Bone remodeling is a tightly controlled mechanism in which osteoblasts (OB), the cells responsible for bone formation, osteoclasts (OC), the cells specialized for bone resorption, and osteocytes, the multifunctional mechanosensing cells embedded in the bone matrix, are the main actors. Increased oxidative stress in OB, the cells producing and mineralizing bone matrix, has been associated with osteoporosis development but the role of autophagy in OB has not yet been addressed. This is the goal of the present study. We first show that the autophagic process is induced in OB during mineralization. Then, using knockdown of autophagy-essential genes and OB-specific autophagy-deficient mice, we demonstrate that autophagy deficiency reduces mineralization capacity. Moreover, our data suggest that autophagic vacuoles could be used as vehicles in OB to secrete apatite crystals. In addition, autophagy-deficient OB exhibit increased oxidative stress and secretion of the receptor activator of NFKB1 (TNFSF11/RANKL), favoring generation of OC, the cells specialized in bone resorption. In vivo, we observed a 50% reduction in trabecular bone mass in OB-specific autophagy-deficient mice. Taken together, our results show for the first time that autophagy in OB is involved both in the mineralization process and in bone homeostasis. These findings are of importance for mineralized tissues which extend from corals to vertebrates and uncover new therapeutic targets for calcified tissue-related metabolic pathologies.

Rationale:Integrins play a crucial role in controlling endothelial cell proliferation and migration during angiogenesis. The Delta-like 4 (Dll4)/Notch pathway establishes an adequate ratio between stalk and tip cell populations by restricting tip cell formation through "lateral inhibition" in response to a vascular endothelial growth factor gradient. Because angiogenesis requires a tight coordination of these cellular processes, we hypothesized that adhesion, vascular endothelial growth factor, and Notch signaling pathways are interconnected.Objective: This study was aimed at characterizing the cross-talk between integrin and Notch signaling in endothelial cells. Methods and Results: