Ephrins and Ephs in cochlear innervation and implications for advancing cochlear implant function

Objectives/Hypothesis Determine if the neuronal pathfinding cues resulting from Eph/ephrin interaction in the inner ear play a role in establishing the tonotopic innervation of the cochlea. Study Design Protein expression of Ephs and ephrins was evaluated in the inner ear of mice and chicks. Subsequently, in vitro, in vivo, and functional electrophysiologic studies were performed to indicate that Ephs and ephrins play a role regulating the normal innervation patterns in the mouse inner ear. Methods Eph and ephrin protein expression was identified in the inner ear by western blotting and localized by fluorescence immunohistochemistry and X-gal staining. Eph/ephrin effects on neurite outgrowth was assessed via co-culture with EphB2 expressing COS-1 cells. Anatomic effects of disrupting Eph/ephrin signaling on cochlear innervation were determined with lipophilic dye tracing and functional effects with auditory brainstem response (ABR). Results Expression of several different Ephs and ephrins were found in the inner ear of chicks and mice. The changes in ephrin-A2 immunoreactivity after gentamicin ototoxicity coincide with the spatio-temporal pattern of hair cell loss and regeneration in the chick cochlea. EphB2 inhibited outgrowth of spiral ganglion cell neurites. Knockout mice with null function of EphB1, EphB2, and EphB3 demonstrated abnormal inner ear innervation and elevated ABR thresholds, indicating hearing loss. Conclusions Ephrin-A2 may be involved in the guidance of ganglion cells to hair cells in the chick. Disruption of Eph/ephrin signaling results in abnormal innervation and hearing loss, suggesting that these proteins play a role in establishing normal innervation patterns in the mouse cochlea.