Influence of Genetic Background on Bone Anabolic Response to Mechanical Loading

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SUMMARY Osteoporosis is a heritable disease of low bone mass and increased fracture risk. Drugs are available to treat low bone mass, yet they carry risks and have limited indications. Thus, there remains a need for new approaches to prevent and treat osteoporosis. Mechanical loading can build and maintain bone mass in humans, and is a potent bone anabolic stimulus in animals. Studies of skeletal loading in mice provide a powerful platform to study bone anabolism at the gene, cell and tissue levels. Yet studies to date have been limited by a lack of genetic diversity – most murine bone loading studies have been done using inbred C57Bl/6 (B6) mice, and the few done with multiple strains used mice with limited genetic diversity. Thus, little is known about the contribution of genetic background to bone's anabolic response to mechanical loading. Our goal is to test the overall hypothesis that the response of bone to mechanical loading is heritable (i.e., depends on genetic background). We will test this using mice from eight genetically diverse inbred mouse strains, including five common laboratory strains and three wild-derived strains. These strains account for 89% of the variation in the mouse genome. In Aim 1, we will apply strain-matched tibial loading to young-adult mice of the eight inbred strains, and determine cortical bone formation responses. We will include female and male mice, to allow assessment of genotype and sex main effects, and sex-genotype interactions. We will then perform RNA-seq analysis on mouse strains identified as low and high responders in order to explore the transcriptomic differences that may drive the differential loading responses. Skeletal loading drives fluid flow in the osteocyte lacunocanalicular network (LCN), and it is widely believed that osteocytes transduce this fluid flow stimulus and regulate the anabolic response to loading. Changes to the LCN occur with osteoporosis and aging, and may alter the osteocyte micromechanical environment, and in turn the anabolic response to loading. In Aim 2, we propose to characterize LCN morphology in bones from the eight inbred strains, and test whether these traits vary between strains. We will use sub-micron resolution X-ray microscopy and confocal imaging to characterize the osteocyte LCN. We will then explore whether inter-strain variations in LCN morphology are associated with variations in anabolic response to loading between strains (from Aim 1). This proposal is appropriate as an R21 Exploratory/ Developmental grant. Aim 1 is developmental; it will establish whether bone's anabolic response to loading is heritable. If so, it will provide rationale for large-scale future studies using genetically diverse mice to discover genes that influence bone's response to loading. Aim 2 will assess the heritability of osteocyte LCN morphology, which may likewise motivate future studies to identify causal genes. It will also explore a fundamental question in bone mechanobiology – whether bone's response to loading depends on osteocyte LCN morphology.