Targeted Mutagenesis of the POU-Domain Gene<i>Brn4/Pou3f4</i>Causes Developmental Defects in the Inner Ear

TLDR

Brn4 is expressed throughout the condensing mesenchyme of the developing inner ear. The study aims to determine whether Brn-4 promotes mesodermal cell survival during inner ear remodeling and regulates inductive signaling in otic mesenchyme. Targeted mutagenesis of Brn4 in mice caused auditory and vestibular deficits, dysplastic inner ear structures, hypoplasia of cochlear mesenchyme‑derived regions, reduced cochlear coiling, and stapes malformations, indicating a key role for Brn4 in mesenchymal patterning and signaling.

Abstract

Targeted mutagenesis in mice demonstrates that the POU-domain gene Brn4/Pou3f4 plays a crucial role in the patterning of the mesenchymal compartment of the inner ear. Brn4 is expressed extensively throughout the condensing mesenchyme of the developing inner ear. Mutant animals displayed behavioral anomalies that resulted from functional deficits in both the auditory and vestibular systems, including vertical head bobbing, changes in gait, and hearing loss. Anatomical analyses of the temporal bone, which is derived in part from the otic mesenchyme, demonstrated several dysplastic features in the mutant animals, including enlargement of the internal auditory meatus. Many phenotypic features of the mutant animals resulted from the reduction or thinning of the bony compartment of the inner ear. Histological analyses demonstrated a hypoplasia of those regions of the cochlea derived from otic mesenchyme, including the spiral limbus, the scala tympani, and strial fibrocytes. Interestingly, we observed a reduction in the coiling of the cochlea, which suggests that Brn-4 plays a role in the epithelial-mesenchymal communication necessary for the cochlear anlage to develop correctly. Finally, the stapes demonstrated several malformations, including changes in the size and morphology of its footplate. Because the stapes anlage does not express the Brn4 gene, stapes malformations suggest that the Brn4 gene also plays a role in mesenchymal-mesenchymal signaling. On the basis of these data, we suggest that Brn-4 enhances the survival of mesodermal cells during the mesenchymal remodeling that forms the mature bony labyrinth and regulates inductive signaling mechanisms in the otic mesenchyme.