PROJECT SUMMARY Osteoporosis is the most prevalent aging metabolic bone disease, characterized by low bone mineral density (BMD) and an increased risk of low trauma osteoporotic fractures (OF). BMD and OF share some—but not all—of their genetic determinants. Animal studies have indicated an important role of gut microbiota in bone. Germ-free mice exhibited increased bone mass with a reduced number of osteoclasts; colonization of these mice with a normal gut microbiota normalized bone mass. Human studies have suggested an association of gut microbiota diversity with osteoporosis, all with small samples. Direct solid evidence of the significance/mechanisms of gut microbiota for osteoporosis has not been obtained, and the mechanisms through which the gut microbiome interacts with the human genome to impact bone metabolism through oteoblasto-/osteoclasto-genic lineages remain unknown. In Project 1, we propose to conduct a large-scale, comprehensive gut metagenome-wide association study to gain fundamental insights into gut microbial composition/function/impact for osteoporosis. Our Goal is to identify/validate significant gut microbial signatures (at both taxonomic and functional levels) underlying osteoporosis risk, including those shared by-or specific to-BMD and/or OF. Further, we will use both bioinformatic and experimental approaches to investigate causal effects and underlying mechanisms of selected bacterial species on bone metabolism. We will accomplish the following Specific Aims: 1) We will leverage the extensive data and sample archive from our current U19 project and Louisiana Osteoporosis Study (LOS) to identify and validate metagenomic signatures (microbial species and pathways) significantly associated with BMD, bone microarchitecture (assessed by trabecular bone score, TBS) and quality (assessed by QCT/finite element analyses) in >5,000 subjects. We will explore potential effects of sex and ethnicity on metagenomic profiles and their association outcomes. 2) We will perform a metagenome-wide association study in 150 subjects with OF and 150 healthy controls (age ≥55) to identify specific microbial species and pathways significantly and intrinsically associated with OF. 3) By cross- referencing the results from Aims 1-2, we will identify significant metagenomic signatures that are unique to–or shared by—BMD/TBS/bone quality and OF. 4) For selected significant bacterial species, we will assess their potential causal effects and underlying mechanisms on BMD and OF by both computational analyses and functional studies in aged mouse models. The data derived from this project will be further integrated (in Core C) with other omics profiles from Projects 2-3 for a comprehensive trans-omics dissection of the molecular (epi-)genetic mechanisms underlying osteoporosis. This project promises to shed light on the significance of the human gut microbiome to osteoporosis, and reveal novel pathophysiological mechanisms of the disease. The findings w...