The Cl--channel TMEM16A is involved in the generation of cochlear Ca2+ waves and promotes the refinement of auditory brainstem networks in mice

Abstract

Before hearing onset (postnatal day 12 in mice), inner hair cells (IHC) spontaneously fire action potentials thereby driving pre-sensory activity in the ascending auditory pathway. The rate of IHC action potential bursts is modulated by inner supporting cells (ISC) of Kölliker's organ through the activity of the Ca2+ activated Cl- channel TMEM16A (ANO1). Here we show that conditional deletion of Ano1 (Tmem16a) in mice disrupts Ca2+ waves within Kölliker's organ, reduces the burst firing activity and the frequency-selectivity of auditory brainstem neurons in the medial nucleus of the trapezoid body (MNTB), and also impairs the functional refinement of MNTB projections to the lateral superior olive (LSO). These results reveal the importance of the activity of Kölliker's organ for the refinement of central auditory connectivity. In addition, our study suggests involvement of TMEM16A in the propagation of Ca2+ waves, which may also apply to other tissues expressing TMEM16A.

Developmental spontaneous activity promotes formation of sensory domains, frequency tuning and proper gain in central auditory circuits

“…Similarly, SGNs in Tmem16a KO mice exhibit altered firing patterns, including near elimination of long burst events and prolongation of inter-spike intervals ( Wang et al, 2015 ). A recent report suggested that TMEM16A enhances ATP release through positive feedback mechanisms in the cochlea ( Maul et al, 2022 ); however, we observed no significant change in the frequency, amplitude, duration, or area of calcium transients in ISCs of Tmem16a cKO mice ( Figures S3A – S3D ; Video S1 ), in accordance with prior observations ( Wang et al, 2015 ). These results indicate that TMEM16A is required to induce periodic, coordinated activation of IHCs throughout the postnatal pre-hearing period, providing a means to selectively disrupt spontaneous activity in the auditory system during this crucial period of development.…”

“…A recent study by Maul et al (2022) used a similar approach to explore the role of spontaneous activity in auditory development by targeting TMEM16A. Genetic inactivation of Tmem16a resulted in a dramatic decrease in spontaneous action potential burst frequency and increased inter-spike intervals in the MNTB, similar to the firing patterns observed in SGNs from isolated Tmem16a KO cochleae ( Wang et al, 2015 ), but action potential firing was not eliminated, likely because of tonic IHC depolarization.…”

“…In the pre-hearing cochlea, supporting cells in Kölliker’s organ initiate spontaneous neural activity through release of ATP, engagement of purinergic P2RY1 receptors, and activation of TMEM16A. Pharmacologic or genetic disruption of this mechanism alters patterns of spontaneous activity in IHCs, SGNs, and neurons of the MNTB and IC, disrupting temporal firing patterns and reducing burst frequency ( Wang et al, 2015 ; Babola et al, 2020 ; Maul et al, 2022 ). However, silencing supporting cell activity reduced, but did not eliminate, the spontaneous activity of IHCs and SGNs or block correlated firing of central auditory neurons prior to hearing onset, suggesting that there are other mechanisms that may contribute to burst firing at this age.…”

“…As a result, IHC membrane potential is near threshold, allowing these cells to generate calcium spikes with minimal stimulus, leading to spurious, spatially uncoordinated activity. This emergent IHC activity may engage central auditory neurons ( Maul et al, 2022 ) but without the precise, spatially restricted coordination afforded by supporting cell-mediated excitation. When deprived of synaptic input, SGNs become hyperexcitable and respond directly to ISC potassium transients ( Babola et al, 2018 ).…”

“…Manipulations that disrupt spontaneous activity by targeting these cells and components during development also result in loss of trophic support, efferent silencing, neuronal degeneration, and deafness, limiting functional in vivo interrogation of the auditory pathway to define the precise roles of pre-hearing spontaneous activity ( Tierney et al, 1997 ; Mostafapour et al, 2000 ; McKay and Oleskevich, 2007 ; Noh et al, 2010 ; Clause et al, 2014 ; Tong et al, 2015 ). These studies suggest that even modest alterations in spontaneous activity can disrupt the tonotopic precision of connectivity and tuning properties of central auditory neurons ( Clause et al, 2014 , 2017 ; Müller et al, 2019 ; Maul et al, 2022 ).…”