Irx3/5 Null Deletion in Mice Blocks Cochlea-Saccule Segregation and Disrupts the Auditory Tonotopic Map

Area of Science:

• Developmental biology
• Neuroscience
• Genetics

Background:

• Inner ear development involves intricate gene regulation for sensory organ formation and function.
• The specific roles of transcription factors like Iroquois homeobox genes in ear development remain largely uncharacterized.

Purpose of the Study:

• To investigate the function of Iroquois homeobox 3 and 5 (Irx3/5) transcription factors in inner ear development.
• To elucidate the role of Irx3/5 in the segregation of auditory and vestibular systems.

Main Methods:

• Generation of a double knockout mouse model lacking Irx3 and Irx5 (Irx3/5 DKO).
• Analysis of inner ear morphology and neuronal projections using histological and imaging techniques.
• Comparison of Irx3/5 DKO mice with wild-type controls.

Main Results:

• Irx3/5 deletion resulted in the fusion of the saccule and cochlear base, with absence of otoconia and tectorial membranes.
• Auditory neurons in Irx3/5 DKO mice showed aberrant projections to both saccule and cochlear hair cells.
• The tonotopic auditory map was obliterated due to incomplete segregation of neuronal projections in the cochlear nucleus.
• A pronounced vestibular-cochlear nerve connection was observed in Irx3/5 DKO mice.

Conclusions:

• Irx3 and Irx5 are essential genes for the proper segregation of auditory and vestibular structures in the mammalian inner ear.
• These genes play a critical role in establishing the tonotopic organization of the cochlea.
• The findings suggest Irx3/5 act as evolutionary branch-point genes, influencing the organization of the saccule and cochlea.