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.

The cellular and molecular events triggering the anabolic response of the skeleton to exogenous parathyroid hormone (PTH) are not well understood. Despite the numerous bone mass studies in rats, few data are available for mice. Therefore, we treated 10-week-old female intact C57BL/6J mice with human PTH(1-34) delivered subcutaneously at a dose of 40 g/kg per day 5 days a week for 3 weeks and 7 weeks. Bone mineral density (BMD) of total bone, femur, tibia, and lumbar vertebrae was measured weekly by PIXImus. Bone turnover was examined by histomorphometry, and gene expression of bone formation and resorption markers and osteoclastogenesis regulators in the excised femur and tibia was assessed by reverse-transcription polymerase chain reaction (RT-PCR) at 3 weeks and 7 weeks. The PTH-stimulated increase in BMD was more prominent in the tibia and femur than in the lumbar vertebrae, with an anabolic effect detected within 1-2 weeks and BMD continuing to increase. The appearance of a detectable PTH-stimulated increase in BMD was slower in the lumbar vertebrae where the increase was only significant after 7 weeks of treatment. Histomorphometric analysis of the proximal tibia at both 3 weeks and 7 weeks indicated significant time-dependent increases in trabecular area, trabecular number, trabecular and cortical widths, and osteoblast and osteoid perimeters. In the lumbar vertebrae, these stimulatory effects of PTH on trabecular area, trabecular number, and cortical width were smaller and not detected until 7 weeks. PTH-stimulated increases in bone turnover were evident by increased gene expression of osteocalcin (

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