Canonical Notch activation in osteocytes causes osteopetrosis

Abstract

Activation of Notch1 in cells of the osteoblastic lineage inhibits osteoblast differentiation/function and causes osteopenia, whereas its activation in osteocytes causes a distinct osteopetrotic phenotype. To explore mechanisms responsible, we established the contributions of canonical Notch signaling (Rbpjκ dependent) to osteocyte function. Transgenics expressing Cre recombinase under the control of the dentin matrix protein-1 ( Dmp1) promoter were crossed with Rbpjκ conditional mice to generate Dmp1-Cre +/−; Rbpjκ Δ/Δ mice. These mice did not have a skeletal phenotype, indicating that Rbpjκ is dispensable for osteocyte function. To study the Rbpjκ contribution to Notch activation, Rosa Notch mice, where a loxP-flanked STOP cassette is placed between the Rosa26 promoter and the NICD coding sequence, were crossed with Dmp1-Cre transgenic mice and studied in the context ( Dmp1-Cre +/−; Rosa Notch; Rbpjκ Δ/Δ) or not ( Dmp1-Cre +/−; Rosa Notch) of Rbpjκ inactivation. Dmp1-Cre +/−; Rosa Notch mice exhibited increased femoral trabecular bone volume and decreased osteoclasts and bone resorption. The phenotype was reversed in the context of the Rbpjκ inactivation, demonstrating that Notch canonical signaling was accountable for the phenotype. Notch activation downregulated Sost and Dkk1 and upregulated Axin2, Tnfrsf11b, and Tnfsf11 mRNA expression, and these effects were not observed in the context of the Rbpjκ inactivation. In conclusion, Notch activation in osteocytes suppresses bone resorption and increases bone volume by utilization of canonical signals that also result in the inhibition of Sost and Dkk1 and upregulation of Wnt signaling.

Notch in skeletal physiology and disease

“…Notch1 activation in osteocytes suppresses the Wnt antagonists sclerostin and dickkopf 1 (Dkk1); through this indirect mechanism Notch increases Wnt/β-catenin signaling [6, 18]. The induction of Wnt signaling seems paradoxical to the direct inhibition of Wnt signaling by Notch in osteoblasts.…”

TNF and Bone Remodeling

“…Consequently, Notch1-deficiency promotes osteoclastogenesis indirectly by enhancing the ability of osteoblast lineage cells to increase RANKL/OPG expression ratio [43, 51]. Notch also markedly suppresses the expression of Wnt antagonists, sclerostin, and DKK1, leading to an enhanced activation of Wnt/β-catenin signaling and increased osteoblastogenesis and bone formation [50, 51]. Additionally, the activated Wnt/β-catenin suppresses osteoclastogenesis and bone resorption by direct action on osteoclast precursors and also indirectly by increasing OPG expression [52–54].…”

Parathyroid hormone inhibits Notch signaling in osteoblasts and osteocytes

“…In contrast to the bone anabolic effects of PTH, induction of Notch1 in mesenchymal precursor cells and mature osteoblasts suppresses cell differentiation and trabecular bone formation, and Notch2 exerts similar inhibitory effects on osteoblast function [21–26]. Conversely, activation of Notch1 in osteocytes induces osteoprotegerin and suppresses sclerostin, leading to an osteopetrotic phenotype [23, 27]. …”