Spontaneous firing activity in pre-hearing inner hair. The maturation of the mechanoelectrical transducer apparatus of auditory hair cells requires a critical period of pre-hearing spiking activity

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+ signalling in the non-sensory cells. It remains unknown whether both these Ca2+-dependent signalling in cochlea are required for the functional maturation of the IHCs. We found that spontaneous Ca2+ AP activity, but not Ca2+ signalling from the non-sensory cells, regulates the maturation of the stereociliary hair bundle in IHCs. We used a mouse model in which the potassium channel Kir2.1 is overexpressed exclusively in IHCs in vivo using doxycycline. Kir2.1 overexpression (Kir2.1-OE) 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 showed that several pathways, including those involved in morphogenesis, actin-filament based processes and Rho-GTPase signalling, were activated 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 period” during which spontaneous Ca2+ APs is required for proper hair bundle maturation. We showed that IHC maturation is finely regulated by their intrinsic electrical activity before sound-induced sensory driven activity.