Mineral Composition is Altered by Osteoblast Expression of an Engineered Gs-coupled receptor

G. J. Kazakia, D. Speer, S. Shanbhag, S. Majumdar, B.R. Conklin, R.A. Nissenson, E.C. Hsiao

Activation of the Gs G protein coupled receptor (GPCR) signaling pathway in osteoblasts can induce dramatic increases in trabecular bone formation. However, the effects of constitutive Gs
signaling on bone tissue quality are not known. The goal of this study is to determine bone tissue quality in mice with osteoblast-specific constitutive Gs signaling activity, using the complementary techniques of FTIR spectroscopy and synchrotron radiation μCT.

Col1(2.3)-tTA/TetO-Rs1 double transgenic (Col1(2.3)/Rs1) mice were created through heterozygote crosses of mice carrying the TetO-Rs1 transgene with mice carrying the Col1(2.3)-tTA transgene. These COL1(2.3)/RS1 mice show osteoblast-specific constitutive Gs signaling activity through the Rs1receptor. Femurs and calvaria of COL1(2.3)/RS1 and WT littermate mice were analyzed by FTIR spectroscopy and imaged by synchrotron radiation μCT.

Significant differences in FTIR spectroscopic measures of bone composition were found between WT and COL1(2.3)/RS1 mice. As compared to the WT littermate controls, mineral-to-matrix ratio was 25% lower (p=0.010), carbonate to-phosphate ratio was 20% higher (p=0.025), crystallinity was 4% lower (p=0.004), and crosslink ratio was 11% lower (p=0.025) in the COL1(2.3)/RS1 bone. Synchrotron imaging revealed dramatic structural
differences between WT and COL1(2.3)/RS1 femurs. Quantitative analysis of the synchrotron data resulted in substantial differences in mean tissue mineral density (TMD) and in spatial distribution of TMD values.

These results demonstrate that chronic Gs activity in murine osteoblasts leads to deposition of immature bone tissue with reduced mineralization. Our findings illustrate that bone tissue quality – in addition to bone quantity and structure – must be assessed in the evaluation of skeletal treatment or phenotype.

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