Possible Roles of Runx1 and Sox9 in Incipient Intramembranous Ossification

Abstract

We evaluated the detailed expression patterns of Runx1 and Sox9 in various types of bone formation, and determined whether Runx1 expression was affected by Runx2 deficiency and Runx2 expression by Runx1 deficiency. Our results indicate that both Runx1 and Sox9 are intensely expressed in the future osteogenic cell compartment and in cartilage. The pattern of Runx1 and Sox9 expression suggests that both genes could potentially be involved in incipient intramembranous bone formation during craniofacial development.Introduction: Runx1, a gene essential for hematopoiesis, contains RUNX binding sites in its promoter region, suggesting possible cross-regulation with Runx2 and potential regulatory roles in bone development. On the other hand, Sox9 is essential for chondrogenesis, and haploinsufficiency of Sox9 leads to premature ossification of the skeletal system. In this study, we studied the possible roles of Runx1 and Sox9 in bone development. Materials and Methods: Runx1, Runx2/Osf2, and Sox9 expression was evaluated by in situ hybridization in the growing craniofacial bones of embryonic day (E)12-16 mice and in the endochondral bone-forming regions of embryonic and postnatal long bones. In addition, we evaluated Runx2/Osf2 expression in the growing face of Runx1 knockout mice at E12.5 and Runx1 expression in Runx2 knockout mice at E14.5. Results: Runx1 and Sox9 were expressed in cartilage, and the regions of expression expanded to the neighboring Runx2-expressing osteogenic regions. Expression of both Runx1 and Sox9 was markedly downregulated on ossification. Runx1 and Sox9 expression was absent in the regions of endochondral bone formation and in actively modeling or remodeling bone tissues in the long bones as well as in ossified craniofacial bones. Runx2 expression was not affected by gene disruption of Runx1, whereas the expression domains of Runx1 were extended in Runx2

Dominance of SOX9 function over RUNX2 during skeletogenesis

“…6d). RUNX3 has been shown to play positive role during chondrocyte hypertrophy along with RUNX2 whereas RUNX1 has been postulated to mediate early events during mesenchymal condensations (12,23,24). At least in the case of RUNX2, autoregulation of its expression has been reported (14).…”

The role of mechanical signals in regulating chondrogenesis and osteogenesis of mesenchymal stem cells

“…Runx2 is a transcription factor essential for osteoblast differentiation and bone development (Ducy et al, 1997;Komori et al, 1997;Otto et al, 1997). In the early mesenchymal condensations stem cells express both of these transcription factors, indicating the potential to differentiate along either the osteogenic or chondrogenic lineage (Ducy et al, 1997;Otto et al, 1997;Bi et al, 1999;Yamashiro et al, 2004). However, since genetic inactivation of these transcription factors leads to loss of one cell phenotype without commitment to another, it suggests that other factors must act upstream of Runx2 and Sox9 in controlling osteogenesis and chondrogenesis (Day et al, 2005).…”

Regulation of skeletogenic differentiation in cranial dermal bone

“…Each of these steps is characterized by the expression of discrete sets of molecular markers. All chondrocytes of the trunk skeleton express the Coll II (also known as Col2a1 and Col II) gene, encoding collagen type II, and the transcription factor Sox9 (Kronenberg, 2003;Harada and Roden, 2003;Zelzer and Olsen, 2003;Yamashiro et al, 2004;Aberg et al, 2005;Shibata et al, 2006). As the skeletal elements mature and growth plates form towards their distal ends, these two genes continue to be expressed at high levels in both proliferating and resting chondrocytes, whereas Sox9 is downregulated during the transition to the hypertrophic chondrocytes.…”

“…RUNX2 also plays a more limited role in chondrocyte hypertrophy during long-bone development (Inada et al, 1999;Kim et al, 1999). The early chondrogenic markers Coll II and Sox9, discussed above, are also expressed in early osteogenic precursors, but are never found in differentiated osteoblasts (Yamashiro et al, 2004;Aberg et al, 2005).…”