Parathyroid hormone (PTH) is well known for its catabolic and anabolic actions in bone. PTH is now in clinical use for its ability to restore bone lost to osteoporosis, and also under investigation for its use in the regeneration of localized osseous defects due to fractures and/or periodontal disease. The mechanisms of PTH action are complex and have eluded bone biologists for decades. The PTH receptor (PTHR1) is present on osteoblasts, but actions of PTH are more than simple activation of these cells that form bone. Accessory cells in the bone marrow are required in an intricate temporal and spatial organization that may utilize both anabolic and catabolic activities of PTH. Preliminary data from the project laboratory strongly support the ability of PTH to stimulate osteoblastic cell proliferation, but in an indirect manner through a hematopoietic cell in the bone marrow microenvironment. PTH has been found to have novel actions to expand cells of the hematopoietic stem cell (HSC) compartment which may function in the impact of PTH in bone. PTH has also been found to up regulate genes important in megakaryocyte differentiation and function. Megakaryocytes are intriguing osteoimmunomodulatory cells and valid candidates of PTH actions in the bone environment. The overall hypothesis of this project is that PTH stimulates osteoblast production of megakaryocyte-associated genes that in turn promote megakaryocytes to support osteoblastic bone formation. Two specific aims will elucidate the action of PTH in models of altered megakaryocyte function in order to link hematopoietic and osteoblastic cells in the actions of PTH in bone. The first specific aim will determine the role of megakaryocyte stimulating factor (MSF/prg4) and interleukin-6 (IL-6) in megakaryocyte differentiation and PTH action in bone. The second specific aim will verify the impact of megakaryocytes in the actions of PTH in bone. Well characterized in vitro and in vivo model systems will be utilized toward these goals. These studies will better clarify the cellular targets of PTH action and extend our existing knowledge of key cellular regulators in bone to include cells of the hematopoietic lineage. Upon the completion of these studies, three key areas will be better understood;1) PTH action in bone, 2) the role of MSF/prg4 in bone, and 3) the role of the megakaryocyte and megakaryocyte stimulatory mediators in bone. The compelling impact of this work is in the application of PTH toward tissue regeneration as in states of osseous defects due to inflammatory diseases such as periodontal disease. Project Narrative: Parathyroid hormone (PTH) is a potent bone forming agent and also supports blood stem cell development, but is not clear how this acts and whether these two known functions are interactive. This project will delineate the target cells important for the action of PTH to build bone and stem cells. This information is critical for the optimization of PTH as a therapy for bone regeneration and the treatment of bone diseases such as periodontal disease and osteoporosis.