Ephrin-B2 reverse signaling is required for topography but not pattern formation of lateral superior olivary inputs to the inferior colliculus.

Graded and modular expressions of Eph-ephrins are known to provide positional information for the formation of topographic maps and patterning in the developing nervous system. Previously we have shown that ephrin-B2 is expressed in a continuous gradient across the tonotopic axis of the central nucleus of the inferior colliculus (CNIC), whereas patterns are discontinuous and modular in the lateral cortex of the IC (LCIC). The present study explores the involvement of ephrin-B2 signaling in the development of projections to the CNIC and LCIC arising from the lateral superior olivary nuclei (LSO) prior to hearing onset. Anterograde and retrograde fluorescent tracing methods in neonatal fixed tissue preparations were used to compare topographic mapping and the establishment of LSO layers/modules in wild-type and ephrin-B2(lacZ/+) mice (severely compromised reverse signaling). At birth, pioneer LSO axons occupy the ipsilateral IC in both groups but are delayed contralaterally in ephrin-B2(lacZ/+) mutants. By the onset of hearing, both wild-type and mutant projections form discernible layers bilaterally in the CNIC and modular arrangements within the ipsilateral LCIC. In contrast, ephrin-B2(lacZ/+) mice lack a reliable topography in LSO-IC projections, suggesting that fully functional ephrin-B2 reverse signaling is required for normal projection mapping. Taken together, these ephrin-B2 findings paired with known coexpression of EphA4 suggest the importance of these signaling proteins in establishing functional auditory circuits prior to experience.

EphA4 and ephrin-B2 expression patterns during inferior colliculus projection shaping prior to experience.

Central processing of complex auditory tasks requires elaborate circuitry. The auditory midbrain, or inferior colliculus (IC), epitomizes such precise organization, where converging inputs form discrete, tonotopically-arranged axonal layers. Previously in rat, we established that shaping of multiple afferent patterns in the IC central nucleus (CNIC) occurs prior to experience. This study implicates an Eph receptor tyrosine kinase and a corresponding ephrin ligand in signaling this early topographic registry. We report that EphA4 and ephrin-B2 expression patterns in the neonatal rat and mouse IC correlate temporally and spatially with that of developing axonal layers. DiI-labeling confirms projections arising from the lateral superior olive (LSO) form frequency-specific layers within the ipsilateral and contralateral mouse CNIC, as has been described in other species. Immunohistochemistry (EphA4 and ephrin-B2) and ephrin-B2 lacZ histochemistry reveal clear gradients in expression across the tonotopic axis, with most concentrated labeling observed in high-frequency, ventromedial aspects of the CNIC. Discrete patches of labeling were also discernible in the external cortex of the IC (ECIC; EphA4 patches in rat, ephrin-B2 patches in mouse). Observed gradients in the CNIC and compartmentalized ECIC expression persisted through the first postnatal week, before becoming less intense and more homogeneously distributed by the functional onset of hearing. EphA4 and ephrin-B2-positive neurons were evident in several auditory brainstem nuclei known to send patterened inputs to the IC. These findings suggest the involvement of cell-cell EphA4 and ephrin-B2 signaling in establishing order in the developing IC.