The molecular mechanisms controlling spontaneous activity in the developing cochlea

Preceding the onset of hearing, inner hair cells (IHCs) in the developing cochlea exhibit spontaneous bursts of activity that propagate throughout nascent auditory centers of the brain. This sound-independent neuronal activity has been widely implicated in promoting neuronal survival and maturation, as well as growth and refinement of their connections to form circuits that can readily extract information from the external world and adapt to the environment. Previous work indicates that this activity is initiated by the release of adenosine triphosphate (ATP) from inner supporting cells (ISCs) within Kölliker’s organ, but the molecular mechanisms that enable this periodic activation of IHCs remain elusive. In this thesis, we combined cellular electrophysiology, molecular biology, Ca2+ imaging, and mouse genetics to determine the molecular mechanisms underlying the spontaneous activity of ISCs, IHCs, and spiral ganglion neurons (SGNs) within the developing cochlea. We found that ATP stimulates purinergic autoreceptors in ISCs, triggering Cl– efflux and crenation by opening TMEM16A Ca2+-activated Cl– channels. Release of Cl– from ISCs is accompanied by K+ efflux, causing transient elevation of extracellular K+ concentration and depolarization of IHCs near sites of ATP release. Genetic deletion of TMEM16A markedly reduced the spontaneous activity of IHCs and SGNs in the developing cochlea, and prevented ATP-dependent currents and crenation of ISCs. The synchronization of activities among nearby IHCs and SGNs was also weakened after deleting TMEM16A. These results indicate that epithelial-like supporting cells in the developing cochlea have adapted a pathway used for fluid secretion in other exocrine organs to induce periodic excitation of hair cells before the onset of hearing. Our work pinpointed a specific molecular target, TMEM16A, as the key regulator of the spontaneous activity during auditory development, and provided an animal model for studying the functional roles of spontaneous activity in the developing auditory circuits