Osteoporosis affects over 200 million women worldwide, one third of whom are predicted to suffer from an osteoporotic fracture in their lifetime. The most promising anabolic drugs involve administration of expensive antibodies. Because mechanical loading stimulates bone formation, our current data, using a mouse model, replicates the anabolic effects of loading in humans and may identify novel pathways amenable to oral treatment. Murine tibial compression produces axially-varying deformations along the cortical bone, inducing highest strains at the mid-diaphysis and lowest at the metaphyseal shell. To test the hypothesis that load-induced transcriptomic responses at different axial locations of cortical bone would vary as a function of strain magnitude, we loaded the left tibiae of 10wk female C57Bl/6 mice in vivo in compression, with contralateral limbs as controls. Animals were euthanized at 1, 3 or 24 h post-loading or loaded for 1 wk (n=4-5/group). Bone marrow and cancellous bone were removed, cortical bone was segmented into the metaphyseal shell, proximal diaphysis and mid-diaphysis, and load-induced differential gene expression and enriched biological processes were examined for the three segments. At each time point, the mid-diaphysis (highest strain) had the greatest transcriptomic response. Similarly, biological processes regulating bone formation and turnover increased earlier and to the greatest extent at the mid-diaphysis. Higher strain induced greater levels of osteoblast and osteocyte genes, whereas expression was lower in osteoclasts. Among the top differentially-expressed genes at 24-hours post-loading, seventeen had known functions in bone biology, of which twelve were present only in osteoblasts, three exclusively in osteoclasts, and two were present in both cell types. Based on these results, we conclude that murine tibial loading induces spatially-unique transcriptomic responses correlating with strain magnitude in cortical bone.
Overall design: Comparative gene expression profiling of RNA-seq data for bone samples from loaded limbs and contralateral control limbs of 10wk old female C57Bl6 mice. At various time points following tibial loading, the cortical bone was segmented into 3 sections: metaphyseal cortical shell, proximal diaphysis and mid-diaphysis. Cancellous bone was removed prior to processing. Differential expression was assessed at each cortical location at 1-h, 3-h, and 24-h following a single loading session. To evaluate long term time points, mice received daily cyclic compressive loading for 7 consecutive days. To replicate standard loading protocols, a final group received 5 consecutive days of loading with euthanasia on the eighth day following the first loading session
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