SP7 Inhibits Osteoblast Differentiation at a Late Stage in Mice

Abstract

RUNX2 and SP7 are essential transcription factors for osteoblast differentiation at an early stage. Although RUNX2 inhibits osteoblast differentiation at a late stage, the function of SP7 at the late stage of osteoblast differentiation is not fully elucidated. Thus, we pursued the function of SP7 in osteoblast differentiation. RUNX2 induced Sp7 expression in Runx2 −/− calvarial cells. Adenoviral transfer of sh-Sp7 into primary osteoblasts reduced the expression of Alpl, Col1a1, and Bglap2 and mineralization, whereas that of Sp7 reduced Bglap2 expression and mineralization at a late stage of osteoblast differentiation. Sp7 transgenic mice under the control of 2.3 kb Col1a1 promoter showed osteopenia and woven-bone like structure in the cortical bone, which was thin and less mineralized, in a dose-dependent manner. Further, the number of processes in the osteoblasts and osteocytes was reduced. Although the osteoblast density was increased, the bone formation was reduced. The frequency of BrdU incorporation was increased in the osteoblastic cells, while the expression of Col1a1, Spp1, Ibsp, and Bglap2 was reduced. Further, the osteopenia in Sp7 or Runx2 transgenic mice was worsened in Sp7/Runx2 double transgenic mice and the expression of Col1a1 and Bglap2 was reduced. The expression of Sp7 and Runx2 was not increased in Runx2 and Sp7 transgenic mice, respectively. The expression of endogenous Sp7 was increased in Sp7 transgenic mice and Sp7-transduced cells; the introduction of Sp7 activated and sh-Sp7 inhibited Sp7 promoter; and ChIP assay showed the binding of endogenous SP7 in the proximal region of Sp7 promoter. These findings suggest that SP7 and RUNX2 inhibit osteoblast differentiation at a late stage in a manner independent of RUNX2 and SP7, respectively, and SP7 positively regulates its own promoter.

Constitutive Activation of Smoothened Leads to Impaired Developments of Postnatal Bone in Mice

“…Consequently, it is likely that suppression of osterix is associated with a reduction in postnatal development in mutant mice. By contrast, it was recently suggested that osterix and Runx2 inhibit osteoblast differentiation at a late stage in an independent manner of Runx2 and osterix, respectively, where osterix positively regulates its own promoter (Yoshida et al, 2012). In this study, we showed that unlike Runx2, the mRNA expression of osterix in mutant mice was quite dynamic and thus its level was higher in the mutant than control mice at 2 weeks of age.…”

“…Ihh expression was also found to up-regulate the function of Runx2 through Gli2 (Shimoyama et al, 2007). In addition, ectopic Ihh expression in chondrocytes mediates Runx2 induction throughout the perichondrium, although Ihh alone is not sufficient to induce bone formation (Komori, 2012). Moreover, sonic hedgehog (Shh) is the most widely characterized of three analogous proteins in mammals and is essential for proper development in both embryonic and adult stages.…”

Roles of chemokine receptor CX3CR1 in maintaining murine bone homeostasis through the regulation of both osteoblasts and osteoclasts

“…Runx2 and Osterix encode essential transcription factors for osteoblast commitment and differentiation at an early stage (Kanatani et al, 2006;Komori et al, 1997;Liu et al, 2001;Nakashima et al, 2002). Recently, Yoshida et al reported that sustained expression of Osterix and Runx2 inhibited osteoblast differentiation from proceeding to a late stage in a reciprocally independent manner (Yoshida et al, 2012). Therefore, it is possible that the inverse temporal downregulation and upregulation of transcription factors such as Runx2 and Osterix, as was observed in vitro (Fig.…”

MicroRNAs and post-transcriptional regulation of skeletal development

“…The expression of Osx begins at around day E13.5 and it promotes the expression of osteoblast markers such as Alpl, Ibsp, and Bglap. The expression of Osx has been shown to be positively regulated by BMP, insulin-like growth factor 1 (IGF1), and MAPK signaling pathways in undifferentiated MSCs , Ortuno et al 2010, and it can also regulate its own expression by interacting with its own promoter (Yoshida et al 2012).…”