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JARO: Journal of the Association for Research in Otolaryngology logoLink to JARO: Journal of the Association for Research in Otolaryngology
. 2021 Oct 6;22(6):601–608. doi: 10.1007/s10162-021-00808-0

Putting the Pieces Together: the Hair Cell Transduction Complex

Jeffrey R Holt 1,✉, Mélanie Tobin 2,3, Johannes Elferich 4, Eric Gouaux 4,5, Angela Ballesteros 6, Zhiqiang Yan 7, Zubair M Ahmed 8, Teresa Nicolson 9
PMCID: PMC8599550  PMID: 34617206

Abstract

Identification of the components of the mechanosensory transduction complex in hair cells has been a major research interest for many auditory and vestibular scientists and has attracted attention from outside the field. The past two decades have witnessed a number of significant advances with emergence of compelling evidence implicating at least a dozen distinct molecular components of the transduction machinery. Yet, how the pieces of this ensemble fit together and function in harmony to enable the senses of hearing and balance has not been clarified. The goal of this review is to summarize a 2021 symposium presented at the annual mid-winter meeting of the Association for Research in Otolaryngology. The symposium brought together the latest insights from within and beyond the field to examine individual components of the transduction complex and how these elements interact at molecular, structural, and biophysical levels to gate mechanosensitive channels and initiate sensory transduction in the inner ear. The review includes a brief historical background to set the stage for topics to follow that focus on structure, properties, and interactions of proteins such as CDH23, PCDH15, LHFPL5, TMIE, TMC1/2, and CIB2/3. We aim to present the diversity of ideas in this field and highlight emerging theories and concepts. This review will not only provide readers with a deeper appreciation of the components of the transduction apparatus and how they function together, but also bring to light areas of broad agreement, areas of scientific controversy, and opportunities for future scientific discovery.

Keywords: hair cell, sensory transduction, mechanotransduction, mechanosensory transduction, TMC1, TMC2, TMIE, TMHS, LHFPL5, PCDH15, CDH23, CIB2, CIB3, TOMPT, tip link, transduction channel

INTRODUCTION

Sensory hair cells of the auditory and vestibular systems are perhaps the most sensitive biological mechanosensors known. At the core of hair cell mechanosensitivity is a collection of 50–100 mechanically gated ion channel complexes that allow for detection of rapid hair bundle movements as small as few angstroms (Denk and Webb 1992). The notion of mechanically gated ion channels in hair cells first emerged from elegant work from Jim Hudspeth’s lab in the late 1970s (Hudspeth and Jacobs 1979; Corey and Hudspeth 1979). These initial biophysical descriptions of mechanosensitive ion channels sent the field searching for these elusive but critical molecules. Forty years hence a handful of molecules have been identified that mediate mechanosensory responses to touch, pain, and most recently hearing and balance. The sections below highlight some of these recent discoveries with attention focused on a chorus of molecules that mediate hair cell mechanosensitivity and how the molecular pieces fit together to function in harmony.

We begin with a review of the biophysical properties of tip links and how they vary tonotopically across the cochlea. Next, we examine the structural nature of the tip-link protein PCDH15 in complex with LHFPL5, a component of the transduction apparatus. We then turn to structural models of TMC proteins and consider how biophysical data support the models. Evidence that TMCs form ion channels when reconstituted in lipid membranes is discussed in the next section. Recent evidence showing that TMCs interact with CIB proteins in auditory and vestibular hair cells is presented. We conclude with a review of data that show how these transduction molecules, including TMIE, assemble in the cell body and traffic to hair bundles in zebrafish hair cells. Together, these paragraphs present the latest insights into the molecular components, structure, and function of the hair cell mechanosensory transduction complex.

Tip-Link Stiffness and Tension Gradients in the Cochlea

Vertebrate hearing relies on acute frequency discrimination over a broad range of sound frequencies. To cope with this requirement, auditory organs are endowed with a few thousand hair cells, each tuned to detect a characteristic frequency within the auditory range and spatially organized in a tonotopic map. In mammals, the characteristic frequency decreases monotonically from the base toward the apex of the cochlea. Thus, hair cells are maximally responsive to frequencies that span the tonotopic axis of the cochlea. However, the mechanism that sets the characteristic frequency of a hair cell is unclear.

