A search for factors specifying tonotopy implicates DNER in hair-cell development

The accurate perception of sound frequency by vertebrates relies upon the tuning of hair cells, which are arranged along auditory organs according to frequency. This arrangement, which is termed a tonotopic gradient, results from the coordination of many cellular and extracellular features. Seeking the mechanisms that orchestrate those features and govern the tonotopic gradient, we used expression microarrays to identify genes differentially expressed between the high- and low-frequency cochlear regions of the chick (Gallus gallus). Of the three signaling systems that were represented extensively in the results, we focused on the notch pathway and particularly on DNER, a putative notch ligand, and PTPζ, a receptor phosphatase that controls DNER trafficking. Immunohistochemistry confirmed that both proteins are expressed more strongly in hair cells at the cochlear apex than in those at the base. At the apical surface of each hair cell, the proteins display polarized, mutually exclusive patterns of localization. Using morpholinos to decrease the expression of DNER or PTPζ as well as a retroviral vector to overexpress DNER, we observed disturbances of hair-bundle morphology and orientation. Our results suggest a role for DNER and PTPζ in hair-cell development and possibly in the specification of tonotopy.

In the uppermost row of images, which portray the cochlear apex, DNER (red) is expressed at the apical surfaces of hair cells in a punctate pattern on the edges adjacent to the kinocilia. Phalloidin (cyan) labels the hair bundles. The middle images demonstrate that DNER expression is strikingly lower in hair cells at the cochlear base. The illustrations at the bottom show that the expression pattern of PTPζ (cyan) is complementary to that of DNER (red). In these illustrations of wholemounts, the kinocilium of each hair cell is oriented toward the right; the scale bars represent 10 µm.