beta-Arrestin2 regulates the differential response of cortical and trabecular bone to intermittent PTH in female mice

Abstract

Cytoplasmic arrestins regulate PTH signaling in vitro. We show that female beta-arrestin2(-/-) mice have decreased bone mass and altered bone architecture. The effects of intermittent PTH administration on bone microarchitecture differed in beta-arrestin2(-/-) and wildtype mice. These data indicate that arrestin-mediated regulation of intracellular signaling contributes to the differential effects of PTH at endosteal and periosteal bone surfaces.

Introduction: The effects of PTH differ at endosteal and periosteal surfaces, suggesting that PTH activity in these compartments may depend on some yet unidentified mechanism(s) of regulation. The action of PTH in bone is mediated primarily by intracellular cAMP, and the cytoplasmic molecule beta-arrestin2 plays a central role in this signaling regulation. Thus, we hypothesized that arrestins would modulate the effects of PTH on bone in vivo.

Materials and methods: We used pDXA, muCT, histomorphometry, and serum markers of bone turnover to assess the skeletal response to intermittent PTH (0, 20, 40, or 80 mug/kg/day) in adult female mice null for beta-arrestin2 (beta-arr2(-/-)) and wildtype (WT) littermates (7-11/group).

Results and conclusions: beta-arr2(-/-) mice had significantly lower total body BMD, trabecular bone volume fraction (BV/TV), and femoral cross-sectional area compared with WT. In WT females, PTH increased total body BMD, trabecular bone parameters, and cortical thickness, with a trend toward decreased midfemoral medullary area. In beta-arr2(-/-) mice, PTH not only improved total body BMD, trabecular bone architecture, and cortical thickness, but also dose-dependently increased femoral cross-sectional area and medullary area. Histomorphometry showed that PTH-stimulated periosteal bone formation was 2-fold higher in beta-arr2(-/-) compared with WT. Osteocalcin levels were significantly lower in beta-arr2(-/-) mice, but increased dose-dependently with PTH in both beta-arr2(-/-) and WT. In contrast, whereas the resorption marker TRACP5B increased dose-dependently in WT, 20-80 mug/kg/day of PTH was equipotent with regard to stimulation of TRACP5B in beta-arr2(-/-). In summary, beta-arrestin2 plays an important role in bone mass acquisition and remodeling. In estrogen-replete female mice, the ability of intermittent PTH to stimulate periosteal bone apposition and endosteal resorption is inhibited by arrestins. We therefore infer that arrestin-mediated regulation of intracellular signaling contributes to the differential effects of PTH on cancellous and cortical bone.

FIG. 1.

Mean percent change (vs. baseline) in total body BMD in VEH- and PTH-treated β-arr2^-/-and WT mice, assessed by peripheral DXA. Error bars represent SE. All increases were significant compared with baseline values (p < 0.01). ^*p < 0.05, ^**p < 0.001 vs. VEH within each genotype.

FIG. 2.

Trabecular bone parameters after intermittent PTH or VEH administration in β-arr2^-/- and WT mice, including trabecular bone volume fraction, thickness, and number in the fifth lumbar vertebral body (A, B, C) and distal femoral metaphysis (D, E, F), assessed by μCT. Error bars represent SE. ^*p < 0.05, ^**p < 0.005 vs. vehicle within each genotype; #p < 0.05 β-arr2^-/- vs. WT within VEH-treated group.

FIG. 3.

Mean values for midfemoral (A) total cross-sectional area (mm²), (B) cortical bone area (mm²), (C) medullary area (mm²), and (D) cortical thickness (μm) in VEH- and PTH-treated WT and β-arr2^-/- mice, assessed by μCT. Error bars represent SE. ^*p < 0.05, ^**p < 0.005 vs. vehicle within each genotype; ^#p < 0.05 β-arr2^-/- vs. WT within the VEH-treated group.

FIG. 4.

Representative histologic image (sagittal section) of femoral cortical bone in WT and β-arr2^-/- after administration of intermittent PTH (80 μg/kg/day) or VEH. Note the calcein double-labeled endocortical surface (ES) in PTH-treated WT and calcein double-labeled periosteal surface (PS) in β-arr2^-/-. Also note the region of bone resorption on the endocortical surface in PTH-treated β-arr2^-/- (shown by arrows). Magnification, ×200.