Long Bone Structure and Strength Depend on BMP2 from Osteoblasts and Osteocytes, but Not Vascular Endothelial Cells

Abstract

The importance of bone morphogenetic protein 2 (BMP2) in the skeleton is well known. BMP2 is expressed in a variety of tissues during development, growth and healing. In this study we sought to better identify the role of tissue-specific BMP2 during post-natal growth and to determine if BMP2 knockout affects the ability of terminally differentiated cells to create high quality bone material. We targeted BMP2 knockout to two differentiated cell types known to express BMP2 during growth and healing, early-stage osteoblasts and their progeny (osterix promoted Cre) and vascular endothelial cells (vascular-endothelial-cadherin promoted Cre). Our objectives were to assess post-natal bone growth, structure and strength. We hypothesized that removal of BMP2 from osteogenic and vascular cells (separately) would result in smaller skeletons with inferior bone material properties. At 12 and 24 weeks of age the osteoblast knockout of BMP2 reduced body weight by 20%, but the vascular knockout had no effect. Analysis of bone in the tibia revealed reductions in cortical and cancellous bone size and volume in the osteoblast knockout, but not in the vascular endothelial knockout. Furthermore, forelimb strength testing revealed a 30% reduction in ultimate force at both 12 and 24 weeks in the osteoblast knockout of BMP2, but no change in the vascular endothelial knockout. Moreover, mechanical strength testing of femurs from osteoblast knockout mice demonstrated an increased Young’s modulus (greater than 35%) but decreased post-yield displacement (greater than 50%) at both 12 and 24 weeks of age. In summary, the osteoblast knockout of BMP2 reduced bone size and altered mechanical properties at the whole-bone and material levels. Osteoblast-derived BMP2 has an important role in post-natal skeletal growth, structure and strength, while vascular endothelial-derived BMP2 does not.

Bmp2 conditional knockout in osteoblasts and endothelial cells does not impair bone formation after injury or mechanical loading in adult mice

“…Experimental groups were comprised of either all male, all female, or male and female mice as reported in the result tables. The choice of sex was based simply on availability; we did not observe any sex-genotype interactions in this study’s results or our previous phenotyping studies [24]. The conditional knockout (cKO) mice carried two floxed Bmp2 alleles and were Cre-positive ( Bmp2 fl/fl ; Cre-positive).…”

“…Lastly, after non-damaging mechanical loading that stimulates lamellar bone formation, BMP2 expression is up-regulated [9]. Taken together with findings that BMP2 is critical for post-natal bone formation [20] and that deletion of BMP2 in osteoblast lineage cells results in osteopenia and reduced bone strength [24,25], this result suggests that BMP2 may be critical in loading-induced bone formation.…”

Contributions of Raman spectroscopy to the understanding of bone strength

“…47 There are a few studies of the correlation between biomechanical parameters and Raman spectroscopic composition parameters in mouse models of disorders, including osteogenesis imperfect (OI) [48][49][50][51] and osteoporosis, 52 as well as on the effects of knocking out matrix proteinase, 53 transcription factor 54 and noncollagenous proteins. [55][56][57][58][59][60][61][62] These are discussed in the following sections.…”

“…The depletion of osteoblast-derived bone morphogenic protein-2 (BMP2) could cause structural and mechanical defects in long bones of Osx-Cre knockout mice, but Raman spectroscopic analyses could not explain these defects. 60 The authors suggested that other parameters related to collagen alignment, cross-links and structure should be examined. In another study, Raman spectroscopy found that calvarial osteoblastic cell cultures treated with BMP2 in the presence or absence of lysyl oxidase resulted in mineralized tissue with distinct collagen secondary structures 61 Moreover,BMP2 and lysyl oxidase treatment yielded in vitro mineralized tissue with increased hardness and elastic modulus, which were comparable to that of 4-week-old calvarial bone.…”

Angiocrine Bmp2 signaling in murine liver controls normal iron homeostasis

“…[4][5][6] Few studies, however, have addressed potential angiocrine functions of Bmp2 in vivo. [7][8][9] Recently, 2 nonredundant signaling pathways have been proposed that control hepatic hepcidin expression and iron homeostasis. 10 One pathway comprises Bmp6 signaling via type I (Bmpr1a/Alk3) and type II (Bmpr2) receptors and Smad phosyphorylation in hepatocytes.…”