Elevated serum IGF-1 levels synergize PTH action on the skeleton only when the tissue IGF-1 axis is intact

Abstract There is growing evidence that IGF-1 and PTH have synergistic actions on bone and that part of the anabolic effects of PTH are mediated by local production of IGF-1. In this study we analyzed the skeletal response to PTH in mouse models with manipulated endocrine or autocrine/paracrine IGF-1. We utilized mice carrying a hepatic IGF-1 transgene (HIT), which results in a 3-fold increase in serum IGF-1 levels and normal tissue IGF-1 expression, and IGF-1 null mice with blunted IGF-1 expression in tissues but 3-fold increases in serum IGF-1 levels (KO-HIT). Evaluation of skeletal growth showed that elevations in serum IGF-1 in mice with igf-1 gene ablation in all tissues except the liver (KO-HIT) resulted in a restoration of skeletal morphology and mechanical properties by adulthood. Intermittent PTH treatment of adult HIT mice resulted in increases in serum osteocalcin levels, femoral total cross-sectional area, cortical bone area and cortical bone thickness, as well as bone mechanical properties. We found that the skeletal response of HIT mice to PTH was significantly higher than that of control mice, suggesting synergy between IGF-1 and PTH on bone. In sharp contrast, although PTH-treated KO-HIT mice demonstrated an anabolic response in cortical and trabecular bone compartments compared to vehicle treated KO-HITs, their response was identical to that of PTH-treated control mice. We conclude that 1) in the presence of elevated serum IGF-1 levels, PTH can exert an anabolic response in bone even in the total absence of tissue IGF-1 and, 2) elevations in serum IGF-1 levels synergize PTH action on bone only if the tissue IGF-1 axis is intact, thus enhancement of PTH anabolic actions is tissue IGF-1-dependent

Measuring the dynamic mechanical response of hydrated mouse bone by nanoindentation

This study demonstrates a novel approach to characterizing hydrated bone's viscoelastic behavior at lamellar length scales using dynamic indentation techniques. We studied the submicron-level viscoelastic response of bone tissue from two different inbred mouse strains, A/J and B6, with known differences in whole bone and tissue-level mechanical properties. Our results show that bone having a higher collagen content or a lower mineral-to-matrix ratio demonstrates a trend towards a larger viscoelastic response. When normalized for anatomical location relative to biological growth patterns in the antero-medial (AM) cortex, bone tissue from B6 femora, known to have a lower mineral-to-matrix ratio, is shown to exhibit a significantly higher viscoelastic response compared to A/J tissue. Newer bone regions with a higher collagen content (closer to the endosteal edge of the AM cortex) showed a trend towards a larger viscoelastic response. Our study demonstrates the feasibility of this technique for analyzing local composition-property relationships in bone. Further, this technique of viscoelastic nanoindentation mapping of the bone surface at these submicron length scales is shown to be highly advantageous in studying subsurface features, such as porosity, of wet hydrated biological specimens, which are difficult to identify using other methods

Serum IGF-1 Affects Skeletal Acquisition in a Temporal and Compartment-Specific Manner

Insulin-like growth factor-1 (IGF-1) plays a critical role in the development of the growing skeleton by establishing both longitudinal and transverse bone accrual. IGF-1 has also been implicated in the maintenance of bone mass during late adulthood and aging, as decreases in serum IGF-1 levels appear to correlate with decreases in bone mineral density (BMD). Although informative, mouse models to date have been unable to separate the temporal effects of IGF-1 depletion on skeletal development. To address this problem, we performed a skeletal characterization of the inducible LID mouse (iLID), in which serum IGF-1 levels are depleted at selected ages. We found that depletion of serum IGF-1 in male iLID mice prior to adulthood (4 weeks) decreased trabecular bone architecture and significantly reduced transverse cortical bone properties (Ct.Ar, Ct.Th) by 16 weeks (adulthood). Likewise, depletion of serum IGF-1 in iLID males at 8 weeks of age, resulted in significantly reduced transverse cortical bone properties (Ct.Ar, Ct.Th) by 32 weeks (late adulthood), but had no effect on trabecular bone architecture. In contrast, depletion of serum IGF-1 after peak bone acquisition (at 16 weeks) resulted in enhancement of trabecular bone architecture, but no significant changes in cortical bone properties by 32 weeks as compared to controls. These results indicate that while serum IGF-1 is essential for bone accrual during the postnatal growth phase, depletion of IGF-1 after peak bone acquisition (16 weeks) is compartment-specific and does not have a detrimental effect on cortical bone mass in the older adult mouse