Contributions of hair bundle properties to tuning in the cochlea have recently been revealed (Tobin et al. 2019). The morphological properties of the hair bundle—the mechanosensory antenna of the hair cell—vary systematically along the tonotopic axis of auditory organs (Wright 1984; Lim 1986; Roth and Bruns 1992; Tilney et al. 1992). In particular, hair bundles range in height from one to several microns from the base to the apex of the mammalian cochlea, respectively. Likewise, the number of stereocilia per hair bundle decreases along the tonotopic axis. These morphological gradients have long been proposed to be associated with mechanical gradients that may help tune the frequency response of the cochlea (Strelioff and Flock 1984).

Recently, Tobin et al. (2019) used mechanical measurements from single hair bundles in the apical half of an excised preparation of the rat cochlea, corresponding to characteristic frequencies between 1 and 15 kHz, to provide new insight into the mechanisms of tuning. They found hair-bundle stiffness of both inner and outer hair cells increased with characteristic frequency, with a steeper gradient in outer hair cells than in inner hair cells (Tobin et al. 2019).

The rotational stiffness of single stereocilia does not depend on the morphology of the hair cell, nor does it vary along the tonotopic axis. While the stiffness associated with single tip links is an intrinsic property that does not depend on hair bundle morphology, tip-link stiffness does vary along the gradient of tonotopic axis (Tobin et al. 2019). Thus, the observed gradients, at the level of the whole hair bundle, do not result from changes in hair-bundle morphology alone, but also depend on intrinsic gradients at the molecular and single tip-link levels.

Tension in the tip links has been estimated by monitoring hair-bundle movements evoked by tip-link disruption (Tobin et al. 2019). There is a steep gradient in total bundle tension for outer hair cells but only a weak gradient for inner hair cells. These gradients are not only due to gradients in the number of tip links within a hair bundle, since tension in single tip links also increases from the apex toward the base of the cochlea, which may affect tip-link stiffness due to strain stiffening (Bartsch et al. 2019). Interestingly, tension in the tip links depends on calcium, such that tension is higher at lower extracellular calcium concentrations. Thus, local and regional variation in endolymph calcium concentration may alter tip-link tension.

To summarize, in the apical region of the rat cochlea, hair cells with higher characteristic frequencies are endowed with hair bundles that are both stiffer and bear more tension. These gradients are not only due to changes in the morphology of the hair bundle but also result from changes in the intrinsic stiffness and tension of single tip links. Curiously, these mechanical gradients are steeper for OHCs than for IHCs. Altogether, these results demonstrate mechanical tuning of the transduction apparatus along the tonotopic axis of the cochlea (Tobin et al. 2019). The molecular and structural basis of tuning in the transduction apparatus remains to be determined.

Architecture and Composition of the Mechanosensory Transduction Complex

Structural information about the various hair cell transduction components may provide novel insight into the form and function of the mechanosensory transduction (MT) complex. Unfortunately, at present, there are no high-resolution, structural insights into the architecture of the hair cell MT complex. The lack of structural information is largely a consequence of the difficulty reconstituting the MT complex in a heterologous system, a prerequisite for most approaches of structure determination. In a study by Ge et al. (2018), systematic co-expression of the putative MT-components PCDH15, TMC, LHFPL5, and TMIE in HEK293 cells yielded a biochemically well-behaved complex only for co-expression of PCDH15 and LHFPL5. This result was in agreement with earlier studies suggesting that PCDH15 and LHFPL5 depend on each other for trafficking to the tips of stereocilia (Mahendrasingam et al. 2017; Xiong et al. 2012). By combining X-ray crystallography of the previously uncharacterized extracellular linker domain (EL; also, PICA, MAD12) with cryo-electron microscopy of the complex of LHFPL5 with two different truncated versions of PCDH15 (ΔEC1–EC7, ΔEC1–EC10), Ge et al. (2018) elucidated the structure of the integral membrane complex of PCDH15 and LHFPL5. In this structure (Fig. 1), PCDH15 and LHFPL5 subunit pairs are related by a twofold axis. The extracellular cadherin domains of PCDH15 are highly mobile with the entire extracellular domain tilting relative to the membrane by up to 55°. Furthermore, the cadherin chain bends up to 90° at the interface between EC9 and EC10, consistent with earlier crystal structures (Araya-Secchi et al. 2016). The cadherin chains are coupled to a rigid, twofold symmetric ‘collar’, formed by the EC11 and EL domains, proximal to the membrane bilayer. Within the membrane, LHFPL5 forms extensive interactions with the PCDH15 transmembrane helices and stabilizes them in an inverted “V” shape, and we speculate that such a pulling force on PCDH15 would separate the PCDH15 TMs on the extracellular side of the membrane. However, any consideration of the mechanism of force transduction into the membrane via PCDH15 will have to take the flexibility of PCDH15 into account.

