Physiological Notch Signaling Maintains Bone Homeostasis via RBPjk and Hey Upstream of NFATc1

Abstract

Notch signaling between neighboring cells controls many cell fate decisions in metazoans both during embryogenesis and in postnatal life. Previously, we uncovered a critical role for physiological Notch signaling in suppressing osteoblast differentiation in vivo. However, the contribution of individual Notch receptors and the downstream signaling mechanism have not been elucidated. Here we report that removal of Notch2, but not Notch1, from the embryonic limb mesenchyme markedly increased trabecular bone mass in adolescent mice. Deletion of the transcription factor RBPjk, a mediator of all canonical Notch signaling, in the mesenchymal progenitors but not the more mature osteoblast-lineage cells, caused a dramatic high-bone-mass phenotype characterized by increased osteoblast numbers, diminished bone marrow mesenchymal progenitor pool, and rapid age-dependent bone loss. Moreover, mice deficient in Hey1 and HeyL, two target genes of Notch-RBPjk signaling, exhibited high bone mass. Interestingly, Hey1 bound to and suppressed the NFATc1 promoter, and RBPjk deletion increased NFATc1 expression in bone. Finally, pharmacological inhibition of NFAT alleviated the high-bone-mass phenotype caused by RBPjk deletion. Thus, Notch-RBPjk signaling functions in part through Hey1-mediated inhibition of NFATc1 to suppress osteoblastogenesis, contributing to bone homeostasis in vivo.

Osteocytes mediate the anabolic actions of canonical Wnt/β-catenin signaling in bone

“…The expression of the ligands Jag1 and 2 and Dll4, and the receptors Notch 1, 2, and 4 is also elevated in daßcat Ot mice, raising the possibility that osteocytic β-catenin initiates a chain of Notch activation throughout the osteocyte network, by which signals are amplified in an autocrine/paracrine fashion. Earlier studies demonstrated that the effects of Notch signaling in the skeleton are complex and, similar to Wnt signaling, are cell-context specific and dependent on the stage of osteoblastic cell differentiation (41)(42)(43)(44). In particular, activation of Notch signaling in osteocytes has a strong impact on bone homeostasis, by mechanisms still unclear and which appear to be different in cancellous versus cortical bone (45).…”

“…Plasma was obtained from blood from the facial vein of mice after 3-h fasting at the indicated age. P1NP and CTX were measured by using ELISA kits (Biomedical Technologies and Immunodiagnostic Systems), respectively (41,42,49). Alkaline phosphatase was measured on a Randox Daytona analyzer (Randox Laboratories Limited) at the General Clinical Research Center of Indiana University School of Medicine.…”

Notch1 maintains dormancy of olfactory horizontal basal cells, a reserve neural stem cell

“…For example, the consequences of Notch1 knockout are more severe during gestation than those of Notch2 (57,58). In specific terms, Notch2 but not Notch1 is responsible for inhibiting endochondral bone formation during limb development (59). Similarly, Notch2 has been shown to play a key role in the establishment and survival of the Sus cell population of the OE (60), but Notch1 does not.…”

NFAT Signaling and Bone Homeostasis

“…Twist, as a negative regulator of osteoblast, binds to RUNX2 and inhibits its ability to activate target genes. Notch and CCL8 and many other proteins not described here also regulate osteoblasts [80][81][82].…”

“…Notch signaling inhibits osteoblasts, and the deletion of transcription factor RBPjk, a mediator of all canonical Notch signaling, and the deletion of Hey1 and HeyL, two target genes of RBPjk, caused high bone formation [80]. Hey1 bond to the promoter of NFATc1 and suppressed its expression.…”