Connexin-mediated signalling in non-sensory cells is crucial for the development of sensory inner hair cells in the mouse cochlea.

Mutations in the genes encoding for gap junction proteins connexin 26 (Cx26) and connexin 30 (Cx30) have been linked to syndromic and non-syndromic hearing loss in mice and humans. The release of ATP from connexin hemichannels in cochlear non-sensory cells has been proposed to be the main trigger for action potential (AP) activity in immature sensory inner hair cells (IHCs), which is crucial for the refinement of the developing auditory circuitry. Using connexin knockout mice, we show that IHCs fire spontaneous APs even in the absence of ATP-dependent intercellular Ca(2+) signalling in the non-sensory cells. However, this signalling from non-sensory cells was able to increase the intrinsic IHC firing frequency. We also found that connexin expression is key to IHC functional maturation. In Cx26 conditional knockout mice (Cx26(Sox10-Cre)) the maturation of IHCs, which normally occurs at around postnatal day 12, was partially prevented. Although Cx30 has been shown not to be required for hearing in young adult mice, IHCs from Cx30 knockout mice exhibited a comprehensive brake in their development, such that their basolateral membrane currents and synaptic machinery retain a pre-hearing phenotype. We propose that IHC functional differentiation into mature sensory receptors is initiated in the pre-hearing cochlea provided that the expression of either connexin reaches a threshold level. As such, connexins regulate one of the most crucial functional refinements in the mammalian cochlea, the disruption of which contributes to the deafness phenotype observed in mice and DFNB1 patients.

SIGNIFICANCE STATEMENT: The correct development and function of the mammalian cochlea relies not only on the sensory hair cells, but also on the surrounding non-sensory cells. Although the non-sensory cells have been largely implicated in the general homeostasis in the mature cochlea, their involvement in the initial functional differentiation of the sensory inner hair cells is less clear. Using mutant mouse models for the most common form of congenital deafness in humans, which are knockouts for the gap-junction channels connexin 26 and connexin 30 genes, we show that defects in non-sensory cells prevented the functional maturation of inner hair cells (IHCs). In connexin knockouts, IHCs remained stuck at a pre-hearing stage of development, and as such are unable to process sound information.