The loss of sensory hair cells from the human inner ear is a leading cause of hearing and balance deficits. The potential for sensory regeneration in the human ear is very limited, but hair cells in the ears of many no mammalian vertebrates can quickly regenerate after injury. A complete understanding of the basic biology of this regenerative process should suggest methods for promoting similar forms of regeneration in the human ear. The overall goals of this study are to identify the molecular signaling pathways that regulate the regeneration of hair cells and their sensory neurons. Our recent work has suggested that cell-cell interactions mediated by the adhesion molecule N-cadherin are a key regulator of sensory regeneration. Experiments proposed here will examine two specific signaling pathways that are known to be activated by N-cadherin interactions. Present data suggest that N-cadherin may regulate either (1) the cellular translocation of Beta-catenin from cell-cell junctions to cell nuclei, and/or (2) activation of the c-Jun kinase (JNK) signaling pathway. We suspect that both of these pathways might act in parallel to regulate the regenerative proliferation of inner ear supporting cells. Other experiments will examine the possible involvement of Sonic Hedgehog (SHH) signaling in the regenerative process. Our pilot data have demonstrated that most of the molecular constituents of the SHH pathway are present in the avian vestibular organs during the early stages of regeneration. We hypothesize that SHH may act as an endogenously produced mitogen in the avian ear during regeneration. A related project will focus on the regeneration of sensory neurons in the vestibular organs. In order to restore sensory function, regenerated hair cells need to establish precise synaptic contacts with afferent neurons. We hypothesize that a signaling molecule that inhibits neuronal growth is produced within the reversal zone of the striola in the utricle. Significantly, we have identified a transcription factor that is uniquely expressed in this region throughout the regenerative process. We propose a series of experiments that are aimed at identification of guidance cues that afferent neurons use to navigate to replacement hair cells during the regenerative process. Knowledge of how afferent neurons are guided to their targets may suggest new methods for enhancing the neural interface of auditory and vestibular prostheses.
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