Fig. 1.

Fig. 1

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Structure of the PCDH15/LHFPL5 complex at the bottom of the mammalian tip link. It remains unclear how this structure couples to TMCs (modeled here using the structure of TMEM16A) and the actin cytoskeleton

The structure of the PCDH15/LHFPL5 complex, together with structural studies of domains of the PCDH15 extracellular domain (Sotomayor et al. 2012; Dionne et al. 2018; Choudhary et al. 2020; De-la-Torre et al. 2018; Araya-Secchi et al. 2016), provides a structural framework for the lower end of the tip-link (Fig. 1). However, they do not address how PCDH15/LHFPL5 interacts with other components of the MT complex and most importantly do not address how force is delivered to the MT-channel pore. Further work may help illuminate the nature of the interaction between the PCDH15/LHFPL5 complex and the channel pore protein TMC (Pan et al. 2018; Jia et al. 2020; Maeda et al. 2014) and how other subunits, such as CIB2 (Giese et al. 2017) and TMIE (Cunningham et al. 2020; Pacentine and Nicolson 2019; Zhao et al. 2014), modulate or facilitate this interaction and perhaps how molecular gradients contribute to structural and biophysical gradients along the tonotopic axis of the cochlea.

TMC1 Structural Model Supports Its Role as a Pore-Forming Subunit of the MT Channel

Although the structure of the hair cell transduction channel has not yet been solved, the evolutionary relationship between the OSCA/TMEM63 mechanosensitive channels, the TMEM16 chloride channels and lipid scramblases, and the TMC proteins, together with structures of the TMEM16 and OSCA/TMEM63 proteins, have provided important structural information for TMC proteins (Brunner et al. 2014; Hahn et al. 2009; Jojoa-Cruz et al. 2018; Kunzelmann et al. 2016; Medrano-Soto et al. 2018). The structures of the OSCA/TMEM63 and TMEM16 proteins revealed a conserved dimeric architecture and the presence of a hydrophilic cavity in each protomer formed by transmembrane (TM) segments 4 to 7 (Brunner et al. 2014; Jojoa-Cruz et al. 2018). This cavity is thought to function as the permeation pathway in both TMEM16 and OSCA/TMEM63 channels. The presence of charged residues in four of the TMs (TM4–TM7) of TMC proteins prevented accurate prediction of their topology until their relationship with TMEM16 and OSCA/TMEM63 channels was identified.

Several TMEM16 and OSCA/TMEM63 structures are available and have been used as templates to model the conserved transmembrane core of TMC proteins (Brunner et al. 2014; Jojoa-Cruz et al. 2018; Paulino et al. 2017). Murine TMC1 models generated using the Mus musculus TMEM16A chloride channel in the calcium-bound (PDB ID: 5OYB) or calcium-free (PDB ID: 5OYG) state, the calcium-bound Nectria haematococca TMEM16 (nhTMEM16) lipid scramblase (PDB ID: 4WIS) or the Arabidopsis thaliana OSCA1.2 (PDB ID: 6MGV) were very similar (rmsd < 1.16 Å) with exception of the TM6 and TM4 conformation (Ballesteros et al. 2018). Consistently, a model of human TMC1 based on TMEM16A (Pan et al. 2018) revealed a similar structure to that of the mouse TMC1 model. A compelling TMC1 model generated using the structure of nhTMEM16 as a template contained 10 TM segments and a large and partially open mainly anionic cavity (Fig. 2). This TMC1 model allows for localization of residues known to alter the properties of MT channels and to cause deafness (Ballesteros et al. 2018; Pan et al. 2018). Mouse models carrying the semi-dominant deafness-causing mutations TMC1 D569N, T416K, or M412K give rise to MT channels with reduced calcium versus cesium permeability when compared to wild-type mice (Pan et al. 2013; Corns et al. 2016; Beurg et al. 2015, 2019, 2021). In addition to reduced calcium permeability, TMC1 D569N mice exhibit reduced MT currents, probably due to the reduced expression of this mutant, or perhaps due to reduced single-channel currents of mutations at this site (Pan et al. 2018). Two recessive deafness-causing mutations, TMC1 D528N and W554L, have been shown to reduce both the MT current and calcium versus cesium permeability. Interestingly, while the reduced MT current in TMC1 W554L is due to the reduced expression of this mutant, the D528N mutation strongly influences both MT current and calcium permeability. The severe effect of the D528N mutation in the MT channel properties is consistent with the predicted removal of a negatively charged residue at the narrowest area of the cavity (Fig. 2).

