Molecular Mechanisms of Type II Spiral Ganglion Neuron Development

PROJECT SUMMARY Hearing function and maintenance relies on the meticulous assembly of spiral ganglion neurons (SGNs). Prior research suggests type II SGNs may act as auditory nociceptors and are primed to detect outer hair cell (OHC) damage. Therefore, type II SGNs may provide a protective mechanism that prevents extreme noise from

damaging OHCs, which are particularly vulnerable. Type II SGNs have a unique projection pattern, with their peripheral axons extending past the inner hair cells (IHCs), making a 90° turn toward the cochlear base followed by synapse formation with 10-15 outer hair cells (OHCs). Overall, the mechanisms related to type II

SGN guidance and synapse formation with OHCs are not well understood. Deciphering how multiple axon guidance cues within the cochlea work together to direct growth cone behavior and synapse formation is fundamental to determining how the neural circuitry in the inner ear develops and can possibly be re-formed in

the context of hearing loss. Based on published and preliminary data, two signaling systems are predicted to facilitate type II SGN patterning in the cochlea: planar cell polarity (PCP) signaling, and Eph/Ephrin signaling. As discussed in this proposal, I will first determine how EPHRIN-A3 and VANGL2 function in type II SGN

guidance by comparing the severity of type II SGN patterning deficiencies in Efna3;Vangl2 double knock out (DKO) mutants to both Efna3 and Vangl2 single nulls. Second, I will examine possible interactions between EPHRIN-A3 and PCP signaling during type II turning and OHC innervation by examining EPPHRIN-A3

distribution in PCP mutants at specific developmental stages. Third, I will assess the mode of Eph/Ephrin signaling in the cochlea by conducting cochlear culture assays at E15.5 and P0. Fourth, I will investigate the impact on type II SGN-OHC synapse formation upon loss of Eph/Ephrin and PCP signaling, and if randomized

type II SGN turning negatively type II SGN-OHC synapse formation. Overall, I hypothesize that Eph/Ephrin signaling acts either downstream or parallel to PCP signaling to guide type II SGN turning. My approach to investigating the molecular basis of type II SGN guidance will include a combination of mouse genetics,

immunohistochemistry, imaging, and morphometric analyses in Imaris. Overall, this research explores a novel question that will expand our knowledge on the cellular and molecular mechanisms controlling type II SGN turning and OHC innervation. This research will benefit therapeutic approaches like gene therapy and cochlear

implant technologies aimed at restoring auditory function. The proposed mentoring, research, and career development training plan will prepare me for flourishing in a career as an independent researcher.