VEGF-Dependent Mechanisms Controlling Osteoblast Differentiation and Bone Formation During Bone Repair

Abstract

Osteoblast-derived vascular endothelial growth factor (VEGF) is important for bone development and postnatal bone homeostasis. Several 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 cortical bone defect. In addition, how VEGF signaling modulates BMP2 functions during bone healing was also examined. To define the roles of osteoblast-derived VEGF in bone repair, a mouse tibial monocortical defect model was used. The effects of deleting Vegfa or Vefgr2 in osteoblast precursors and their descendants on the bone repair process were analyzed at various time points after surgery. To study how VEGF modulates the osteogenic activity of BMP2, BMP2, with or without the soluble VEGFR (sFlt1, VEGF decoy receptor), was delivered to the cortical defects in VE-cadherin-cre;tdTomato mice. The results indicate that osteoblast-derived VEGF is important at various stages during healing of the cortical defect. In the inflammation phase, osteoblast-derived VEGF controls neutrophil release into the circulation and macrophage-related angiogenic responses. VEGF is required, at optimal levels, for angiogenesis-osteogenesis coupling in areas where repair occurs by intramembranous ossification (IO). In this role, VEGF likely functions as a paracrine factor since deletion of Vegfr2 in osteoblast precursors and their progeny enhances osteoblastic maturation and mineralization. Furthermore, osteoblast- and hypertrophic chondrocyte-derived VEGF stimulates recruitment of blood vessels and osteoclasts, and promotes cartilage resorption at the repair site during the periosteal endochondral ossification stage. Finally, osteoblast-derived VEGF stimulates osteoclast formation in the final remodeling phase of the repair process. Our data also indicate that skeletal stem cells at different locations respond differently to BMP2, and that the osteogenic activity of BMP2 is modulated by extracellular VEGF. In the cortical defect, delivery of recombinant BMP2 inhibits intramembranous bone formation in the intramedullary space while it enhances endochondral bone formation in the injured periosteum. Inhibition of extracellular VEGF by sFlt1 reverses the inhibitory effects of BMP2 on intramembranous ossification-mediated bone repair. These findings add to the understanding of VEGF functions and provide a basis for clinical strategies to improve bone regeneration and treat cases of compromised bone healing.