Calcitonin (CT) is used clinically to treat conditions that are characterized by abnormally high rates of osteoclastic bone reabsorption. Osteoclast activity is modulated by positive and negative regulatory factors, but the responsible intracellular signal transduction mechanisms are incompletely elucidated. The CT receptor (CTR) is a representative member of a distinct G protein-coupled receptor (GPCR) family that includes other receptors that play a role in regulating the metabolism of calcium bone, such as the PTH/PTHrP type I receptor. It presents a unique window on osteoclast (OC) physiology, that it is the best characterized GPCR in these cells. Recombinant CTR isoforms can activate multiple signaling pathways by coupling to heterotrimeric G/s, G/q and G/i proteins, although the specific pattern of signal transduction that is observed is both isoform- and cell type- specific. Recent results reveal that CT can induce a complex array of signaling events, including the activation of Erk1 and Erk2 and phospholipases A/2 and D, and the tyrosine phosphorylation of several proteins. One of these is HEF-1, a recently identified adaptor protein that is phosphorylated in response to integrin ligation in B and T cells. The goal of this proposal is to better characterize the role of HEF1 in CTR (and by extension other GPCR) signaling. The specific aims are: 1) to identify the signaling pathways that couple of CTR to HEF1 phosphorylation, and 2) to begin to characterize the function of HEF1 in osteoclast signal transduction. The results of these studies will reveal heretofore unknown aspects of CT-related signal transduction that can potentially allow a more refined manipulation of osteoclastic bone resorption and renal calcium excretion. Such insights into CTR signaling are also likely to be relevant to signaling mechanisms that are activated by other family members, such as the PTH/PTHrP receptor, and may identify noel targets for the design of pharmaceutical agents to modulate the activities of cells, such as osteoclasts, which are regulated by CT.