Chondrocyte-specific ablation of Osterix leads to impaired endochondral ossification

Abstract

Osterix (Osx) is an essential transcription factor required for osteoblast differentiation during both intramembranous and endochondral ossification. Endochondral ossification, a process in which bone formation initiates from a cartilage intermediate, is crucial for skeletal development and growth. Osx is expressed in differentiating chondrocytes as well as osteoblasts during mouse development, but its role in chondrocytes has not been well studied. Here, the in vivo function of Osx in chondrocytes was examined in a chondrocyte-specific Osx conditional knockout model using Col2a1-Cre. Chondrocyte-specific Osx deficiency resulted in a weak and bent skeleton which was evident in newborn by radiographic analysis and skeletal preparation. To further understand the skeletal deformity of the chondrocyte-specific Osx conditional knockout, histological analysis was performed on developing long bones during embryogenesis. Hypertrophic chondrocytes were expanded, the formation of bone trabeculae and marrow cavities was remarkably delayed, and subsequent skeletal growth was reduced. The expression of several chondrocyte differentiation markers was reduced, indicating the impairment of chondrocyte differentiation and endochondral ossification in the chondrocyte-specific Osx conditional knockout. Taken together, Osx regulates chondrocyte differentiation and bone growth in growth plate chondrocytes, suggesting an autonomous function of Osx in chondrocytes during endochondral ossification.

A vertebrate specific and essential role for sp7/osterix in osteogenesis revealed by gene knock-out in the teleost medaka

“…S9). Although the ectopic cartilage observed in mouse osx mutants (Oh et al, 2012) could be a consequence of delayed Osx-controlled perichondral ossification, we suggest that osx is dispensable for chondrocyte maturation in medaka.…”

“…Thus, in this respect, medaka osx mutants recapitulate bone formation and mineralization defects reported in mouse mutants (Nakashima et al, 2002), confirming that osx is a crucial player in bone formation in non-mammalian vertebrates. However, unlike in the mouse model (Oh et al, 2012), the formation of cartilage appeared to be normal in medaka osx mutants (Fig. S9).…”

Smpd3 Expression in both Chondrocytes and Osteoblasts Is Required for Normal Endochondral Bone Development

“…In our breeding experiments, we used Osx-Cre and Col2a1-Cre transgenic lines to either ablate Smpd3 in both late differentiation-stage chondrocytes and osteoblasts or just in chondrocytes, respectively. Although OSX was initially identified to be an osteoblastspecific transcription factor, more recent works have demonstrated that it is also expressed in the late-stage chondrocytes (14,32,33). The expression of OSX in the mineralizing cell types of the developing skeleton justifies the use of the Osx-Cre line in our breeding experiments that aimed to verify our original hypothesis that SMPD3 has a nonsystemic role in the skeletal tissues.…”

Ezh2 is required for neural crest-derived cartilage and bone formation

“…5G,H). Similarly, expression analysis of early markers of osteogenic and chondrogenic differentiation revealed that the transcription factors Runx2 and Osterix (Sp7 -Mouse Genome Informatics), as well as the osteoblast marker alkaline phosphatase (ALP) (Nishimura et al, 2012;Oh et al, 2012), were significantly downregulated in Ezh2 cko compared with control BA1, as shown by quantitative RT-PCR (Fig. 5I).…”