Spontaneous firing activity in pre-hearing inner hair cells regulates the maturation of the mechanoelectrical transducer apparatus over a critical time-period

Sensory-independent Ca2+ spiking activity is crucial for the development of mammalian sensory systems. In the cochlea, spontaneous Ca2+ action potentials (APs) are present in the pre-hearing inner hair cells (IHCs), the frequency and pattern of which is modulated by ATP-dependent intercellular Ca2+ waves in the non-sensory cells. It remains unknown whether both these Ca2+-dependent signalling mechanisms in the cochlea are required for the functional maturation of the IHCs. We found that the intrinsic Ca2+ AP activity, but not that triggered by Ca2+ signalling in the non-sensory cells, regulates the maturation and maintenance of the stereociliary hair bundle in IHCs. We used a mouse model in which the potassium channel Kir2.1 is reversibly overexpressed in IHCs in vivo. Kir2.1 overexpression (Kir2.1-OE mice) prevented spontaneous Ca2+ spikes in IHCs, but not their modulation by ATP-induced signalling from non-sensory cells. Without spontaneous Ca2+ APs, mechanoelectrical transduction in IHCs disappeared prior the onset of hearing at P12 due to the progressive loss of stereocilia and their fusion. RNA-sequencing data showed that several pathways, including those involved in morphogenesis, actin-filament based processes, and Rho-GTPase signalling, were upregulated during the second postnatal week. By manipulating the in vivo expression of Kir2.1 channels, we identified the second post-natal week as the “critical point” after which spontaneous Ca2+ APs are required for proper hair bundle maintenance. We showed that the final stage of IHC maturation is finely regulated by their intrinsic electrical activity just before sound-induced sensory driven activity. The sensory epithelium (Kir2.1) from 4 mice were micro dissected in DNase free ice-cold PBS 1X and immediately snap frozen in liquid nitrogen.