Genetic randomization reveals functional relationships among morphologic and tissue-quality traits that contribute to bone strength and fragility

We examined femora from adult AXB/BXA recombinant inbred (RI) mouse strains to identify skeletal traits that are functionally related and to determine how functional interactions among these traits contribute to genetic variability in whole-bone stiffness, strength, and toughness. Randomization of A/J and C57BL/6J genomic regions resulted in each adult male and female RI strain building mechanically functional femora by assembling unique sets of morphologic and tissue-quality traits. A correlation analysis was conducted using the mean trait values for each RI strain. A third of the 66 correlations examined were significant, indicating that many bone traits covaried or were functionally related. Path analysis revealed important functional interactions among bone slenderness, cortical thickness, and tissue mineral density. The path coefficients describing these functional relations were similar for both sexes. The causal relationship among these three traits suggested that cellular processes during growth simultaneously regulate bone slenderness, cortical thickness, and tissue mineral density so that the combination of traits is sufficiently stiff and strong to satisfy daily loading demands. A disadvantage of these functional interactions was that increases in tissue mineral density also deleteriously affected tissue ductility. Consequently, slender bones with high mineral density may be stiff and strong but they are also brittle. Thus, genetically randomized mouse strains revealed a basic biological paradigm that allows for flexibility in building bones that are functional for daily activities but that creates preferred sets of traits under extreme loading conditions. Genetic or environmental perturbations that alter these functional interactions during growth would be expected to lead to loss of function and suboptimal adult bone quality

Genetic Variation in Mouse Femoral Tissue-Level Mineral Content Underlies Differences in Whole Bone Mechanical Properties

A/J mice, as compared to C57BL/6J (B6) mice, have a significantly greater total femoral mineral (ash) content which correlates with an increased femoral stiffness (resistance to deformation), but also with an increased brittleness (catastrophic failure). To determine if this whole bone variation in mineral content is indicative of significant mineral and/or matrix variation at the tissue level, femora from 16-week-old female A/J and B6 mice were isolated, embedded in PMMA, sectioned and mounted on barium fluoride infrared windows for FTIRI analyses. In addition, preliminary studies of femora from C3H/HeJ (C3H) mice were conducted, since they have an ash content intermediate to A/J and B6. Mean values for mineral-to-matrix ratio were significantly different for A/J (8.4 ± 0.8) and B6 (7.5 ± 0.4), as were values for collagen cross-link maturity (1.8 ± 0.05 and 3.2 ± 0.1, respectively). C3H mice appeared to have a mineral-to-matrix ratio intermediate of A/J and B6, and cross-link maturity greater than both A/J and B6. B6 femora had similar carbonate-to-amide ratios, carbonate-to-mineral ratios and acid phosphate levels. Thus, whole bone differences in mineral content are concurrent with tissue-level variation in mineral content and collagen maturity in inbred mice. The greater stiffness and brittleness of A/J femora are likely due to differential biological control (osteoblast activity) of the amount of mineral

Elevated serum IGF-1 levels synergize PTH action on the skeleton only when the tissue IGF-1 axis is intact

