The roles of Irx3 and Irx5 in inner ear neurosensory patterning

Abstract

Iroquois (Irx) genes encode evolutionary conserved homeodomain transcription factors that are involved in multiple developing processes. Irx3 and Irx5 are IrxB cluster genes, linked in chromosome from fly to mammal. In mouse embryo, both Irx3 and Irx5 are expressed in the central nervous system, limb bud and otocyst, but their functions in inner ear development are poorly understood. In this study, Irx3LacZ/+ and Irx5EGFP/+ were analyzed to examine the expression pattern of these two genes, respectively. Both Irx3 and Irx5 were expressed in the otic placode. In otic vesicle, the Irx genes shared a broad expression domain in the otic epithelium and periotic mesenchyme. In the cochlear epithelium, Irx3 was expressed in both sensory and non-sensory domains, while Irx5 became gradually restricted to non-sensory regions, accompanying with the progression of hair cell differentiation. To further understand the functions of Irx3 and Irx5 during inner ear development, phenotype analyses have been done in the Irx3LacZ/LacZ, Irx5EGFP/EGFP and Irx3/5-/- mutants. The gross morphology of Irx3LacZ/LacZ and Irx5EGFP/EGFP inner ears were relatively normal, while Irx3/5-/- mutant displayed shortened cochlear duct, enlarged cochlear lumen and lack of the non-sensory structure that segregate the vestibule and cochlea. Furthermore, the vestibular macular of saccule and the cochlear sensory structures were fused together in the compound mutant. In addition, spatial expansion of neurosensory competent domain and temporal elongation of neurosensory competence could be observed in Irx3/5-/- otic epithelium. In conclusion, our study suggests that Irx3 and Irx5 are required for inner ear patterning and neurosensory fate determination.