Vascular Endothelial Growth Factor (VEGF) and Semaphorin 3A (Sema3A) signaling for vascularized bone grafts

One of the major challenges for the treatment of critical size bone defects is to ensure a rapid and efficient vascularization of tissue-engineered bone grafts upon implantation in vivo.
The biological processes of osteogenesis and angiogenesis are intimately coupled, and many factors play important roles in this cross-talk. Among them, Vascular Endothelial Growth Factor (VEGF), the master regulator of vascular growth, is crucial during bone development, homeostasis and repair, and it is a key molecular target for the generation of vascularized bone grafts. However, in order to exploit its therapeutic potential, VEGF dose and spatial-temporal distribution have to be precisely controlled.
Semaphorin 3A (Sema3A) regulates osteoblasts and osteoclasts to promote bone synthesis through the Neuropilin-1 receptor (NP-1) and it has important roles in angiogenesis. We previously found that VEGF dose-dependently inhibits endothelial Sema3A expression in skeletal muscle.
Here we investigated the role of VEGF and Sema3A in the coupling of angiogenesis and osteogenesis in engineered bone grafts in order to provide rational bases for novel, safe and effective therapeutic strategies for the repair of bone tissue
To this purpose, osteogenic constructs were prepared with human bone marrow mesenchymal cells (BMSCs) in combination with fibrin matrices decorated with recombinant VEGF or Sema3A engineered with a transglutaminase substrate sequence (TG-VEGF and TG-Sema3A) to allow cross-linking into fibrin hydrogels and controlled release.
We found that VEGF-dose dependently regulates both angiogenesis and osteogenesis. Low VEGF doses accelerated vascular invasion and ensured efficient bone depositio. High VEGF doses delayed vascular ingrowth, increased osteoclast recruitment and decreased bone formation by impairing the differentiation of the implanted human osteogenic progenitor cells. Moreover, we showed that VEGF-dose dependently downregulates Sema3A expression and that Sema3A is critical for both vascularization and intramembranous bone formation in osteogenic grafts.
These results confirm the importance of both VEGF and Sema3A in bone biology and provide the basis for the design of novel rational strategies to generate vascularized bone grafts with the aim to improve the healing of critical-size bone defects.