Macrophages Modulate Osteoclast Activity through Plasmin Regulation

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Abstract Proper bone healing following tooth extraction is critical to restore function. Complications can lead to infection, prolonged pain, and fewer treatment options for patients. During healing, macrophages are recruited to the site of inflammation, which peaks 24 hours after injury and persists through day 7. Additionally, macrophage presence has been shown to support bone formation and efferocytosis (phagocytosis of apoptotic cells) is associated with bone healing. Osteoclastic resorption is another essential step in early stages of osseous wound healing. Patients treated with anti-resorptives are at risk for osteonecrosis of the jaw (ONJ), and the possibility of developing ONJ dramatically increases with tooth extraction. Preliminary data by the project investigator showed a significant up-regulation of plasminogen activator inhibitor 2 (protein: PAI2, gene: SerpinB2) in macrophages when they were exposed to apoptotic bone cells, and not when exposed to non- bone cells. PAI2, a member of the serpin superfamily, inhibits plasminogen activators resulting in reduced plasmin concentrations. Plasmin has an established role as a proteolytic enzyme in the fibrinolytic system. Plasmin also cleaves substrates in the extracellular matrix and is implicated in degradation of the osteoid layer covering the bone surface, thus increasing susceptibility to osteoclastic resorption. The overall goal of this project is to study the integration of the fibrinolytic system and osteoimmunology in oral wound healing. It is hypothesized that efferocytosing macrophages produce PAI2, which inhibits plasminogen activation resulting in decreased bone resorption and delayed wound healing. Two aims are proposed: 1) to identify the conditions of PAI2 production by macrophages in response to apoptotic osteoblast engulfment, and 2) to determine the effect of PAI2 deficiency on osteoclastic activity and wound healing. To accomplish these aims, first an in vitro co-culture system that allows for macrophage engulfment of apoptotic bone marrow cells will be used. This will be a valuable tool to measure increases in PAI2 gene and protein changes in a controlled environment. Furthermore, we will delineate whether PAI2 is intracellular or secreted. Next, we will determine the dependence of oral socket bone healing on PAI2 using a murine knock out model. We anticipate there to be an effect on osteoclast activity due to the role of plasmin in initiating bone resorption. Therefore, we will focus evaluation of these samples on measures of osteoclastic activity. Understanding bone microenvironmental mechanisms for recovery broadens our knowledge of basic bone biology and pathology, providing early targets for future therapeutic interventions. When completed, the experiments described in this proposal will be novel in establishing the role of PAI2 in bone biology and homeostasis, providing new mechanistic insights into the bone microenvironment and signaling pathways. In addition, these studies will be foundational in establishing scientific experiences that will launch a successful scientific career for the project principal investigator.