Fig. 2.

Fig. 2

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Mutation of residues in the TMC1 cavity alters the mechanosensory transduction channel properties. a Ribbon representation of the murine TMC1 model based on the nhTMEM16 structure. One protomer is colored in yellow, and the other in gray. The transmembrane segments are numbered, and the intracellular (IC) and extracellular (EC) sides are indicated. The approximated location of the lipid carbonyls of the plasma membrane as calculated by the PPM server is represented with two planes of gray spheres. b Surface representation of the TMC1 cavity formed by TM4–TM7 depicting deafness-causing mutations that alter MT channel properties in magenta (M412K, T416K, D528N, D569N, W554L), cysteine mutants altering channel properties after treatment with a water-soluble cysteine cross-linker reagent (MTSET) in yellow (N404, S408, G411, I440, T531, T532, T535), and cysteine cross-linking mutants with no effect on MT after MTSET treatment in green (D419, S571, I570, N573, V574, G596)

Comparable results were obtained when performing cysteine accessibility experiments in organ of Corti explants from TMC1 and TMC2 double knockout mice expressing TMC1 cysteine mutants after treatment with water-soluble methanethiosulfonate (MTS) thiol-reactive reagents (Pan et al. 2018). Seventeen mutations were examined in residues located at TM4–TM8, which are not accessible to water-soluble reagents unless they are part of a permeation pathway. Accessible cysteine residues will react with the MTS reagent and may alter the electrophysiological properties of the permeation pathway. Cysteine mutation of murine TMC1 residues D569 reduced the MT current, whereas mutations of G411, M412, D528, or T532 reduced the MT current and decreased the permeability of calcium versus cesium after treatment with thiol-reactive compounds. Similar reactions for cysteine substitutions at N404, S408, I440, T531, or T535 reduced calcium versus cesium permeability. Furthermore, as expected for residues that do not line the permeation pathway, cysteine substitutions at D419, S571, I570, N573, V574, and G596 did not alter the properties of the MT channel after treatment with thiol-reactive compounds (Fig. 2). Overall, these data suggest that the TMC1 protein could function as a channel and that the cavity formed by the TM4–TM6 segments may function as the permeation pathway, as in the related TMEM16 and OSCA/TMEM63 proteins, which supports the hypothesis that TMC1 is a pore-forming subunit of the inner ear MT channel complex.

TMC Channel Activity in Proteoliposomes

While structural models and biophysical characterization of hair cell transduction have provided intriguing evidence, until recently, the molecular identity of the hair cell mechanosensory transduction channel remained elusive. TRPA1 (Corey et al. 2004) and Piezo (Coste et al. 2010) proteins attracted attention as candidate transduction molecules, but mouse knockouts of these genes have normal hearing (Bautista et al. 2006; Wu et al. 2017). Other genes that cause human deafness such as TMHS/ LHFPL5, TMIE and TMC1/2 have remained as viable candidates.

Though TMCs 1 and 2 are predicted to be transmembrane proteins, electrophysiological recording of TMCs 1 and 2 to test the hypothesis that they form ion channels has been extremely difficult in heterologous systems, mainly because of the poor expression of TMC proteins in the cell membrane in heterologous cell cultures.

To overcome the technical challenges, Jia et al. (2020) screened 21 TMC proteins from different species. TMC1 from the green sea turtle Chelonia mydas (CmTMC1) and TMC2 from the budgerigar Melopsittacus undulatus (MuTMC2) both showed a high level of membrane expression. Following liposome reconstitution of CmTM1 and MuTMC2, both proteins exhibited robust channel activity. Successful reconstitution of TMC1 and TMC2 also allowed for tests of mechanosensitivity. By applying pressure to the reconstituted channels, TMC1 and TMC2 proteins were shown to respond to mechanical force directly. Further, deafness-related CmTMC1 mutants exhibited reduced or no ion channel activity. Thus, Jia et al. (2020) concluded that TMC1 and TMC2 proteins are pore-forming subunits of mechanosensitive ion channels. Together with previous studies that show TMC1/2 mutant mice have no mechanosensitive current (Kawashima et al. 2011) in hair cells, TMCs 1 and 2 localize to the tip of stereocilia where transduction occurs (Kurima et al. 2015), and TMC1 mutations alter the biophysical properties of hair cell MT (Pan et al. 2018), the liposome data support the proposal that TMC1 and TMC2 proteins are pore-forming subunits of mechanosensitive ion channels that mediate vertebrate hearing. Future work will be needed to determine whether mammalian TMC proteins also form ion channels and are mechanosensitive.

