The cochlea is innervated by the bipolar sensory neurons of the spiral ganglia that relay sound information from sensory receptor hair cells to central auditory targets. Deafness due to acoustic trauma is associated with pathologies in both spiral ganglion neurons and the hair cells which they innervate and an important aspect of repairing the deafened cochlea is coaxing spiral ganglion neurons to re-innervate their hair cell partners. It is generally anticipated that hair cell re-innervation will involve similar cellular and molecular mechanisms to those guiding nascent hair cell innervation. Therefore, understanding all aspects of spiral ganglion development and hair cell innervation are important prerequisites of regeneration-based therapeutic strategies. A subset of neurons in the spiral ganglion is dedicated to a fundamentally important feedback circuit that provides neuroprotection in extreme noise and facilitates hearing and speech discrimination in background noise. This circuit is dependent on the Type2 spiral ganglion neurons (SGN2) that innervate the outer hair cells. The morphological development of SGN2s is unique because their peripheral axon projects beyond the inner hair cells before making a distinct 90 turn towards the base of the cochlea in order to synapse with 8 to 10 outer hair cells. While many aspects of SGN2 development and outer hair cell innervation are not known, our laboratory has found evidence that the planar cell polarity protein Vangl2 contributes to at least one step in this process; the turning event that directs the SGN2 peripheral axon to the base of the cochlea. The goal of this Exploratory/Developmental Research grant is to establish two basic properties of Vangl2 function during SGN2 peripheral axon turning with the expectation that this will form the foundation of a larger, independent line of research addressing spiral ganglion development. The first is to distinguish between autonomous and non-cell autonomous sites of Vangl2 function in the peripheral axon growth cone or organ of Corti. This will be accomplished using a vangl2 conditional knockout line previously generated by the lab in combination with Cre lines selected to spatially restrict vangl2 gene deletion. The second is to assay the relative contribution of two alternative non-canonical Wnt receptors and signaling pathways that have been demonstrated to function upstream of Vangl2 in other contexts. This will be established through genetic interaction assays based upon the hypothesis that if Vangl2 and an upstream receptor function in the same pathway, then removing both will enhance SGN2 turning phenotypes. While these experiments are focused on developmental processes guiding axon pathfinding and target cell innervation we anticipate that these events must be recapitulated during hair cell re-innervation and repair, and therefore the proposed research will advance therapies for repairing the deafened cochlea.
Type2 Spiral Ganglion neurons (SGN2) innervate outer hair cells in of the cochlea and provide sensory input to an efferent feedback loop that is neuroprotective and facilitates hearing background noise. This project is designed to determine mechanisms guiding SGN2 peripheral axon pathfinding, in particular a stereotyped turning event towards the base of the cochlea that allows SGN2 axons to form the outer spiral bundle and contact multiple outer hair cells. Understanding the developmental mechanisms underlying auditory circuitry will contribute to therapies for repairing these circuits following acoustic trauma and promoting the re-innervation of hair cells during the course of regeneration-based deafness therapies.
| Ghimire, Satish R; Ratzan, Evan M; Deans, Michael R (2018) A non-autonomous function of the core PCP protein VANGL2 directs peripheral axon turning in the developing cochlea. Development 145: |
