Expression of VEGF and the VEGF receptor neuropilin-1 during the endochondral ossification

Although angiogenesis and osteoblast migration into the cartilage are crucial components of endochondral ossification, the process where cartilage is replaced by bone, little is known about how they are regulated. This thesis provides evidence that vascular endothelial growth factor (VEGF), a well-characterized angiogenic factor and stimulator of osteoblast activity, and neuropilin-1 (NRP1), characterized by our laboratory as a VEGF receptor (VEGFR), are involved in endochondral ossification. Chondrocytes in culture express VEGF, and VEGF expression is up-regulated as these chondrocytes mature and undergo hypertrophy. In the growth plate in vivo, VEGF is most abundantly synthesized by late maturing and hypertrophic chondrocytes adjacent to the metaphysis, where vascularization of the growth plate occurs. In addition, cultured osteoblasts; bind VEGF in an isoform specific manner (binding VEGF165 but not VEGF121) that is characteristic of the VEGFR NRP1. I report the novel finding that osteoblasts bind VEGF through NRP1, as they do not express VEGFR-1 or VEGFR-2. In the developing bone, NRP1 is expressed by osteoblasts in regions of active bone deposition. NRP1 is present on osteoblasts migrating adjacent to metaphyseal blood vessels invading the growth plate, and also on osteoblasts lining the developing edge of trabeculae. Endothelial cells (EC) of blood vessels in the metaphysis and other areas of the bone, also express NRP1, as do osteoclasts; in the metaphysis, and chondrocytes within the growth plate.;The in vivo expression patterns of VEGF and NRP1 suggest that VEGF/NRP1 interactions may play a multiple role during endochondral ossification. VEGF secreted by hypertrophic chondrocytes could bind to the VEGFRs NRP1 and VEGFR-2 expressed on EC of blood vessels and mediate angiogenesis into the growth plate. Chondrocyte expressed VEGF may also bind to NRP1 on osteoblasts, and NRP1 and VEGFR-1on osteoclasts, inducing migration of these cells into the growth plate to replace the cartilage with bone.