Project Summary/Abstract Loss of bone mass throughout the body, and especially adjacent to inflamed joints, is a major cause of morbidity and fracture in veterans with rheumatic diseases. Although bisphosphonates are sometimes used to protect bone in veterans with rheumatoid arthritis, there is no evidence-based indication that this therapeutic maneuver is effective. The long- term goal of the studies proposed in this application is to elucidate the mechanisms causing inflammation-associated bone loss. Osteoclasts are the only cells capable of degrading bone and RANKL is a protein produced by cells that support osteoclast differentiation that is essential for this process. Therefore, the studies proposed in this application seek to understand the mechanisms that control RANKL gene expression and identify the cell types that express it during inflammation by addressing the following hypothesis: inflammation- associated bone loss is due to stimulation of the RANKL gene in fibroblasts and stromal/osteoblastic cells, but not in activated T cells, and stimulation of RANKL expression is mediated by distant transcriptional regulatory elements. The studies in Specific Objective 1 will determine whether RANKL expression in lymphocytes contributes to bone loss in murine models of inflammation and sex steroid deficiency by deleting this gene specifically in T cells or B cells. Studies in Specific Objective 2 will identify the mechanisms by which inflammation stimulates RANKL gene transcription in stromal/osteoblastic cells and T lymphocytes by identifying the transcriptional responsive regions of the RANKL gene. Lastly, the studies in Specific Objective 3 will determine the role of a transcriptional enhancer of the RANKL gene, identified previously, in the bone loss associated with murine models of inflammation and sex steroid deficiency. It is important to note that the studies proposed in this application to elucidate the mechanisms causing inflammation-associated bone loss would also be relevant to other bone losing conditions such as periodontal tooth loss (the most common cause of adult loss of dentition), as well as the bone loss that occurs around prosthetic joints and results in their failure. In summary, the studies proposed in this Merit application are relevant to several major causes of localized as well as systemic bone loss and are thus important to the veterans patient health care mission. PUBLIC HEALTH RELEVANCE: Potential Impact on Veterans Health Care. Loss of bone mass throughout the body, and especially adjacent to inflamed joints, is a major cause of morbidity and fracture in veterans with rheumatic diseases. Although bisphosphonates are sometimes used to protect bone in veterans with rheumatoid arthritis, there is no evidence-based indication that this therapeutic maneuver is effective. TNF antagonists are another therapeutic option, but as many as 40% of rheumatoid arthritis patients do not respond to anti-TNF therapy. This, and the high cost and uncertain long- term safety of anti-TNF proteins, suggests that novel approaches are still required. Studies that address the mechanism behind the bone loss that occurs with inflammation are therefore of major clinical importance in the VA healthcare mission. Furthermore, studies proposed in this application to elucidate the mechanisms causing inflammation-associated bone loss would also be relevant to other bone losing conditions such as periodontal tooth loss (the most common cause of adult loss of dentition), as well as the bone loss that occurs around prosthetic joints and results in their failure. In summary, the studies proposed in this Merit application are relevant to several major causes of localized as well as systemic bone loss and are thus important to the veterans patient health care mission.