Cells in the osteoblast lineage are crucial for controlling bone formation and bone resorption throughout life. An in-depth understanding of how signaling processes within osteoblast-lineage cells are linked to bone formation and bone resorption is essential to provide new insights into skeletal physiology and to open up new avenues for osteoporosis therapy. In this regard, the G protein signaling pathway is crucial in that the only FDA-approved anabolic treatment for osteoporosis is intermittent parathyroid hormone which activates two G proteins- Gs and Gq- through a G protein coupled receptor (GPCR) in osteoblasts. Previous studies have focused largely on the role of Gs signaling, and little is known about the function of other osteoblast G protein pathways such as Gi and Gq. It is vital that we increase our understanding of how specific G protein signals in osteoblasts are linked to bone formation and bone resorption, and to define their pathophysiological significance. The complexity of the skeletal environment required for normal bone formation and turnover make it necessary to use in vivo models to address these critical issues. Our laboratory has established the utility of an approach to dissect the role of specific signaling pathways using transgenic mice expressing engineered GPCRs termed RASSLs (Receptors Activated Solely by Synthetic Ligands). Our results so far demonstrate that activation of the Gs signaling pathway in mature osteoblasts results in a massive increase in the formation of trabecular bone with a marked reduction in marrow elements and erosion of the cortex. By contract, activation of the Gi signaling pathway in mature osteoblasts results in trabecular osteopenia due to decreased rates of bone formation. Recently, we have developed a method to block endogenous Gi signaling in osteoblasts in vivo using the targeted expression of the catalytic subunit of pertussis toxin. In female mice, blockade of osteoblast Gi signaling results in increased trabecular bone formation during bone growth, and completely prevents age-related trabecular bone loss. These exciting findings indicate that endogenous Gi signaling in osteoblasts negatively regulates bone formation, and suggest that this pathway is an important contributor to age-related bone loss. In the present proposal, we will extend these observations and probe their pathophysiological relevance. Specifically, we will: 1) determine the pathophysiological importance of osteoblast Gi signaling in age-related bone loss, and assess whether osteoblast Gi signaling limits the effectiveness of PTH as an anabolic agent; 2) identify the mechanisms whereby osteoblast Gi signaling negatively regulates bone formation; and 3) determine the role of Gq signaling in mature OBs in regulating skeletal homeostasis. The knowledge gained from these studies will provide new insights into the control of osteoblastic bone formation, and these will be valuable in the design and development of improved anabolic therapies for osteoporosis.