The Roles of Irx3 and Irx5 in Cochlear Epithelial Cell Fate Specification

Abstract

The Iroquois (Irx) genes are homeodomain transcription factors that play various roles in embryonic development. Of the six Irx genes, Irx3 and Irx5 appear to cooperate for normal development in various systems. Irx genes are shown to be widely expressed in the chick inner ear. Mutations in Irx5 has also been shown to be linked to craniofacial defects and sensorineural hearing loss in human patients, but their functions in the development of mammalian cochlear epithelium remain unknown. To explore the roles of Irx3/5 in cochlear epithelium development, phenotypical analysis of mouse mutants of Irx3LacZ/+, Irx5EGFP/+, and Irx3/5-/- and conditional double knockouts using Emx2Cre were performed. Irx3 and Irx5 are generally coexpressed within the early developing cochlear epithelium before slightly segregating as the cochlea further develops. The double knockout (DKO) shows an enlarged cochlear duct that varies greatly from those of the single mutants and control. Furthermore, we utilized Sox2, Myo7a, Bmp4, Otx2, CD44, Cxn26, and Kcnq1 to characterize the ventral sensory and dorsal nonsensory cell types of the cochlear epithelium in detail. The ventral cochlear epithelium cell composition of the DKO was altered as both the greater epithelial ridge and lesser epithelial ridge were lost and the Organ of Corti was expanded. These results suggest that both Irx3 and Irx5 are required for correct cell fate specification within the ventral cochlear epithelium.