Cortical bone gain following loading is site-specifically enhanced by prior and concurrent disuse in aged male mice
The primary aim of bone anabolic therapies is to strategically increase bone mass in skeletal regions likely to experience high strains. This is naturally achieved by mechanical loading of the young healthy skeleton. However, these bone anabolic responses fail with age. Here, we applied site specificity analysis to map regional differences in bone anabolic responses to axial loading of the tibia (tri-weekly, for two weeks) between young (19-week-old) and aged (19-month-old), male and female mice. Loading increased bone mass specifically in the proximal tibia in both sexes and ages. Young female mice gained more cortical bone than young males in specific regions of the tibia. However, these site-specific sex difference were lost with age such that bone gain following loading was not significantly different between old males and females. Having previously demonstrated that prior and concurrent disuse enhances bone gain following loading in old females, we established whether this \"rescue\" is sex-specific. Old male mice were subjected to sciatic neurectomy or sham surgery, and tri-weekly loading was initiated four days after surgery. Disuse augmented cortical bone gain in response to loading in old male mice, but only in the regions of the tibia which were load-responsive in the young. Increased understanding of how locally-activated load-responsive processes lead to site-specific bone formation, and how the age-related diminution of these processes can be site-specifically enhanced by disuse, may lead to the next generation of strategic bone anabolic therapies.\n\nHighlightsO_LISex differences in cortical tissue area of young and old mice are not site-specific\nC_LIO_LIThe loading response in young, but not old, mice is sex- and site-specific\nC_LIO_LIThe cortical loading response is site-specifically enhanced by disuse in old mice of both sexes\nC_LIO_LIThe trabecular loading response can be rescued by disuse in old male, but not female, mice\nC_LI