Runx2 activity in committed osteoblasts is not essential for embryonic skeletogenesis

Abstract

Runx2 transcription factor is essential for the development of mineralized tissue, and is required for osteoblast commitment and chondrocyte maturation. Mice with global deletion of Runx2 exhibit complete failure of bone tissue formation, while chondrocyte-specific Runx2-deficient mice lack endochondral ossification. However, the function of Runx2 after commitment of mesenchymal cells to the osteoblast lineage remains unknown. Here, we elucidate the osteoblast-specific requirements of Runx2 during development of the tissue. Runx2 was deleted in committed osteoblasts using Cre-recombinase driven by the 2.3kbCol1a1 promoter. Surprisingly, Runx2ΔE8/ΔE8 mice were born alive and were essentially indistinguishable from wild-type littermates. At birth, we failed to detect any alterations in skeletal patterning or extent of bone development in homozygous mutants. However, by 4 weeks of age, mutant mice showed obvious growth deficiencies, and weighed 20–25% less than sex-matched wild-type littermates. Micro-CT analysis of the hindlimb revealed a dramatic decrease of 50% in both cortical and trabecular bone volume compared with wild-type mice. Consistent with this observation, trabecular number and thickness were decreased by 51% and 21%, respectively, and trabecular space was increased by 2-fold in limbs of Runx2ΔE8/ΔE8 mice. In addition to poor acquisition of bone mass, the average density of hydroxyapatite was markedly decreased in bone of Runx2ΔE8/ΔE8 mice. Together, these findings demonstrate that loss of Runx2 activity in committed osteoblasts impairs osteoblast function, and that Runx2 is critical for postnatal, but not embryonic endochondral ossification.

Loss of Runx2 in Committed Osteoblasts Impairs Postnatal Skeletogenesis

“…Specific deletion of Runx2 in chondrocytes results in failed endochondral ossification and perinatal lethality owing to respiratory failure . However, mice with osteoblast‐specific Runx2 deficiency are viable with grossly normal development at birth . These results suggest that Runx2 exerts different regulatory functions in osteoblasts and chondrocytes during embryonic skeletogenesis.…”

“…Deletion of exon 8 results in a mutant Runx2 protein (Δ369) that lacks 159 amino acids at the C‐terminus. Notably, the Δ369 mutant does not act as a dominant negative Runx2 protein and yields a null phenotype . Most of the N‐terminal domains, including the Q/A (poly‐glutamic‐alanine), RHD (DNA binding domain), the NLS (nuclear localization signal), and part of the transcriptional activation/suppression domain are preserved in the Δ369 protein.…”

Substrate Stiffness Controls Osteoblastic and Chondrocytic Differentiation of Mesenchymal Stem Cells without Exogenous Stimuli

“…Whether this reflects in vivo differentiation on osteoclast resorbed bone surfaces, which are primarily collagen and non-collagenous proteins like osteopontin rather than stiffer fully mineralized bone [41,42], isn’t clear. The lower RUNX2 in HOBs on certain substrates suggests that the cells are less active osteoblasts [43,44] than on other substrates.…”

MiR-30a inhibits osteolysis by targeting RunX2 in giant cell tumor of bone

“…RunX2 is considered to regulate osteoblast and osteolysis activities in normal tissues and GCT [17,18]. As we have proved miR30a to be a direct regulator for RunX2, we investigated the role of miR-30a during the bone resorption in GCT.…”