Osteoporosis is a silent disease characterized by reduced bone mass and increased fracture risk. Unfortunately, 50% of women and 25% of men over the age of 50 in the US will suffer an osteoporotic fracture in their lifetime. As such, novel approaches to promote bone health are highly desirable. Recent work has highlighted the gut microbiota and intestinal barrier as important targets and regulators of bone health. The work in this dissertation establishes gut-targeted therapies to promote bone health and prevent glucocorticoid-induced osteoporosis (GIO), the leading cause of secondary osteoporosis. Gut-targeted therapies tested include dried plums (prunes), indolepropionic acid (IPA), Lactobacillus reuteri ATCC PTA 6475 (LR 6475), and Korean Red Ginseng (KRG) extract. Little is known about how dietary prune (DP) signals through the gut-bone axis to benefit bone health over time, particularly in healthy animal models. Therefore, the temporal effects of DP supplementation were investigated in healthy female mice. Continually feeding 25% DP increased femur trabecular bone volume fraction (BV/TV) in 5-days (by 30%) with continued increases to 220% after 16-weeks. Functionally, DP enhanced biomechanical properties of the tibia after 16-weeks. Gut microbiota composition demonstrated profound and specific shifts, and intestinal barrier function was significantly enhanced as early as 5-days after DP initiation. Next, gut and bone response kinetics were examined in healthy female mice fed DP for 4-weeks, followed by a patterned feeding approach (on, on/off, on/off/on) of DP discontinuation and DP reintroduction. In the femur, DP enhancement of trabecular BV/TV was completely lost after 2- or 3-weeks of DP discontinuation. The gut microbiota response was complex, but similar to that observed in the bone. Intestinal barrier benefits were lost within 1-week of discontinuation. Remarkably, bone and gut benefits were significantly enhanced when DP was reintroduced for 1- or 2-weeks after discontinuation. Importantly, the gut microbiota composition and intestinal barrier responses were highly correlated to bone status. These novel findings highlight the dynamic and kinetic effects of DP on the gut-bone axis in healthy female mice. Next, DP was investigated as a novel approach to combat GIO in female mice. Results demonstrated that DP (as low as 5% in the diet) prevented distal femur trabecular GIO after 8-weeks. This was accompanied by specific shifts in gut microbiota composition that were strongly correlated to the bone response, consistent with results in animal models and humans. Further analysis revealed IPA, a bacterially produced tryptophan metabolite, was only present in the serum of DP treated animals. Given IPA’s beneficial effects on gut microbiota, intestinal barrier function, and bone health, we investigated if its effects on the gut-bone axis were similar to DP in healthy animals. Daily treatment of IPA for 8-weeks, however, did not reproduce the beneficial effects of DP on the gut-bone axis. As additional approaches to target the gut-bone axis, probiotic (LR 6475) and botanical (KRG extract) were tested in an oral model of GIO in growing male mice. Results demonstrated that LR 6475 prevented trabecular GIO and intestinal barrier dysfunction while KRG extract prevented trabecular GIO, altered gut microbiota composition, and altered immune populations. Taken together, these findings identify multiple novel approaches to target the gut-bone signaling axis. Our results further strengthen the gut microbiota composition and intestinal barrier function as key regulators of bone health. These results motivate future studies to better understand the effect of these novel targeting approaches on human gut and bone health.
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