Bone Abstracts

Sp7 and Runx2 are essential transcription factors for osteoblast differentiation at an early stage, and Runx2 inhibits osteoblast maturation at a late stage. However, the functions of Sp7 in bone development are not fully elucidated. Thus, we pursued the functions of Sp7 in bone development by generating osteoblast-specific Sp7 transgenic (tg) mice using 2.3 kb Col1a1 promoter. Sp7 tg mice showed osteopenia and woven-bone like structure in the cortical bone, which was thin and less mineralized. 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 tg mice was worsened in Sp7/Runx2 double tg mice and the expression of Col1a1 and Bglap2 was reduced. Histological analysis showed that blood vessels were increased in the cortical bone, the numbers of canaliculi and osteocyte processes were reduced, and the osteocytes were accumulated around the blood vessels in Sp7 tg mice. Immunohistochemical analysis showed an increase in CD34-positive vascular endothelial cells, and micro-CT analysis showed an increase of vascular volume in the cortical bone. In contrast, the introduction of sh-Sp7 into WT metatarsal bones suppressed the angiogenesis in the cortical bones in organ cultures. Immunohistochemical and western blot analyses showed that vascular endothelial growth factor (VEGF) A expression was increased in the osteoblasts in Sp7 tg mice. Immunohistochemical analysis using WT bone sections showed that osteoblasts, which were positive for endogenous Sp7, were also detected by VEGF antibody. These findings indicated that Sp7 inhibits osteoblast maturation at a late stage, and suggested that Sp7 regulates angiogenesis in bone through the regulation of VEGF.