Neural Stem/Progenitor Cell Properties of Glial Cells in the Adult Auditory Nerve [development]

Spiral ganglion neurons (SGNs) and the associated components of the auditory nerve are primary carriers of auditory information from hair cells to the brain. Loss of SGNs occurs with many pathological conditions, resulting in permanent sensorineural hearing loss. Neural stem/progenitors (NSPs) have been well-characterized in several locations of adult brain and retina. However, it is unclear whether NSPs are present in the adult auditory nerve. Here we examined the self-renewal potential of the adult auditory nerve using ouabain application as a well-established mouse model of acute SGN injury. The observed increase in cell proliferation, alteration in enchromatin/heterochromatin ratio and down-regulation of histone deacetylase expression in glial cells suggest that the quiescent glial cells convert to an activated state after SGN degeneration. This was further confirmed by global gene expression analysis of injured auditory nerves, which showed up-regulation of numerous neurogenesis- and/or development-associated genes shortly after ouabain exposure. These genes include molecular markers commonly used for the identification of NSPs. Under a strict culture regimen, auditory nerve-derived cells of adult mouse ears gave rise to neurospheres, suggesting that multipotent NSPs are present in adult cochlear nerve. Neurosphere assays on Sox2 transgenic mice revealed that Sox2+ glial cells are the source for NSPs. Our data also showed that acute injury or hypoxia enhances neurosphere formation. Taken together, our study revealed that glial cells of adult cochlea exhibit several NSP characteristics, and hence these mature non-neuronal cells may be important targets for promoting self-repair of degenerative auditory nerves. Analysis was conducted on auditory nerve samples from stages encompassing maturation and onset of hearing. Cochleas were collected from euthanized mice (CBA/CaJ) at perinatal (P) stages P0, P3, P7, P10, P14 and P21. Cochleas underwent microdissection to remove the outer bony cochlear shell and cochlear lateral wall, thus preserving the modiolus portion of cochlea which contains mainly the auditory nerve. Paired cochleas (i.e., from left and right ears) from each mouse were pooled to make individual samples. All sample types were done in experimental duplicate (n=2).