Osteoblast-derived VEGF regulates osteoblast differentiation and bone formation during bone repair
Kai Hu, Bjorn R. Olsen
Kai Hu, Bjorn R. Olsen
Published January 5, 2016
Citation Information: J Clin Invest. 2016;126(2):509-526. https://doi.org/10.1172/JCI82585.
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Research Article Bone biology

Osteoblast-derived VEGF regulates osteoblast differentiation and bone formation during bone repair

Abstract

Osteoblast-derived VEGF is important for bone development and postnatal bone homeostasis. Previous studies have demonstrated that VEGF affects bone repair and regeneration; however, the cellular mechanisms by which it works are not fully understood. In this study, we investigated the functions of osteoblast-derived VEGF in healing of a bone defect. The results indicate that osteoblast-derived VEGF plays critical roles at several stages in the repair process. Using transgenic mice with osteoblast-specific deletion of Vegfa, we demonstrated that VEGF promoted macrophage recruitment and angiogenic responses in the inflammation phase, and optimal levels of VEGF were required for coupling of angiogenesis and osteogenesis in areas where repair occurs by intramembranous ossification. VEGF likely functions as a paracrine factor in this process because deletion of Vegfr2 in osteoblastic lineage cells enhanced osteoblastic maturation and mineralization. Furthermore, osteoblast- and hypertrophic chondrocyte–derived VEGF stimulated recruitment of blood vessels and osteoclasts and promoted cartilage resorption at the repair site during the periosteal endochondral ossification stage. Finally, osteoblast-derived VEGF stimulated osteoclast formation in the final remodeling phase of the repair process. These findings provide a basis for clinical strategies to improve bone regeneration and treat defects in bone healing.

Authors

Kai Hu, Bjorn R. Olsen

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Figure 4

Postnatal deletion of Vegfa in osteoblast lineage cells impairs intramembranous bone formation in cortical defects.

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Postnatal deletion of Vegfa in osteoblast lineage cells impairs intramem...
(A) Reduced body size and weight in 8-week-old Vegfa CKO mice (18.4 ± 0.7 g) compared with Vegfafl/fl mice without doxycycline (DOX) treatment (23.4 ± 0.7 g); n = 8–10, P < 0.01. This reduction was eliminated in DOX-treated mice (20.8 ± 0.3 g and 21.7 ± 1.1 g in the 2 groups); n = 4–6. (B) Absence of Cre-activated ZsG in diaphysis (DIA) and metaphysis (META) of 8-week-old Vegfafl/fl Osx-Cre/ZsG mice continuously fed with DOX; ZsG induced in mice by DOX withdrawal at 4 weeks. GP, growth plate. Representative images from 3 mice for each genotype. (C) μCT analysis of mineralized bone formed in hole region of 9-week-old Vegfa CKO mice with DOX withdrawn at 1 week shows reduced BV as percentage of total volume (BV/TV, 5.2% ± 1.9%), reduced Tb.N (5.4/mm ± 0.7/mm), but increased Tb.Sp (0.21 ± 0.03 mm) compared with that of Vegfafl/fl mice (BV/TV, 19.7% ± 2.4%; Tb. N, 11.9/mm ± 1.2/mm; Tb. Sp, 0.09 ± 0.01 mm). No significant differences were seen in Tb.Th; n= 6–10 for each genotype. BV/TV and Tb.N, P < 0.01; Tb.Sp, P < 0.05. (D) Reduced density of aniline blue staining (19.5% ± 4.2%) and decreased mineralization/collagen ratio (15.6% ± 5.1%) in hole region of 9-week-old Vegfa CKO mice with DOX withdrawn at 1 week, compared with Vegfafl/fl mice (40.8% ± 1.9% and 43.6% ± 7.2%); n= 6–10 for each genotype. Aniline blue staining, P < 0.01; mineralization/collagen ratio, P < 0.05. Yellow stippled rectangle: hole region. Scale bars: 100 μm (B and C), 200 μm (D). Unpaired 2-tailed Student’s t test was used for comparisons in C and D.