PROJECT SUMMARY/ABSTRACT Loss of estrogens at menopause is a major cause of osteoporosis due to an increase in number and activity of the cells that resorb bone called osteoclasts. Estrogens inhibit bone resorption in part via direct effects on osteoclasts. However, the cellular and molecular mechanisms responsible for the direct effects of estrogens on osteoclasts remain unclear. The work proposed in this application seeks to identify the molecular mechanisms by which estrogens directly control osteoclast number, function, or both. We found that estrogens prevent an early stimulatory effect of RANKL – the indispensable cytokine for osteoclast differentiation – on mitochondria respiration and ATP production in osteoclast progenitors. Evolutionarily Conserved Signaling Intermediate in Toll Pathway (ECSIT) is a mitochondria complex I-associated protein that regulates immune responses in macrophages in response to inflammatory signals. We found that ECSIT is essential for RANKL-induced stimulation of mitochondria activity and ATP production and a target for the anti-osteoclastogenic effects of estrogens. Estrogens also decrease NAD+ levels and NAD+/NADH ratio and promote the mitochondria pathway of apoptosis. The above observations form the foundation of the hypothesis that estrogens reduce osteoclast number and bone resorption by inhibiting RANKL stimulation of mitochondria, decreasing NAD+, and promoting mitochondria-mediated apoptosis. The work proposed will elucidate the independent contribution of ECSIT, NAD, and apoptosis to osteoclastogenesis and the effects of estrogens. We will also identify possible interactions among these pathways. To test the hypothesis, we will determine whether ECSIT stimulation of mitochondria activity is required for the bone loss caused by estrogen deficiency. This will be accomplished using mice lacking ECSIT in osteoclast lineage cells. In addition, we will examine the contribution of NAD to the estrogen deficiency-dependent increas