The role of the cytokine SDF-1 (CXCL12) in bone marrow mesenchymal stem/progenitor cells (BMSCs) function and bone homeostasis was the focus of our previous work in the original PO1 Project 3. The novel outcomes from those studies are driving the current goals in this competitive renewal. The mechanisms involved in aging-related osteoporosis remain poorly defined, however, recent studies from our group, and others, suggest that it is a stem cell disease. A critical problem is that aging triggers impaired localization, proliferation, survival, and differentiation of the osteogenic progenitor cell population in the bone marrow (BM), specifically BMSCs. A critical barrier to preventing these changes is identifying key regulatory pathways, and being able to alter or correct them. Our goal is to use information generated from our original PO1, where we have identified epigenetic changes in a subset of miRNAs in BMSCs, and to target these miRNAs to limit, or reverse, bone loss. Age-associated changes in SDF-1 systemically alter epigenetic regulatory systems, specifically miRNAs, in BMSCs, which in turn directly, and via nutrient signaling pathways, affect cell survival, osteogenesis and bone formation. SDF-1 is tightly regulated translationally, transcriptionally, and post- transcriptionally. We have identified age-associated changes in the SDF-1 axis. Here we show that SDF-1 is not only a target of novel age-associated miRNA epigenetic regulation, but itself alters expression of miRNAs that change with age in BMSCs and target the SDF-1 axis. Significantly, a key miRNA that we identified to increase with age in BMSCs (miR-29b-1-5p) belongs to the miR29 family of miRNAs that have been shown to be critical in extracellular matrix homeostasis and osteogenesis. What is novel, and a challenge to the existing paradigm, is that miR29b-1-5p is the ?passenger strand?, which is normally thought to be degraded leaving the complementary miR-29b-1-3p ?guide strand? as the functional miRNA. Understanding the mechanism that drives this previously unknown age-associated increase in the passenger strand is a critical goal because we have demonstrated that miR29b-1-5p regulates SDF-1 expression and suppresses BMSC osteogenesis and survival pathways. We propose to test the hypothesis that the elevated age-related levels of circulating SDF-1 and decreased bone marrow compartment levels and activity regulate the increased expression of miR-29b-1- 5p and downstream effects on bone homeostasis. Our objectives are to test new hypotheses derived from the findings of the original PO1 by manipulating the miRNAs we identified as changing with age in human/murine BMSCs to determine their effects on bone formation and turnover in vivo and at the molecular level on BMSC osteogenic function. We will test novel methods to reduce expression of these miRNAs, including altering nutritional pathways leading to bone loss by using specific forms of dietary amino acid and delivering synthetic anti-miRNAs to inhibit them. The impact of this project will be demonstrating the role of age-associated changes of BMSC miRNAs and identifying novel targets for reducing, or reversing, age related bone loss.
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