Abstract：
Maintenance of bone integrity is mediated by the balanced actions of osteoblasts and osteoclasts. Because macroautophagy/autophagy regulates osteoblast mineralization, osteoclast differentiation, and their secretion from osteoclast cells, autophagy deficiency in osteoblasts or osteoclasts can disrupt this balance. However, it remains unclear whether upregulation of autophagy becomes beneficial for suppression of bone-associated diseases. In this study, we found that genetic upregulation of autophagy in osteoblasts facilitated bone formation. We generated mice in which autophagy was specifically upregulated in osteoblasts by deleting the gene encoding RUBCN/Rubicon, a negative regulator of autophagy. The rubcnflox/flox;Sp7/Osterix-Cre mice showed progressive skeletal abnormalities in femur bones. Consistent with this, RUBCN deficiency in osteoblasts resulted in elevated differentiation and mineralization, as well as an increase in the elevated expression of key transcription factors involved in osteoblast function such as Runx2 and Bglap/Osteocalcin. Furthermore, RUBCN deficiency in osteoblasts accelerated autophagic degradation of NOTCH intracellular domain (NICD) and downregulated the NOTCH signaling pathway, which negatively regulates osteoblast differentiation. Notably, osteoblast-specific deletion of RUBCN alleviated the phenotype in a mouse model of osteoporosis. We conclude that RUBCN is a key regulator of bone homeostasis. On the basis of these findings, we propose that medications targeting RUBCN or autophagic degradation of NICD could be used to treat age-related osteoporosis and bone fracture.Abbreviations: ALPL: alkaline phosphatase, liver/bone/kidney; BCIP/NBT: 5-bromo-4-chloro-3\'-indolyl phosphate/nitro blue tetrazolium; BMD: bone mineral density; BV/TV: bone volume/total bone volume; MAP1LC3/LC3: microtubule-associated protein 1 light chain 3; MTOR: mechanistic target of rapamycin kinase; NICD: NOTCH intracellular domain; RB1CC1/FIP200: RB1-inducible coiled-coil 1; RUBCN/Rubicon: RUN domain and cysteine-rich domain containing, Beclin 1-interacting protein; SERM: selective estrogen receptor modulator; TNFRSF11B/OCIF: tumor necrosis factor receptor superfamily, member 11b (osteoprotegerin).
摘要：
骨完整性的维持由成骨细胞和破骨细胞的平衡作用介导。因为巨自噬/自噬调节成骨细胞矿化，破骨细胞分化，以及它们从破骨细胞中分泌出来的，成骨细胞或破骨细胞中的自噬缺陷会破坏这种平衡。然而,目前尚不清楚自噬的上调是否有利于抑制骨相关疾病.在这项研究中,我们发现成骨细胞自噬的遗传上调促进了骨形成。我们通过删除编码RUBCN/Rubicon的基因，产生了自噬在成骨细胞中特异性上调的小鼠，自噬的负调节因子。rubcnflox/flox;Sp7/Osterix-Cre小鼠在股骨中显示出进行性骨骼异常。与此一致，成骨细胞RUBCN缺乏导致分化和矿化升高，以及参与成骨细胞功能的关键转录因子如Runx2和Bglap/Osteocalcin的升高表达增加。此外，成骨细胞RUBCN缺乏加速NOTCH胞内结构域(NICD)的自噬降解,下调NOTCH信号通路,负调节成骨细胞分化。值得注意的是,RUBCN的成骨细胞特异性缺失减轻了骨质疏松症小鼠模型的表型。我们得出结论，RUBCN是骨稳态的关键调节剂。根据这些发现,我们建议针对RUBCN或NICD自噬降解的药物可用于治疗年龄相关性骨质疏松症和骨折.缩写：ALPL：碱性磷酸酶，肝/骨/肾；BCIP/NBT：5-溴-4-氯-3\'-吲哚基磷酸/硝基蓝四唑；BMD：骨矿物质密度；BV/TV：骨体积/总骨体积；MAP1LC3/LC3：微管相关蛋白1轻链3；MTOR：雷帕霉素激酶的机制靶标；NICD：富含CCRINB1-半胱氨酸结构域：含有TCRNOH/RBeclin1-相互作用蛋白;SERM:选择性雌激素受体调节剂;TNFRSF11B/OCIF:肿瘤坏死因子受体超家族,11b成员（骨保护素）。
