Full Length ArticleOsteoblast-specific Notch2 inactivation causes increased trabecular bone mass at specific sites of the appendicular skeleton

- Notch2 was specifically inactivated in cells of the osteoblast and osteoclast lineage.
- Notch2 inactivation in osteoclasts did not cause skeletal abnormalities in vivo.
- Notch2 inactivation in osteoblasts caused excessive trabecular bone mass in the proximal femur and distal tibia.
- Osteocyte differentiation was not impaired by Notch2 inactivation in osteoblasts.
- Notch2 inactivation in osteoblasts caused increased osteogenesis ex vivo.

Notch signaling is a key pathway controlling various cell fate decisions during embryogenesis and adult life. It is activated by binding of specific ligands to four different Notch receptors that are subsequently cleaved by presenilins to release an intracellular domain that enters the nucleus and activates specific transcription factors. While the skeletal analysis of various mouse models with activated or inactivated Notch signaling has demonstrated a general impact of this pathway on bone remodeling, the more recent identification of NOTCH2 mutations in individuals with Hajdu-Cheney syndrome (HCS) has highlighted its human relevance. Since HCS is primarily characterized by skeletal defects, these latter findings led us to analyze the specific role of Notch2 in skeletal remodeling. After observing Notch2 expression in osteoblasts and osteoclasts, we utilized Runx2-Cre and Lyz2-Cre mice to inactivate Notch2 in cells of the osteoblast or osteoclast lineage, respectively. Whereas Notch2fl/fl/Lyz2-Cre mice did not display significant alterations of skeletal growth, bone mass or remodeling, Notch2fl/fl/Runx2-Cre mice progressively developed skeletal abnormalities in long bones. More specifically, these mice displayed a striking increase of trabecular bone mass in the proximal femur and the distal tibia at 6 and 12 months of age. Whereas undecalcified sectioning of the respective regions did not reveal impaired osteocyte differentiation as a potential trigger for the observed phenotype, ex vivo experiments with bone marrow cells identified an increased osteogenic capacity of Notch2fl/fl/Runx2-Cre cultures. Collectively, our findings demonstrate that Notch2 physiologically regulates bone remodeling by inhibiting trabecular bone formation in the appendicular skeleton. Understanding the underlying mechanisms may help to improve diagnosis and therapy of HCS.