Date Published: January 24, 2017
Publisher: Public Library of Science
Author(s): Koji Nishimura, Teppei Noda, Alain Dabdoub, Olivia Bermingham-McDonogh.
Primary auditory neurons (PANs) connect cochlear sensory hair cells in the mammalian inner ear to cochlear nucleus neurons in the brainstem. PANs develop from neuroblasts delaminated from the proneurosensory domain of the otocyst and keep maturing until the onset of hearing after birth. There are two types of PANs: type I, which innervate the inner hair cells (IHCs), and type II, which innervate the outer hair cells (OHCs). Glial cells surrounding these neurons originate from neural crest cells and migrate to the spiral ganglion. Several transcription factors are known to regulate the development and differentiation of PANs. Here we systematically examined the spatiotemporal expression of five transcription factors: Sox2, Sox10, Gata3, Mafb, and Prox1 from early delamination at embryonic day (E) 10.5 to adult. We found that Sox2 and Sox10 were initially expressed in the proneurosensory cells in the otocyst (E10.5). By E12.75 both Sox2 and Sox10 were downregulated in the developing PANs; however, Sox2 expression transiently increased in the neurons around birth. Furthermore, both Sox2 and Sox10 continued to be expressed in spiral ganglion glial cells. We also show that Gata3 and Prox1 were first expressed in all developing neurons, followed by a decrease in expression of Gata3 and Mafb in type I PANs and Prox1 in type II PANs as they matured. Moreover, we describe two subtypes of type II neurons based on Peripherin expression. These results suggest that Sox2, Gata3 and Prox1 play a role during neurogenesis as well as maturation of the PANs.
Primary auditory neurons (PANs), also known as spiral ganglion (SG) neurons, receive chemical signals from cochlear hair cells and transmit the information to the central cochlear nucleus (CN) in the brainstem (see review by Dabdoub and Fritzsch ). Neuronal precursors delaminate from the otocyst at the early stage of inner ear development and form PANs . A number of transcription factors that are involved in neurogenesis as well as specification of PANs have been identified: Neurog1 [3, 4], NeuroD1 , Gata3 [6, 7] and Sox2 [8–10]. There are two types of PANs. Type I cells comprise 95% of all PANs within the cochlear nerve and have bipolar neurites that connect a single inner hair cell (IHC) with the central CN. Type II PANs make up the remaining 5% of all PANs and have pseudo-unipolar neurites that connect multiple outer hair cells (OHCs) with the CN [11, 12]. The biological differences between type I and type II neurons have been reported [13–21] (also see review on type II PANs by Zhang and Coate ). More recently, it was argued that type II PANs should be nociceptors mediating auditory pain and do not drive the olivocochlear reflex [23, 24]; however, it remains unknown how they develop from a common neuroblast.
Sox2, which belongs to the high-mobility group (HMG) box transcription factors, is unique as it is the only transcription factor that is expressed in embryonic and adult stem cells as well as progenitor cells . Furthermore, adult stem cells expressing Sox2 originate from Sox2 expressing progenitor cells indicating that Sox2 initially plays a role in the development of progenitor cells and continues to be expressed in derivative adult tissues where it indicates stem cells . With regard to inner ear development, Sox2 is the earliest definitive marker of the prosensory domain  and remains expressed in the supporting cells and cells in the spiral ganglion at postnatal stages [8, 53–56]. We examined spatiotemporal expression of Sox2 and Sox10 in the developing cochlea focusing on cells in the spiral ganglion. Our results demonstrated Sox2 expression levels had two peaks at the delamination of neuroblasts from otocyst and around birth, in general agreement with the previously reported expression patterns of Sox2 in developing avian  and mouse  inner ear, and with lineage tracing studies of Sox2 . In summary, Sox2 was first expressed in delaminating neuroblasts and its downregulation is necessary for progression of neurogenesis by Neurogenin1 and NeuroD1 . Sox2 expression was later upregulated in PANs around birth and finally declined to undetectable levels by adult stages. Antagonistic interactions between Sox2 and bHLH transcription factors have been well documented [53, 57, 58], and might explain the upregulation of Sox2 in PANs around birth. For example, the transient downregulation of Neurog1 around birth  (http://goodrich.med.harvard.edu/microarray-data.html) could allow for the upregulation in Sox2 expression we observed.