The molecular basis of spiral ganglion neurons : diversity and development

Abstract: Hearing, one of our main senses, allows us to socialize, listen and enjoy sounds around us. The critical transmitters of the sound information are the spiral ganglion neurons (SGNs); located in the cochlea, they transmit the auditory signals from the hair cells to the brain. This thesis aims to extend our present understanding of the diversity (study I and II) and the development (study III) of the SGNs. To contextualize, this thesis first reviews the relevant literature in the Introduction chapter, followed by the presentation of the significant findings. In studies I and II, we use single-cell RNA sequencing to analyze the molecular profiles of adult SGNs in mice. We identify three new subtypes of type I SGNs (Ia, Ib, and Ic) and new markers for type II neurons that we confirm with immunological and in situ hybridization labeling. We also correlate those new subtypes to previously known physiologically different subtypes of SGNs. Finally, we observe that those neuronal subtypes can already be identified soon after birth in mice. Results of study I and of previous research in the field are summarized in study II. In study III, we use single-cell RNA sequencing to analyze the molecular profiles of mouse embryonic SGNs during their early development. We observe the molecular diversification of the different SGN lineages, starting with an unspecialized population at E14.5, giving rise, through successive bifurcations, to different developmental trajectories leading to Ic neurons, then the type II neurons (E15.5-16.5), followed by Ib and Ia (E16.5-E17.5). The sequencing analysis also revealed the dynamic change of genes and gene regulatory networks of potential importance for the SGNs diversification, among which Neurod1 was identified as essential for the Ic pathway differentiation program. Altogether, the data included in this thesis add new insights into the crucial molecular aspects regulating the development and maturation of the SGNs and evidence regarding the existence of molecular types of SGNs.