# Dicer Deletion in the Ear Can Cut Most Neurons and Their Innervation of Hair Cells to Project to the Ear and the Brainstem

## TL;DR

Deleting Dicer in the ear disrupts most auditory neurons and their connections, leading to hearing loss and highlighting the role of microRNAs in ear development.

## Contribution

The study reveals how Dicer deletion differentially affects auditory and vestibular neurons and their projections, emphasizing miRNA roles in ear development.

## Key findings

- Dicer deletion eliminates spiral ganglion neurons but spares some vestibular neurons.
- Hair cell development is disrupted in the cochlea and canal cristae but preserved in the utricle and saccule.
- Central projections from Dicer null mice reach cochlear and vestibular nuclei but not the cerebellum.

## Abstract

Dicer is crucial for the generation of microRNAs (miRNAs), which are essential for regulating gene expression and keeping neuronal health. Dicer’s conditional deletion cuts all spiral ganglion neurons but spares a small fraction of vestibular ganglion neurons, innervating the utricle and part of the saccule. Hair cells develop in the utricle, saccule, posterior crista, and the cochlea in Pax2Cre; Dicerf/f. Cochlear hair cells develop at the base and expand the OHC and IHC in the middle, or split into a base/middle and the apex. In contrast, Foxg1Cre; Dicerf/f cuts all canal cristae and cochlea hair cells, leaving a reduced utricle and an exceedingly small saccule. Likewise, Foxg1Cre; Gata3f/f shows no cochlear hair cells and is absent in the horizontal and reduced in the posterior crista. In contrast, the utricle, saccule, and anterior crista are nearly normal, underscoring the intricate regulatory networks involved in hair cell and neuronal development. The central projections have been described as the topology of various null deletions. Still, without spiral ganglion neurons, fibers from Dicer null mice navigate to the cochlear nuclei and expand into the vestibular nuclei to innervate the caudal brainstem. Beyond a ramification around the CN, no fibers expand to reach the cerebellum, likely due to Pax2 and Foxg1 that cut these neurons. Genetic alterations, such as Dicer deletion, can lead to hearing loss and impairments in auditory signal processing, illustrating the critical role of microRNAs in the development and function of auditory and vestibular neurons. Further studies on this topic could help in understanding potential therapeutic targets for hearing loss associated with neuronal degradation of miRNA.