Runx2 Regulates Endochondral Ossification Through Control of Chondrocyte Proliferation and Differentiation

Abstract

Synthesis of cartilage by chondrocytes is an obligatory step for endochondral ossification. Global deletion of the Runx2 gene results in complete failure of the ossification process, but the underlying cellular and molecular mechanisms are not fully known. Here, we elucidated Runx2 regulatory control distinctive to chondrocyte and cartilage tissue by generating Runx2 exon 8 floxed mice. Deletion of Runx2 in chondrocytes caused failure of endochondral ossification and lethality at birth. The limbs of Runx2ΔE8/ΔE8 mice were devoid of mature chondrocytes, vasculature, and marrow. We demonstrate that the C-terminus of Runx2 drives its biological activity. Importantly, nuclear import and DNA binding functions of Runx2 are insufficient for chondrogenesis. Molecular studies revealed that despite normal level of Sox9 and PTHrP, chondrocyte differentiation and cartilage growth is disrupted in Runx2ΔE8/ΔE8 mice. Loss of Runx2 in chondrocytes also impaired OPG-RANKL signaling and chondroclast development. Dwarfism observed in Runx2 mutants was associated with the near absence of proliferative zone in the growth plates. Finally, we show Runx2 directly regulates a unique set of cell cycle genes Gpr132, Sfn, c-Myb, and Cyclin A1 to control proliferative capacity of chondrocyte. Thus, Runx2 is obligatory for both proliferation and differentiation of chondrocytes.

MiR‐455‐3p regulates early chondrogenic differentiation via inhibiting Runx2

“…We also showed the regulatory role of miR-455-3p as an inhibitor of the expression of Runx2 during chondrogenic differentiation. Considering the important roles of Runx2 in the acceleration of hypertrophic chondrocyte differentiation and endochondral ossification [5,16,17,21,27], we concluded from these data that miR-455-3p potentially alters chondrogenic differentiation.…”

“…Runx2 is an important regulator in chondrogenic differentiation and endochondral ossification, and it was shown that Runx2 regulated endochondral ossification by controlling chondrocyte proliferation and differentiation [1,27,[32][33][34]. Runx2 was highly expressed in hypertrophic chondrocytes and osteoblasts [19,27,28], and C/EBPb could enhance Runx2 activity in ossification [21].…”

“…Runx2 was first shown to be a key transcription factor associated with osteoblast differentiation, but it also has an important role in the acceleration of hypertrophic chondrocyte differentiation and endochondral ossification [16][17][18][19]. Deletion of Runx2 in chondrocytes causes failure of endochondral ossification and lethality at birth, and it directly regulates a set of cell cycle genes Gpr132, Sfn, c-Myb, and Cyclin A1 to control the proliferative capacity of chondrocytes [20]. Runx2 also mediates chondrogenesis and ossification as a co-regulator with Smad, TGF-b, and CCAAT/enhancer binding protein beta (C/EBPb) [5,21].…”

Runx2 deletion in smooth muscle cells inhibits vascular osteochondrogenesis and calcification but not atherosclerotic lesion formation

“…In contrast to our studies, Sun et al 21 reported a role for SMC autonomous Runx2 in macrophage accumulation and atheromatous lesion formation using a very different Runx2 targeting construct. 41 Their construct targeted exon 8 of the Runx2 gene and produced a functional, truncated Runx2 protein upon Cre-mediated recombination of the floxed-Runx2 allele. 21,41 Importantly, this truncated protein contained an intact Runt DNA binding domain and domains interacting with Runx2 partner proteins, such as CBF and Smad3/4.…”

“…41 Their construct targeted exon 8 of the Runx2 gene and produced a functional, truncated Runx2 protein upon Cre-mediated recombination of the floxed-Runx2 allele. 21,41 Importantly, this truncated protein contained an intact Runt DNA binding domain and domains interacting with Runx2 partner proteins, such as CBF and Smad3/4. 21,42 Using SM22aCre to remove Runx2 in vascular SMCs of ApoE À/À mice with this model, the authors found significantly reduced SMC elaboration of RANKL, a tumour necrosis factor family member important for monocyte infiltration and differentiation of mineral-resorbing osteoclasts, 43,44 along with a near complete blockage of monocyte/macrophage recruitment and atherosclerosis.…”