Mechanisms of Regeneration: Facial Nerve Injury and Repair

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Project Summary/Abstract This proposal presents a five-year research career development program focused on the study of Hedgehog (Hh) pathway signaling in facial nerve injury response. The candidate is currently an Assistant Professor of Otolaryngology at the Stanford School of Medicine. The outlined proposal builds on the candidate’s previous research and clinical experience under the supervision of a world-renowned expert in Hh pathway signaling, Dr. Phil Beachy. The proposed experiments and didactic work will position the candidate with a unique set of skills that will enable his transition to independence as a physician scientist in the mechanisms of nerve injury response and nerve fibrosis. Facial paralysis results in significant disability for affected patients, impacting both facial function and overall quality of life. For patients with permanent paralysis, surgery is the most effective treatment to improve facial symmetry and – in some cases – restore partial movement. However, existing treatments do not completely restore normal facial movement. Clinically, the injured facial nerve becomes fibrotic, which can impede recovery and treatment. Despite the clinical importance of this, the mechanism underlying nerve fibrosis is poorly understood. Given the critical role of the hedgehog pathway in perineurial development, we explored the function of hedgehog-responsive elements in the murine facial nerve after injury. We first verified the presence of hedgehog-responsive fibroblasts (Gli1+) within the murine facial nerve. Gli1+ fibroblasts within the facial nerve expand impressively after injury and compose the majority of the perineurium of the regenerating nerve. These cells closely associate with activated Schwann cells and regenerating axons and appear to promote angiogenesis in the first two weeks after injury. By 10 weeks after injury, however, Gli1+ fibroblasts appear to form fibrotic tissue within the nerve. Following on these findings, we hypothesize that Hh signaling promotes early nerve regeneration after injury but transitions to a pro-fibrotic state at later time points after injury. To assess these hypotheses, we propose the following aims: 1) Characterize the role of Hh signaling after facial nerve injury using powerful genetic and pharmacologic models, 2) Describe changes in Gli1+ cells during facial nerve regeneration and fibrosis via Next Generation Sequencing. Completion of the research in this proposal will enable a thorough dissection of the role of Hh signaling in facial nerve injury, vet this pathway as a mediator of both regeneration and fibrosis after injury, and reveal new therapeutic targets for clinical use.