Dimorphic effects of Notch signaling in bone homeostasis

Abstract

Notch signaling is a central mechanism for controlling embryogenesis. However, its in vivo function during mesenchymal cell differentiation, and specifically, in bone homeostasis remains largely unknown. Here, we show that osteoblast-specific gain of Notch function causes severe osteosclerosis due to increased proliferation of immature osteoblasts. Under these pathological conditions, Notch stimulates early osteoblastic proliferation by up-regulating Cyclin D, Cyclin E, and Osterix. Notch also regulates terminal osteoblastic differentiation by directly binding Runx2 and repressing its transactivation function. In contrast, loss of all physiologic Notch signaling in osteoblasts, generated by deletion of Presenilin 1 and 2 in bone, is associated with late onset, age-related osteoporosis resulting from increased osteoblast-dependent osteoclastic activity due to decreased production of Osteoprotegerin. Together, these findings highlight the potential dimorphic effects of Notch signaling in bone homeostasis and may provide direction for novel therapeutic applications.Evolutionarily conserved Notch signaling plays a critical role in cell fate determination, and various developmental processes by translating cell-cell interactions into specific transcriptional programs 1, 2 . Temporal and spatial modulation of this pathway can significantly affect proliferation, differentiation and apoptotic events 3 . Moreover, the timing of Notch signaling can lead to diverse effects within the same cell lineage 4, 5 . In mammals, activation of up to four Notch receptors by membrane-bound ligands initiates a process leading to presenilin-mediated cleavage and release of the Notch intracellular domain (NICD) from the membrane that then traffics to the nucleus. NICD subsequently regulates the expression of genes in cooperation with the transcription factor RBP-Jκ and Mastermind-like proteins.The observation that mutations in the Notch ligand Delta homologue-3 (Dll-3) and γ-secretase Presenilin1 both cause axial skeletal phenotypes originally linked Notch signaling with skeletal development 6, 7 . Recently, several in vitro studies with conflicting results implicated the Notch pathway in the regulation of osteoblast differentiation, but the in vivo role of Notch signaling in bone homeostasis still remains unknown 8-12 .Corresponding Author: Brendan Lee, M.D., Ph.D., One Baylor Plaza, Rm 635E, Houston, Tx 77030,, Email E-mail: blee@bcm.tmc.edu. In this study, we investigate the tissue, cellular, and molecular consequences of both gain and loss of function of Notch signaling in committed osteoblasts. NIH Public Access RESULTS Gain of function of Notch signaling results in severe osteosclerosisTo determine the pathological consequences of in vivo gain of Notch function during bone formation and homeostasis, we generated transgenic mice expressing the Notch1 intracellular domain (N1ICD) under the control of the type I collagen (Col1a1) promoter (Suppl. Fig. 1a,b). Here, gain of Notch function would occur in committed osteoblastic ce...

Identification of a novel alpha‐fetoprotein‐expressing cell population induced by the Jagged1/Notch2 signal in murine fibrotic liver

“…Notch signaling is essential for maintaining the stem cell niche in various organs 28, 29, 30, 31. Regarding its functions in the liver, it was originally implicated in the regulation of the terminal differentiation of HPCs into biliary epithelial cells23 and/or ductal structure formation in liver development 32, 33, 34.…”

Inhibition of Notch1 signaling by Runx2 during osteoblast differentiation

“…(38,48) For this reason, it was expected that Notch1 could be a possible regulator of Runx2 signaling in osteoblast differentiation. However, one can ask whether normal osteoblast differentiation can occur regardless of the presence of Notch1.…”

“…Engin and colleagues suggested that Notch1 represses osteoblast maturation through the binding of Notch1-IC to Runx2. (38) However, several reports have shown that osteoblast cells expressed Notch1, and BMP-2 did not have an appreciable effect on Notch1 expression. (39)(40)(41) Moreover, osteoblast differentiation can occur despite Notch1 expression.…”

c-MYC overexpression with loss of Ink4a/Arf transforms bone marrow stromal cells into osteosarcoma accompanied by loss of adipogenesis

“…Key genes for the regulation of embryonic organogenesis (Hoxa5, Hoxa9, Foxc1 and Foxf2) and the commitment of immature cells to multiple lineages (Pparg, Cebpa, Plxnd1, Egfl7, Mgp, Plxdc2, Ebf1 and Tcf3) were expressed at higher levels in AO cells than in AX cells, suggesting that AO cells have characteristics of immature mesenchymal cells (Wang and Reed, 1993;Luo et al, 1997;Wu et al, 1999;Rosen et al, 2000;Parker et al, 2004;Torres-Vazquez et al, 2004;Miller et al, 2007;Cole et al, 2008). The profile of AX cells, on the other hand, was indicative of both cells that are committed to the osteocyte lineage (Dlx3, Sp7, Dkk1, Ostn and Bmp3) and cells that are inclined to tumorigenesis in vivo (Cd276, Ptgs2, Vegfa, Epha2, Frzb and Sfrp1) (Kim et al, 1993;Daluiski et al, 2001;Dannenberg et al, 2001;Thomas et al, 2003;Lee et al, 2004 Zang et al, 2007;Brantley-Sieders et al, 2008;Engin et al, 2008;Li et al, 2008).…”