Bone morphogenetic proteins in bone stimulate osteoclasts and osteoblasts during bone development.
In this study, overexpression of noggin, a BMP antagonist, in developing bone caused significantly decreased osteoclast number as well as bone formation rate, resulting in increased bone mass with immature bone quality. BMP signaling plays important roles in normal bone development and regulation of bone resorption.


BACKGROUND
Bone morphogenetic proteins (BMPs) act on various types of cells. Although involvement of BMP signals in osteoblast differentiation has been studied extensively, the effects of BMPs on osteoclasts have not been widely researched. Consequently, the net effects of BMPs on bone remain unclear. The purpose of this study was to delineate more fully the role of BMPs in skeletal biology.


METHODS
We generated transgenic mice that express BMP4 or noggin in bone under the control of the 2.3-kb alpha1(I) collagen chain gene (Col1a1) promoter, and analyzed their bone phenotype. We also analyzed bone of transgenic mice expressing BMP4 specifically in cartilage.


RESULTS
Mice overexpressing BMP4 in bone developed severe osteopenia with increased osteoclast number. Mice overexpressing noggin, a BMP antagonist, in bone showed increased bone volume associated with decreased bone formation rate and decreased osteoclast number. The noggin-transgenic tibias exhibited reduced periosteal bone formation and reduced resorption of immature bone in marrow spaces, associated with frequent fractures at the diaphysis. Co-culture of primary osteoblasts prepared from noggin-transgenic calvariae and wildtype spleen cells resulted in poor osteoclast formation, which was rescued by addition of recombinant BMP2, suggesting that noggin inhibits osteoclast formation by attenuating BMP activities in noggin-transgenic mice. The expression levels of Rankl were not decreased in primary osteoblasts from noggin transgenic mice. Immunoblot analysis showed increased phosphorylation of Smad1/5/8 in osteoclast precursor cells after 20-minute treatment with BMPs, suggesting that these cells are stimulated by BMPs. Mice overexpressing BMP4 in cartilage had enlarged bones containing thick trabeculae, possibly because of expansion of cartilage anlagen.


CONCLUSIONS
Overexpression of noggin in bone revealed that BMP signals regulate bone development through stimulation of osteoblasts and osteoclasts.
Okamoto
Mina
M
Department of Orthopaedic Surgery, Osaka University Graduate School of Medicine, Suita, Osaka, Japan.
Murai
Junko
J
Yoshikawa
Hideki
H
Tsumaki
Noriyuki
N
D000818:Animals; D001846:Bone Development / Q000235:genetics; D055396:Bone Morphogenetic Protein 2; D055415:Bone Morphogenetic Protein 4; D019485:Bone Morphogenetic Proteins / Q000037:antagonists & inhibitors / Q000378:metabolism*; D001862:Bone Resorption / Q000235:genetics / Q000378:metabolism; D002352:Carrier Proteins / Q000096:biosynthesis* / Q000235:genetics; D002478:Cells, Cultured; D015870:Gene Expression; D008562:Membrane Glycoproteins / Q000096:biosynthesis; D051379:Mice; D008822:Mice, Transgenic; D010006:Osteoblasts / Q000166:cytology / Q000378:metabolism*; D010010:Osteoclasts / Q000166:cytology / Q000378:metabolism*; D010766:Phosphorylation; D011499:Protein Processing, Post-Translational / Q000235:genetics; D053245:RANK Ligand; D053246:Receptor Activator of Nuclear Factor-kappa B; D015398:Signal Transduction / Q000235:genetics; D051785:Smad Proteins / Q000378:metabolism; D016212:Transforming Growth Factor beta / Q000378:metabolism*; D000099176:Noggin Protein
D055396:Bone Morphogenetic Protein 2; D055415:Bone Morphogenetic Protein 4; D019485:Bone Morphogenetic Proteins; D002352:Carrier Proteins; D008562:Membrane Glycoproteins; D053245:RANK Ligand; D053246:Receptor Activator of Nuclear Factor-kappa B; D051785:Smad Proteins; D016212:Transforming Growth Factor beta; D000099176:Noggin Protein; C525719:Bmp2 protein, mouse; C525810:Bmp4 protein, mouse; C505687:Tnfrsf11a protein, mouse; C505689:Tnfsf11 protein, mouse