Bernd Fritzsch

The mammalian inner ear houses the vestibular and cochlear sensory organs dedicated to sensing balance and sound, respectively. These distinct sensory organs arise from a common prosensory region, but the mechanisms underlying their divergence remain elusive. Here, we showed that two evolutionarily conserved homeobox genes, Irx3 and Irx5 , are required for the patterning and segregation of the saccular and cochlear sensory domains, as well as for the formation of auditory sensory cells. Irx3/5 were highly expressed in the cochlea, their deletion resulted in a significantly shortened cochlea with a loss of the ductus reuniens that bridged the vestibule and cochlea. Remarkably, ectopic vestibular hair cells replaced the cochlear non-sensory structure, the Greater Epithelial Ridge. Moreover, most auditory sensory cells in the cochlea were transformed into hair cells of vestibular identity, with only a residual organ of Corti remaining in the mid-apical region of Irx3/5 double knockout mice. Conditional temporal knockouts further revealed that Irx3/5 are essential for controlling cochlear sensory domain formation before embryonic day 14. Our findings demonstrate that Irx3/5 regulate the patterning of vestibular and cochlear sensory cells, providing insights into the separation of vestibular and cochlear sensory organs during mammalian inner ear development.The mammalian inner ear houses the vestibular and cochlear sensory organs dedicated to sensing balance and sound, respectively. These distinct sensory organs arise from a common prosensory region, but the mechanisms underlying their divergence remain elusive. Here, we showed that two evolutionarily conserved homeobox genes, Irx3 and Irx5 , are required for the patterning and segregation of the saccular and cochlear sensory domains, as well as for the formation of auditory sensory cells. Irx3/5 were highly expressed in the cochlea, their deletion resulted in a significantly shortened cochlea with a loss of the ductus reuniens that bridged the vestibule and cochlea. Remarkably, ectopic vestibular hair cells replaced the cochlear non-sensory structure, the Greater Epithelial Ridge. Moreover, most auditory sensory cells in the cochlea were transformed into hair cells of vestibular identity, with only a residual organ of Corti remaining in the mid-apical region of Irx3/5 double knockout mice. Conditional temporal knockouts further revealed that Irx3/5 are essential for controlling cochlear sensory domain formation before embryonic day 14. Our findings demonstrate that Irx3/5 regulate the patterning of vestibular and cochlear sensory cells, providing insights into the separation of vestibular and cochlear sensory organs during mammalian inner ear development.

Hearing loss is the most common form of sensory deficit. It occurs predominantly due to hair cell (HC) loss. Mammalian HCs are terminally differentiated by birth, making HC loss incurable. Here, we show the pharmacogenetic downregulation of Cldn9, a tight junction protein, generates robust supernumerary inner HCs (IHCs) in mice. The putative ectopic IHCs have functional and synaptic features akin to typical IHCs and were surprisingly and remarkably preserved for at least fifteen months >50% of the mouse’s life cycle. In vivo, Cldn9 knockdown using shRNA on postnatal days (P) P1-7 yielded analogous functional putative ectopic IHCs that were equally durably conserved. The findings suggest that Cldn9 levels coordinate embryonic and postnatal HC differentiation, making it a viable target for altering IHC development pre- and post-terminal differentiation.

The inner ear is the hub where hair cells transduce sound, gravity, and head acceleration stimuli carried by neural codes to the brain. Of all the senses, hearing and balance, which rely on mechanosensation, are the fastest sensory signals transmitted to the central nervous system. The mechanoelectrical transducer (MET) channel in hair cells is the entryway for the sound-balance-brain interface, but the channel's composition has eluded biologists due to its complexity. Here, we report that the mouse utilizes Piezo1 (Pz1) and Piezo2 (Pz2) isoforms as central components of the MET complex. The Pz channel subunits are expressed in hair-cell stereocilia, are co-localized and co-assembled, and are essential components of the MET complex in vitro and in situ, including integration with the transmembrane channel (Tmc1/2) protein. Mice expressing non-functional Pz1 and Pz2, but not functional Pz1 at the ROSA26 locus under the control of hair-cell promoters, have impaired auditory and vestibular traits that can only be explained if Pz channel multimers are integral to the MET complex. We affirm that Pz protein subunits constitute MET channels and that functional interactions with components of the MET complex yield current properties resembling hair-cell MET currents. Our results demonstrate Pz is a MET channel component central to interacting with MET complex proteins. Results account for the MET channel pore and complex.