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Bioenergetic programs of cancellous and cortical bone are distinct and differ with age and mechanical loading

ABSTRACT: Mechanical loading induces bone formation in young rodents, but mechanoresponsiveness is reduced with age. Glycolytic activity and mitochondrial dysfunction increase with age and may change bone mechanotransduction. To evaluate load-induced changes to bioenergetic activity in young and adult animals, we loaded the tibia of 10-wk and 26-wk female mice and examined transcriptomic responses at the mid-diaphysis, metaphyseal cortical shell, and cancellous core. Across all biological processes, oxidative phosphorylation and mitochondrial pathways were most often enriched with loading and had opposite enrichment in young and adult animals. Following loading, young animals had temporally-coordinated differential expression of mitochondrial-associated genes, with greatest expression at the mid-diaphysis. In adults, bioenergetic gene expression was lower compared to young animals.

ORGANISM(S): Mus musculus

PROVIDER: GSE281942 | GEO | 2024/11/15

REPOSITORIES: GEO

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Project description: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.

Project description:Studies from global loss-of-function mutants suggest that alternative NF-kB downstream of NF-kB inducing kinase (NIK) is a cell-intrinsic negative regulator of osteogenesis. However, the interpretation of the osteoblast and/or osteocyte contribution to the bone phenotype is complicated by simultaneous osteoclast defects in these models. Therefore, we turned to a transgenic mouse model to investigate the direct role of NIK in the osteolineage. Osx-Cre;NT3 animals (NT3-Cre+), which bear a constitutively active NIK allele (NT3) driven by Osx-Cre, were compared to their Cre-negative, Control (Ctrl) littermates. NT3-Cre+ mice had elevated serum P1NP and CTX levels. Despite this high turnover state, microCT showed that constitutive activation of NIK resulted in a net increase in basal bone mass in both cortical and cancellous compartments. Furthermore, NT3-Cre+ mice exhibited a greater anabolic response following mechanical loading compared to controls. We next performed RNA-Seq on non-loaded and loaded tibiae to elucidate possible mechanisms underlying the increased bone anabolism seen in NT3-Cre+ mice. Hierarchical clustering revealed two main transcriptional programs: one loading-responsive and the other NT3 transgene-driven. Gene ontology (GO) analysis indicated a distinct upregulation of receptor, kinase and growth factor activities including Wnts, as well as a calcium-response signature in NT3-Cre+ limbs. The promoters of these GO-term associated genes, including many known to be bone-anabolic, were highly enriched for multiple kB recognition elements (kB-RE) relative to the background frequency in the genome. The loading response in NT3-Cre+ mice substantially overlapped (>90%) with Ctrl. Surprisingly, control animals had 10-fold more differentially expressed genes (DEGs) in response to loading. However, most top DEGs shared between genotypes had a high incidence of multiple kB-RE in their promoters. Therefore, both transcriptional programs (loading-responsive and NT3 transgene-driven) are modulated by NF-kB. Our studies uncover a previously unrecognized role for NF-kB in the promotion of both basal and mechanically-stimulated bone formation.

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Bioenergetic programs of cancellous and cortical bone are distinct and differ with age and mechanical loading

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