Molecular Aspects of the Development and Function of Auditory Neurons

Abstract

This review provides an up-to-date source of information on the primary auditory neurons or spiral ganglion neurons in the cochlea. These neurons transmit auditory information in the form of electric signals from sensory hair cells to the first auditory nuclei of the brain stem, the cochlear nuclei. Congenital and acquired neurosensory hearing loss affects millions of people worldwide. An increasing body of evidence suggest that the primary auditory neurons degenerate due to noise exposure and aging more readily than sensory cells, and thus, auditory neurons are a primary target for regenerative therapy. A better understanding of the development and function of these neurons is the ultimate goal for long-term maintenance, regeneration, and stem cell replacement therapy. In this review, we provide an overview of the key molecular factors responsible for the function and neurogenesis of the primary auditory neurons, as well as a brief introduction to stem cell research focused on the replacement and generation of auditory neurons.

Keywords: auditory pathways; cochlea; genetic mutations; single-cell RNAseq; transcription factor.

Figure 1

Auditory neurons form the spiral ganglion in the cochlea and connect hair cells in the organ of Corti to cochlear nuclei in the brain stem. (A) Diagram shows innervation of the organ of Corti. Type I neurons extend radial fibers toward the inner hair cells (5–30 type I neurons innervate one inner hair cell), and type II neurons provide diffuse innervations to the outer hair cells and supporting cells within the cochlea. Efferent axons from superior olivary complex innervate the outer hair cells. (B) Whole-mount immunolabeling of the mouse cochlea shows outer hair cells (OHC, labeled by anti-prestin) and inner hair cells (IHC, labeled by anti-calretinin) forming the organ of Corti (OC). (C) Whole-mount immunostaining of the spiral ganglion (SG) neurons (anti-NeuN, a neuronal soma marker) shows the dendrites extending in the periphery to the organ of Corti (OC) and the axons traveling to the central nervous system (anti-acetylated alpha-tubulin-labeled nerve fibers). Scale bars, 50 μm. Confocal images (B) taken from [6] and (C) unpublished data. (D) Schematic drawing of synapses of molecular subtypes of type I neurons on the inner hair cell. Type I neurons are segregated into three distinct groups: types Ia (blue), Ib (magenta), and Ic (green-blue) of the spiral auditory neurons, which extend peripheral processes that are spatially segregated on the surface of the IHC. HS: Hoechst nuclear staining.

Figure 2

Tonotopic organization. (A) An illustration of the cochlea and its tonotopy across the frequency spectrum. High-frequency sounds maximally stimulate the base of the cochlea (red), whereas low-frequency sounds maximally stimulate the apex (green). (B) The tonotopic organization of the cochlear nucleus can be visualized by lipophilic dye tracing. Diagram of applications of different colored dyes (magenta, vestibular organs; red, base; and green, apex) to label distinct bundles of neuronal fibers of the auditory nerve projecting to the cochlear and vestibular nuclei. Representative image of triple-dye labeling from the cochlea shows the trajectory of low- (green from the apex) and high-frequency (red from the base) auditory nerve fibers and the tonotopic organization of the cochlear nucleus subdivisions, the anteroventral (AVCN) and dorsal cochlear nucleus (DCN), in mouse embryos at embryonic day 18.5. Image from Reference [6].

Figure 3

Morphogenesis of the inner ear ganglia. (A) Neurons forming cochleovestibular ganglion in the mouse embryo at E10.5 are visualized by the detection of Isl1 mRNA in situ hybridization. Image from Reference [41]. (B) NEUROD1⁺ delaminating neuroblasts detected in the proneurosensory epithelium of the otocyst. Confocal image with permission from Reference [39]. Scale bar, 100 μm. (C) The stages of inner ear morphogenesis are shown schematically from the otocyst to the mature three-dimensional structure. In parallel, the neurons delaminate to form first a cochlear-vestibular ganglion (CVG), followed by the gradual separation of the vestibular (VG, green) and spiral (SG, red) ganglia, which eventually innervate the vestibular and auditory sensory epithelia. cd, cochlear duct; HS, Hoechst nuclear staining; SVG, superior vestibular ganglion; and IVG, inferior vestibular ganglion.

Figure 4

Differential innervation phenotype in the cochlea as the effect of the elimination of transcription factors. (A) Whole-mount immunostaining with anti-Myo7a (a marker of hair cells, HCs) and anti-ß-tubulin (nerve fibers) antibodies shows the innervation and the formation of the sensory epithelium and hair cells in the mouse control cochlea at postnatal day 0. (B) Delayed deletion of the SRY (sex-determining region Y)-box 2 (Sox2) [41], (C) Neurod1 [6], and (D) double deletion of Neurod1 and Atoh1 [52] were generated by Isl1^Cre and result in abnormalities in the number of neurons, formation of sensory epithelium, and cochlear innervation. Scale bars, 50 μm. (D) With permission from Reference [52].

Figure 5

Stem cell applications. Mouse- and human-derived embryonic and induced pluripotent stem cells can be reprogramed in order to generate sensory cells and neurons for stem cell therapy and cochlear organoids for in vitro analyses (adapted from [80]). PSC: pluripotent stem cell.