The pivotal role of the cellular and molecular crosstalk in bone regeneration

Bone is a unique tissue with particular properties and features such as an abundant and heterogenous vasculature, which is divided in two subtypes (type H and type L vessels) and a high regeneration capability. As expected, angiogenesis is intimately linked to bone regeneration and the cellular and molecular crosstalk between endothelial cells (ECs) and bone forming cells is the main mechanism that guarantees that functional coupling. Such cell-to-cell interaction is principally characterized by the release of paracrine signals, which nature has been widely investigated both in vitro (2D or 3D co-culture systems) and in vivo (mice models). The aim of my work has been deepening the osteo-regenerative crosstalk features and how to apply them in clinics. The most important factors involved in this interplay are angiogenic, morphogenetic and pro-osteogenic molecules such as Noggin, SLIT3, HIF-1a and VEGF. Type H endothelium, although it represents a minimal percentage equal to 1.77% of total vasculature, provides a bone specialized microvascular niche that promotes both angiogenesis and osteogenesis. Therefore, type H ECs are directly involved in the regenerative crosstalk with bone cells. Pro-osteogenic type H microvascular niches are positively regulated by Notch signals. Notch activation leads not only to an upregulation of VEGF, a key player both in angiogenesis and bone regeneration, but in particular to an increased release of Noggin, a BMP antagonist. Noggin is secreted by type H ECs and acts directly on osteoblast-lineage cells affecting osteo-differentiation and bone cells maturation. On the other hand, osteoblasts regulate skeletal osteogenic endothelium by secreting a soluble factor called SLIT3 that binds the membrane receptor ROBO1 on type H ECs and activates pro-angiogenic signalling pathways: the consequent effect is a significant increase of bone formation rate. Targeting Notch, SLIT3 and VEGF, as they are the protagonists of this crosstalk, could be a potential and promising therapeutical approach in bone regeneration; indeed an in vivo delivery of SLIT3 and VEGF showed positive effects both in promoting new blood vessel development and bone formation. It has been demonstrated that SLIT3 treatment is able to enhance bone fracture healing and to reverse osteoporotic phenotype in murine models. Whereas VEGF delivery is more challenging since it has showed contrasting effects depending on the treatment strategy used. A further interesting application of the cellular crosstalk properties could be the bone tissue engineering (BTE); the purpose is the development of a potentially implantable bone substitute after a proper choice of a biocompatible scaffold. BTE current strategies focus attention on how to overcome an inadequate blood supply in the tissue-engineered constructs. Therefore, a scaffold able to allow the pro-angiogenic and pro-osteogenic cellular dialogue may have a key role in bone regeneration in the future.