There is growing evidence that insulin-like growth factor 1 (IGF-1) and parathyroid hormone (PTH) have synergistic actions on bone and that part of the anabolic effects of PTH is mediated by local production of IGF-1. In this study we analyzed the skeletal response to PTH in mouse models with manipulated endocrine or autocrine/paracrine IGF-1. We used mice carrying a hepatic IGF-1 transgene (HIT), which results in a threefold increase in serum IGF-1 levels and normal tissue IGF-1 expression, and Igf1 null mice with blunted IGF-1 expression in tissues but threefold increases in serum IGF-1 levels (KO-HIT). Evaluation of skeletal growth showed that elevations in serum IGF-1 in mice with Igf1 gene ablation in all tissues except the liver (KO-HIT) resulted in a restoration of skeletal morphology and mechanical properties by adulthood. Intermittent PTH treatment of adult HIT mice resulted in increases in serum osteocalcin levels, femoral total cross-sectional area, cortical bone area and cortical bone thickness, as well as bone mechanical properties. We found that the skeletal response of HIT mice to PTH was significantly higher than that of control mice, suggesting synergy between IGF-1 and PTH on bone. In sharp contrast, although PTH-treated KO-HIT mice demonstrated an anabolic response in cortical and trabecular bone compartments compared with vehicle-treated KO-HIT mice, their response was identical to that of PTH-treated control mice. We conclude that (1) in the presence of elevated serum IGF-1 levels, PTH can exert an anabolic response in bone even in the total absence of tissue IGF-1, and (2) elevations in serum IGF-1 levels synergize PTH action on bone only if the tissue IGF-1 axis is intact. Thus enhancement of PTH anabolic actions depends on tissue IGF-1. (C) 2010 American Society for Bone and Mineral Research

Sex-specific regulation of body size and bone slenderness by the acid labile subunit

Insulin-like growth factor-1 (IGF-1) is a crucial mediator of body size and bone mass duringgrowth and development. In serum, IGF-1 is stabilized by several IGF-1 binding proteins(IGFBPs) and the acid labile subunit (ALS). Previous research using ALS knockout (ALSKO)mice indicated a growth retardation phenotype and clinical reports of humans have indicated shortstature and low bone mineral density (BMD) in patients with ALS deficiency. To determine thetemporal and sex-specific effects of ALS deficiency on body size and skeletal development duringgrowth we characterized control and ALSKO mice from 4 to 16 weeks of age. We found thatfemale ALSKO mice had an earlier onset reduction in body size (4 weeks), but that both femaleand male ALSKO mice were consistently smaller than control mice. Interestingly, skeletalanalyses at multiple ages showed increased slenderness of ALSKO femora that was more severe infemales than in males. Both male and female ALSKO mice appeared to compensate for their moreslender bones through increased bone formation on their endosteal surfaces during growth, butALSKO females had increased endosteal bone formation compared to ALSKO males. This studyrevealed age and sex-specific dependencies of ALS deficiency on body size and bone size. Thesefindings may explain the heterogeneity in growth and BMD measurements reported in humanALS deficient patients

Measuring the dynamic mechanical response of hydrated mouse bone by nanoindentation

This study demonstrates a novel approach to characterizing hydrated bone's viscoelastic behavior at lamellar length scales using dynamic indentation techniques. We studied the submicron-level viscoelastic response of bone tissue from two different inbred mouse strains, A/J and B6, with known differences in whole bone and tissue-level mechanical properties. Our results show that bone having a higher collagen content or a lower mineral-to-matrix ratio demonstrates a trend towards a larger viscoelastic response. When normalized for anatomical location relative to biological growth patterns in the antero-medial (AM) cortex, bone tissue from B6 femora, known to have a lower mineral-to-matrix ratio, is shown to exhibit a significantly higher viscoelastic response compared to A/J tissue. Newer bone regions with a higher collagen content (closer to the endosteal edge of the AM cortex) showed a trend towards a larger viscoelastic response. Our study demonstrates the feasibility of this technique for analyzing local composition-property relationships in bone. Further, this technique of viscoelastic nanoindentation mapping of the bone surface at these submicron length scales is shown to be highly advantageous in studying subsurface features, such as porosity, of wet hydrated biological specimens, which are difficult to identify using other methods