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.

Osteoporosis constitutes a major worldwide public health burden characterized by enhanced skeletal fragility. Bone metabolism is the combination of bone resorption by osteoclasts and bone formation by osteoblasts. Whereas increase in bone resorption is considered as the main contributor of bone loss that may lead to osteoporosis, this loss is accompanied by increased bone marrow adiposity. Osteoblasts and adipocytes share the same precursor cell and an inverse relationship exists between the two lineages. Therefore, identifying signaling pathways that stimulate mesenchymal stem cells osteogenesis at the expense of adipogenesis is of major importance for developing new therapeutic treatments. For this purpose, we identified by transcriptomic analysis the oxytocin receptor pathway as a potential regulator of the osteoblast/adipocyte balance of human multipotent adipose-derived stem (hMADS) cells. Both oxytocin (OT) and carbetocin (a stable OT analogue) negatively modulate adipogenesis while promoting osteogenesis in both hMADS cells and human bone marrow mesenchymal stromal cells. Consistent with these observations, ovariectomized (OVX) mice and rats, which become osteoporotic and exhibit disequilibrium of this balance, have significant decreased OT levels compared to sham-operated controls. Subcutaneous OT injection reverses bone loss in OVX mice and reduces marrow adiposity. Clinically, plasma OT levels are significantly lower in postmenopausal women developing osteoporosis than in their healthy counterparts. Taken together, these results suggest that plasma OT levels represent a novel diagnostic marker for osteoporosis and that OT administration holds promise as a potential therapy for this disease.