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P&F Grant Awards

Year 6

Grant # 11

The Effects of Systemic Hedgehog Pathway Modulation on Fracture Healing

PI: Michael J. Gardner, MD


Specific Aims

Five to ten percent of the 5.6 million fractures that occur annually in the United States show evidence of impaired healing,1 and these place a significant burden on society. Therefore, elucidation of novel strategies to promote osteogenesis is critical and may potentially benefit a large number of patients. BMP-2, BMP-7, and teriparatide have been developed for clinical use as therapeutics that target inherent biological pathways, but each has disadvantages that have limited their widespread use. The BMP’s, for example, require surgery for direct implantation and local delivery, and the optimal dose and delivery remain unclear. The hedgehog (Hh) signaling pathway is critical to developmental osteogenesis2, 3, chondrogenesis4-6, and bone homeostasis7. Further, the Hh pathway is known to regulate angiogenesis, which may in part explain its role in osteogenesis8-10. These observations suggest a possible central role for Hh signaling in fracture healing, which involves coordinated processes of chondro-, angio- and osteogenesis. Although Hh upregulation has been indirectly associated with fracture healing,11-13 the functional role of Hh signaling in fracture healing in not known. Preliminary studies in our lab indicate that inhibition of Hh signaling impairs non-endochondral bone formation in stress fractures, although it is not known whether a similar effect occurs for endochondral bone formation in complete fractures. On the other hand, activation of Hh signaling using SAG, a small molecule agonist of Smoothened (Smo), through which all Hh signals are transduced, upregulates early markers of bone formation in vitro14. Whether activation of Hh signaling enhances fracture healing has not been studied. Understanding the role of Hh signaling in osteogenesis holds substantial promise in providing therapeutic targets for novel drugs to be used by surgeons to enhance bone healing. Our proposed experiment is designed to provide proof-of-concept for developing biological interventions to augment fracture healing using the Hh pathway. As such, this study is ideally suited for a Pilot & Feasibility Study, from which the data may be used as a basis for a comprehensive, federally-funded grant. Our findings will potentially provide a rationale for larger scale and more mechanistic studies targeting the Hh pathway with pharmacologic agents to improve bone healing in challenging clinical situations. Our central hypothesis is that Hh signaling can be modulated to affect fracture healing. Although preliminary studies are promising and published work is supportive, based on current gaps in our knowledge of this pathway, the proposed study is necessary to support or refute the hypothesis. We will test this hypothesis using a murine, stabilized femoral fracture model that is established in our hands.

Specific Aim 1: Determine the effect of activation of the Hh pathway on fracture healing in a healing-challenged model. In this aim, we will test the hypothesis that fracture healing can be enhanced by systemically activating the Hh pathway in a healing-challenged fracture model. Mice will receive treatment with either vehicle or SAG, a potent Hh agonist. We will create femur fractures in aged (18 month old) mice. Older rodents demonstrate less robust fracture healing, and of particular relevance have a lower and delayed peak of Hh expression in fracture healing15, 16. We hypothesize that SAG will overcome the age-related decline in Hh signaling and improve radiographic, histologic, biomechanical, and vascular aspects of fracture healing.