Degradation of the NOTCH intracellular domain by elevated autophagy in osteoblasts promotes osteoblast differentiation and alleviates osteoporosis.

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).