Loss of insulin receptor in osteoprogenitor cells impairs structural strength of bone

Abstract

Type 1 diabetes mellitus (T1D) is associated with decreased bone mineral density, a deficit in bone structure, and subsequently an increased risk of fragility fracture. These clinical observations, paralleled by animal models of T1D, suggest that the insulinopenia of T1D has a deleterious effect on bone. To further examine the action of insulin signaling on bone development, we generated mice with an osteoprogenitor-selective (osterix-Cre) ablation of the insulin receptor (IR), designated OIRKO. OIRKO mice exhibited an 80% decrease in IR in osteoblasts. Prenatal elimination of IR did not affect fetal survival or gross morphology. However, loss of IR in mouse osteoblasts resulted in a postnatal growth-constricted phenotype. By 10-12 weeks of age, femurs of OIRKO mice were more slender, with a thinner diaphyseal cortex and, consequently, a decrease in whole bone strength when subjected to bending. In male mice alone, decreased metaphyseal trabecular bone, with thinner and more rodlike trabeculae, was also observed. OIRKO mice did not, however, exhibit abnormal glucose tolerance. The skeletal phenotype of the OIRKO mouse appeared more severe than that of previously reported bone-specific IR knockdown models, and confirms that insulin receptor expression in osteoblasts is critically important for proper bone development and maintenance of structural integrity.

Figure 1

OIRKO mice are smaller in size, with slighter bones. PCR of the IR locus revealed that recombination of floxed IR alleles occurs only in bone ((a), ΔIR; calvaria, tibia). OIRKO mice exhibited an ~80% decrease in IR in whole bone by Western blot (n = 3 IR flox, 3 OIRKO tibiae, (b)). Loss of the insulin receptor in osteoblasts reduced the size of male and female mice. Postnatal growth curves for male ((c), top) and female ((c), bottom) IR flox and OIRKO mice are shown (*P < 0.05, **P < 0.01, and ***P < 0.001). Disruption in the growth plate was not apparent (a representative H & E stain is shown, (d)). μCT images of the femur midshaft demonstrated that femurs from OIRKO mice are narrower than from comparably aged IR flox mice (e).

Figure 2

OIRKO femurs are weaker and more slender. Loss of IR in osteoblasts caused the bone to be more slender (ratio of length to total cross-sectional area) relative to body weight (Length/Tt.AR per BW; (a), male; (b), female). Loss of IR in osteoblasts also caused a decrease in the structural strength of the femur in bending (peak force; (c), male; (d), female). Data is displayed as a scatter-plot, with mean and SEM indicated.