Axon guidance and morphogenesis in developing auditory brainstem circuits

Highly ordered neural circuits are essential for appropriate auditory processing in vertebrates. The formation of these auditory neurons and their precise connections requires careful orchestration of molecular mechanisms during embryogenesis. This dissertation describes the role of Eph receptor axon guidance molecules in development of these circuits in the avian auditory brainstem. Our hypotheses on the functions of Eph signaling are based on known expression patterns within the circuits of investigation. We manipulated Eph-ephrin signaling in the auditory brainstem of chick embyros using complementary gain and loss of function techniques then tested the effects on auditory circuit formation. In the central auditory pathway used for sound localization, from n. magnocellularis (NM) to n. laminaris (NL), we found that forward EphB signaling is required for appropriate targeting of the contralateral projecting branch of NM axons to the ventral neuropil of NL and for defining the lateral border of these projections, and that morphogenesis of the nuclei is dependent on Eph-ephrin signaling. We further investigated differential targeting of axons from the vestibulocochlear (VIIIth) nerve. We designed a method to selectively trace auditory VIIIth nerve axons and tested the role of Eph-ephrin signaling in guidance of all VIIIth nerve axons in the hindbrain. We found that Eph receptor subclasses differentially regulate central patterning and borders of central VIIIth nerve axons, and that EphB signaling is important for development of the peripheral auditory sensory organ of the inner ear. Despite these peripheral and central changes, auditory axons still traveled in the appropriate hindbrain pathway at the level of nerve entry when Eph signaling was disrupted. To identify additional factors that might guide VIIIth nerve targeting, we performed an expression survey of multiple candidate axon guidance molecules in development of these projections. We found differences in spatiotemporal expression patterns of netrin-1, slit-2, cadherin-6, semaphorin-3A and neuropilin-1, in the VIIIth nerve and/or auditory circuits when the circuitry is being formed. Together, these data suggest that coordinated Eph-ephrin signaling is important for targeting of NM-NL projections and in appropriate compartmentalization of VIIIth nerve axons in the hindbrain. Additional candidates that may be involved in VIIIth nerve targeting have been identified for further investigation.