A comparison of bone microarchitectural and transcriptomic changes in murine long bones in response to hindlimb unloading and aging
Age and disuse-related bone loss both result in decreases in bone mineral density, cortical thickness, and trabecular thickness and connectivity. Disuse induces physiological changes in bone similar to those seen with aging. Animal studies used 6, and 22-month-old C57BL/6J male and female mice that were hindlimb unloaded (HLU) for 3 weeks. Ingenuity pathway analysis (IPA) included 4-month-old male mice unloaded for 3 weeks and a publicly available dataset (GLDS647) from 3-month female C57BL/6N mice unloaded for 7 days. There were age- and sex-dependent changes in bone structure and mechanical properties in response to HLU. The cortical bone and epiphyseal trabecular bone compartments were most affected. There were greater changes in bone mechanical properties due to age in males, and HLU in females.  RNA extracted from whole-bone marrow-flushed tibiae was sequenced and analyzed. Gene ontology analysis demonstrated that mitochondrial function was downregulated after HLU whereas cell-cycle transition was downregulated with aging. IPA was used to identify the leading canonical pathways and upstream regulators in each HLU age group. IPA identified “Senescence Pathway” as the third leading canonical pathway enriched in mice exposed to HLU. HLU induced activation of the senescence pathway in 3 month and 4-month-old mice, inhibited it in 6-month-old mice and did not change it in 22 month old mice. In conclusion, we demonstrate that hindlimb unloading initiates changes in bone microarchitecture and gene expression characteristic of aging, but with distinct differences. Overall design: 6-month-old and 22-month-old male and female mice were subjected to hindlimb unloading (HLU) for a period of 3 weeks. RNA was extracted from marrow-flushed left tibias and sequenced.