BMP3 Suppresses Osteoblast Differentiation of Bone Marrow Stromal Cells via Interaction with Acvr2b

Abstract

Enhancing bone morphogenetic protein (BMP) signaling increases bone formation in a variety of settings that target bone repair. However, the role of BMP in the maintenance of adult bone mass is not well understood. Targeted disruption of BMP3 in mice results in increased trabecular bone formation, whereas transgenic overexpression of BMP3 in skeletal cells leads to spontaneous fracture, consistent with BMP3 having a negative role in bone mass regulation. Here we investigate the importance of BMP3 as a mediator of BMP signaling in the adult skeleton. We find that osteoblasts (OBL) and osteocytes are the source of BMP3 in adult bone. Using in vitro cultures of primary bone marrow stromal cells, we show that overexpression of BMP3 suppresses OBL differentiation, whereas loss of BMP3 increases colony-forming unit fibroblasts and colony-forming unit OBL. The ability of BMP3 to affect OBL differentiation is due to its interaction with activin receptor type 2b (Acvr2b) because knockdown of endogenous Acvr2b in bone marrow stromal cells reduces the suppressive effect of BMP3 on OBL differentiation. These findings best fit a model in which BMP3, produced by mature bone cells, acts to reduce BMP signaling through Acvr2b in skeletal progenitor cells, limiting their differentiation to mature OBL. Our data further support the idea that endogenous BMPs have a physiological role in regulating adult bone mass.

Development of the Endochondral Skeleton

“…Recent studies have shown that BMP3 engages the BMP type II receptor Acvr2b to inhibit signaling by BMP2 or BMP4 (Kokabu et al 2012). In the postnatal skeleton, BMP3 is mainly produced by osteoblasts and osteocytes and it in turn reduces differentiation from the earlier-stage osteoblast lineage cells (Kokabu et al 2012). Thus, BMP3 may function in a negative-feedback mechanism to ensure the production of a proper number of osteoblasts.…”

Specification of osteoblast cell fate by canonical Wnt signaling requires Bmp2

“…In situ hybridization with digoxigenin-labeled Grhl3 probe (kindly provided by Stephen Jane, Monash University, Australia) was carried out as described previously (Gamer et al, 2009). Hindlimbs from E16 mice were embedded in optimal cutting temperature compound (OCT) on dry ice and frozen sections were prepared for β-galactosidase staining as previously described (Kokabu et al, 2012).…”

Expression of TLE3 by bone marrow stromal cells is regulated by canonical Wnt signaling

“…After epiphyses were removed, bone was flushed with 15 ml aMEM medium using a 20 ml syringe fitted with a 23-gauge needle. Flushed bone marrow cells were cultured with aMEM medium containing 15% FBS for 4 days [8]. Cells were treated with 100 ng/ml rhWnt3a (R&D Systems).…”

“…BMSCs have been shown to acquire the osteoblast phenotype in culture [4] and form bone in vivo after implantation in diffusion chambers [5,6]. We previously reported that W20-17, a cell line derived from adult mouse bone marrow responds to BMP2 by differentiating into osteoblast-like cells [7,8].…”