Dissecting Age-related Changes in Stem Cell-based Bone-Gut Crosstalk - Age-related musculoskeletal deterioration, leading to osteoporosis and osteoarthritis, is a major public health challenge posed by an aging society, driving morbidity, mortality, and elevated healthcare costs. The gut microbiome (GM) has emerged as a critical regulator of host physiology, including bone homeostasis. Alterations in GM-derived circulating metabolites, e.g., bile acids (BA), have been proposed to mediate tissue-specific dysfunction. While studies have linked GM composition to bone remodeling, precise mechanisms connecting GM dysbiosis to the function of stem cells maintaining skeletal and hematopoietic integrity remain unclear. Specifically, the effects of aged GM alterations on skeletal stem cells (SSCs) and their regulation of the BM niche are largely unknown. In foundational preliminary studies, I established an antibiotic (ABX)-mediated GM depletion model in mice and found that erasing the aged (24-month-old) GM significantly improved aged SSC function (e.g., injury response, osteogenesis in vitro and in vivo). Concurrently, aged ABX-treated mice exhibited reduced myeloid-biased hematopoietic progenitors and osteoclast activity within the BM, alongside a more balanced lymphoid-to-myeloid ratio in circulation, suggesting reduced bone resorption and systemic inflammation. Crucially, hematopoietic reconstitution experiments demonstrated these effects on HSC lineage output are mediated indirectly via the BM niche environment, rather than through intrinsic changes to HSCs, pointing to SSCs as key mediators of GM-bone crosstalk. Based on our transcriptomic data showing specific expression of the BA receptor Vitamin D Receptor (VDR) on SSCs, I hypothesize that age-related dysbiosis shifts the systemic BA pool towards inflammatory species (i.e., altered conjugation) that impair SSC function. I further hypothesize that modulation of VDR signaling or youthful BAs can modulate SSC niche activity. These hypotheses will be tested through two specific aims. Aim 1 will mechanistically define the BA-VDR signaling axis in SSCs using in vitro assays and a novel skeletal-specific inducible VDR knockout model to confirm lineage- specific causality. Aim 2 will define age-dependent changes in the BA pool using targeted liquid chromatography (LC) tandem mass spectrometry (MS), known as LC-MS/MS, and determine the therapeutic efficacy of dietary BA supplementation to reinstate youthful SSC niches and hematopoietic lineage output in aged animals. This study combines advanced genetic models, targeted metabolomics, and stem cell functional assays to elucidate a novel gut-bone signaling axis and assess the therapeutic potential of targeting the GM for the treatment of age- related skeletal disorders. For an aspiring surgeon-scientist, this project provides a unique training opportunity, combining a clinically relevant problem with rigorous, mechanism-centered discovery research that has the potential to impact future therapies.