CIB Proteins and Their Role in Inner Ear Mechanosensory Transduction

In addition to TMCs, PCDH15, and LHFPL5, calcium and integrin-binding protein 2 (CIB2) plays a critical role in hair MT. CIB2 binds to the n-terminus of TMCs 1 and 2. Disruption of CIB2-TMC1/2 interaction with a single CIB2 missense mutation results in deafness and the loss of mechanosensory transduction in mammalian auditory hair cells, suggesting that CIB2 is also an essential component of the MT complex in these cells (Giese et al. 2017). Despite prominent loss of hearing and auditory hair cell MT, Cib2 knockout mice display no obvious indications of vestibular dysfunction, such as circling, hyperactivity, or head bobbing. The absence of an overt vestibular phenotype in Cib2 mutants indicates that CIB2 function is likely to be redundant in mouse vestibular hair cells. Using quantitative RT–PCR, the expression of the Cib gene family (Cib1-4), Giese et al. (2017) analyzed cochlear and vestibular sensory epithelia at P12. It was found that Cib2 is not the only CIB family member expressed in vestibular hair cells. Cib3 has an almost eightfold higher expression in the vestibular samples as compared to cochlear samples. Therefore, it is logical to suggest that hair cells may use functionally overlapping CIB2 and CIB3 protein expression to shape MT in different hair cell types, as is the case for TMC1 and TMC2 expression (Kawashima et al. 2011). Indeed, a recent study showed that similar to CIB2 and CIB3 can also interact with TMC1/2 in vitro (Tang et al. 2020), and thus both proteins may be functionally redundant for MT in vestibular hair cells.

To investigate the potential redundancy, Cib2:Cib3 double-mutant mice were generated. Cib3 knockout (Cib3KO) mice were generated using CRISPR/Cas9 technology, and their hearing function was assessed via auditory-evoked brainstem responses (ABRs). Preliminary data indicate the Cib3KO mice did not display hearing deficits and lacked obvious indications of vestibular dysfunction, such as circling, hyperactivity, or head bobbing. The Cib3KO mice performed well when exploratory behavior was tested and they had normal vestibule-ocular reflexes. Potential functional redundancy of CIB2 and CIB3 in the auditory and vestibular end organs was investigated by introducing the Cib3KO allele into mice with a Cib2KO background. Visual observation of the Cib2:Cib3 double-knockout mice indicated altered exploratory behavior in an open-field test, suggesting impairment of the vestibular system. To assess the implications of the Cib2:Cib3 double-knockout alleles on vestibular function, a series of tests was performed, including quantification of the vestibulo-ocular reflex (VOR), balance beam testing, and head movement analysis. Results of these studies confirmed vestibular dysfunction in Cib2:Cib3 double-mutant mice. Although there was no overt vestibular hair cell loss in Cib2:Cib3 double-knockout mice, the MT function in these mutants was impaired, assessed via channel permeating dye, FM1-43. These studies indicate that CIB2 and CIB3 have functionally redundant but compulsory roles in the vestibular hair cell MT complex. As such, a growing body of evidence now supports a role for CIBs 2 and 3 as molecular components of the hair cell MT complex.

Assembly of the Mechanosensory Transduction Complex

Although key molecular components of the transduction complex have now been identified, how they assemble and are transported to the tips of hair cell stereocilia is not well understood. Several of the components at the lower end of the tip link are integral membrane proteins, necessitating the need for trafficking through the secretory pathway to reach the apical plasma membrane. As described above, at least two membrane proteins, PCDH15 and LHFPL5, are bound together as a complex of four proteins with dimers of PCDH15 sandwiched between two LHFPL5 proteins (Ge et al. 2018). The number of copies of the TMCs and TMIE in the complex is not clear, but a recent study of purified TMC1 proteins suggests that the TMCs exist as multimers (Pan et al. 2018). As discussed above, the collective evidence also suggests that TMCs are pore-forming subunits of the mechanotransduction complex (Pan et al. 2018; Jia et al. 2020). Based on the resemblance of TMCs to TMEM16A channels (Ballesteros et al. 2018), which exist as dimers, and based on FRET interactions between fluorescently tagged TMC1 and TMC2 (Pan et al. 2018), native TMC proteins may interact with each other in vivo. Whether the entire membrane complex of all five components is assembled at the ER membrane and trafficked as a large particle to the plasma membrane is not known.

