BMPs were discovered as potent inducers of ectopic bone formation when implanted subcutaneously (Urist, 1965
). Studies of human disorders chondrodysplasia (Thomas et al., 1996
) and fibrodysplasia ossificans progressiva (Shore et al., 2006
) indicate the importance of BMP signaling in cartilage and muscle, respectively, suggesting that BMP signaling plays an important role in controlling mineralization in musculoskeletogenesis. Bone is the primary component in skeletogenesis, and osteoblasts are the predominant cell type in bone. However, the mechanism by which BMP signaling in osteoblasts contributes the skeletogenesis has not been fully described.
The majority of bones, including long and ectopic bone, are formed through an endochondral process (Kronenberg, 2003
). Condensed mesenchymal cells differentiate into chondrocytes to form a cartilage template that is later replaced by osteoblasts (Mackie et al., 2008
; Maes et al., 2007
). Mesenchymal cells and chondrocytes respond to BMP signaling to differentiate and maintain their features in vivo (Bandyopadhyay et al., 2006
; Tsuji et al., 2006
; Yoon et al., 2005
). By contrast, during the alternate process of intramembranous bone formation, mesenchymal cells differentiate directly into osteoblasts without going through a cartilaginous phase. In this study, we genetically altered BMP signaling in osteoblasts using a mouse model. To avoid secondary effects from chondrocytes on osteoblasts in the endochondral process, we primarily examined intramembranous bone formation (e.g. calvaria) and demonstrated the direct effects that BMP signaling has on osteoblasts.
BMP receptor type IA (BMPR1A), which is abundantly expressed in bone, is activated by major BMP ligands BMP2 and BMP4. Conventional knockout of BMP2, BMP4 and BMPR1A in mice leads to embryonic death before bone development (Mishina et al., 1995
; Winnier et al., 1995
; Zhang and Bradley, 1996
). We previously disrupted Bmpr1a
during adult stages in an osteoblast-specific manner using Og2-Cre
mice (Mishina et al., 2004
). This study suggests that the response of osteoblasts to loss of BMP signaling is age dependent, as bone volume decreased in young mice but increased in old mice. Similarly, the mechanism by which BMP signaling regulates bone mass is not straightforward, as loss-of-function of BMP2 and gain-of-function of BMP4 both reduce bone mass (Okamoto et al., 2006
; Tsuji et al., 2006
). Bone mass is determined by the balance of bone formation and resorption, and osteoblasts regulate both processes. Thus, we focused on osteoblasts and addressed the complicated effect of BMP signaling on bone mass.
Human genetic studies have shown that loss-of-function mutations in components of Wnt signaling, such as the Wnt co-receptor low-density lipoprotein receptor-related protein 5 (LRP5), is associated with osteoporosis (Gong et al., 2001
; Patel and Karsenty, 2002
). Dominant missense LRP5 mutations are associated with high bone mass (HBM) diseases (Boyden et al., 2002
; Little et al., 2002
; Van Wesenbeeck et al., 2003
), indicating that canonical/β-catenin Wnt signaling enhances bone mass (Baron et al., 2006
; Glass and Karsenty, 2006
; Krishnan et al., 2006
). In vitro, Wnt signaling induces BMP expression (Bain et al., 2003
; Winkler et al., 2005
), whereas BMPs induce Wnt expression (Chen et al., 2007
; Rawadi et al., 2003
), suggesting that both BMP and Wnt signaling may synergistically regulate each other in osteoblast, possibly through autocrine/paracraine loop. Both BMP and Wnt signaling induce bone mass; however, the mechanism by which BMP and Wnt signaling cooperate to affect bone mass is not well understood, particularly during embryonic development when bone mass dramatically increases.
Here, we have employed a tamoxifen-inducible Cre-loxP system under the control of a 3.2 kb type I collagen promoter and have disrupted or upregulated BMP signaling through BMPR1A in osteoblasts during embryonic bone development. We unexpectedly found increased bone mass in response to loss of BMPR1A in osteoblasts and a new interaction between BMP and Wnt signaling through sclerostin.