice were indistinguishable from wild-type littermates at birth.…”