Studies of the highly conserved zebrafish orthologues suggest that there is a division between the trafficking pathways of the MT components (Fig. 3). Interestingly, the tip-link proteins Pcdh15a and Lhfpl5a are trafficked independently of Tmie and the Tmc1/2 proteins in zebrafish hair cells (Maeda et al. 2017; Erickson et al. 2020; Smith et al. 2020). Furthermore, there is a strong interplay between Tmie and Tmcs. The levels of Tmie protein greatly influence the abundance of the Tmc subunits in the hair bundle. Loss of Tmie results in loss of GFP-tagged Tmc1 and Tmc2b in stereocilia, whereas overexpression of Tmie strongly bolsters the level of Tmc1 and 2b in stereocilia (Pacentine and Nicolson 2019). The presence of Tmie within the mature bundle and the reduction of microphonic potentials due to mutations or chimeras near or within the second transmembrane domain suggest that it is a critical component of the complex (Pacentine and Nicolson 2019). In addition, the deafness gene, transmembrane-O-methyltransferase (tomt), is required for trafficking of the Tmcs into stereocilia (Erickson et al. 2017). A tagged version of the Tomt protein localizes to the Golgi apparatus, suggesting that modification or regulation of the Tmcs is critical at this point in the secretory pathway. Pcdh15a has been shown to form a protein–protein interaction with the N-termini of Tmcs (Maeda et al. 2014), and this interaction may occur either en route or upon arrival at the site of MT, thus bringing the complex together.

Fig. 3.

Fig. 3

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Cross-sectional view of the transmembrane domains of the mechanosensory transduction components in zebrafish stereocilia. The TEM micrograph shows a typical tip link and upper insertion plaque in two neighboring stereocilia in a zebrafish hair cell. The two independently trafficking protein complexes (i) Pcdh15a and the Lhfpl5a/b proteins and the (ii) Tmc/Tmie complex are indicated, along with the Golgi protein Tomt, which is critical for trafficking of the Tmc1/2 proteins to the hair bundle. Ten transmembrane domains are predicted for the Tmcs based on homology to Tmem16a channels (see sections above). The transmembrane domain of Pcdh15a, including internal and external residues, physically interacts with the N termini of the Tmc proteins (dashed arrow)

CONCLUSIONS

While the evidence is mounting that CDH23, PCDH15, LHFPL5, TMIE, TMC1/2, and CIB2/3 all contribute to mechanosensitivity in auditory and vestibular hair cells, precisely how these molecules are organized into a cohesive molecular assembly is still emerging. We are optimistic that the coming years will reveal the molecular structure and precise function of one of biology’s most exquisitely sensitive mechanotransduction complexes. As presenters of this ARO symposium and co-authors of this JARO review, we look forward to discussing progress on these fronts with readers of JARO and members of ARO at future ARO meetings.

Acknowledgements

M.T. is an alumnus of the Frontiers in Life Science PhD program of Université Paris Diderot and thanks the Fondation Agir pour l’Audition for a doctoral fellowship. E.G. is an investigator of the Howard Hughes Medical Institute.

Funding

This work was supported by NIH/NIDCD grant R01-DC013521 (J.R.H.), by the French National Agency for Research (ANR-11-BSV5 0011 and ANR-16-CE13-0015) and the Labex Celti-sphybio ANR-10-LABX-0038 (M.T.). A.B. was supported by the Intramural Research Program number NS002945 of the NINDS, NIH, Bethesda, MD, to Kenton J. Swartz. T.N. was supported by NIH/NIDCD grants R01-DC013572 and R01-DC013531.

Declarations

Conflict of Interest

J.R.H. holds patents on TMC1 gene therapy and is an advisor to several biotech firms focused on inner-ear therapies. The authors report no other conflicts.

Footnotes

Publisher's Note

Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.

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