The effects of Tmc1 Beethoven mutation on mechanotransducer channel function in cochlear hair cells

Identification of Druggable Binding Sites and Small Molecules as Modulators of TMC1

Our ability to hear and maintain balance relies on the proper functioning of inner ear sensory hair cells, which translate mechanical stimuli into electrical signals via mechano-electrical transducer (MET) channels, composed of TMC1/2 proteins. However, the therapeutic use of ototoxic drugs, such as aminoglycosides and cisplatin, which can enter hair cells through MET channels, often leads to profound auditory and vestibular dysfunction. Despite extensive research on otoprotective compounds targeting MET channels, our understanding of how small-molecule modulators interact with these channels remains limited, hampering the discovery of novel drugs. Here, we propose a structure-based screening approach, integrating 3D-pharmacophore modeling, molecular dynamics simulations of the TMC1+CIB2+TMIE complex, and experimental validation. Our pipeline successfully identified several novel compounds and FDA-approved drugs that reduced dye uptake in cultured cochlear explants, indicating MET-modulation activity. Simulations, molecular docking and free-energy estimations allowed us to identify three potential drug-binding sites within the channel pore, phospholipids, key amino acids involved in modulator interactions, and TMIE as a flexible component of the MET complex. We also identified shared ligand-binding features between TMC and structurally related TMEM16 proteins, providing novel insights into their distinct inhibition. Our pipeline offers a broad application for discovering modulators for mechanosensitive ion channels.

Human TMC1 and TMC2 are mechanically gated ion channels

Mammalian transmembrane channel-like proteins 1 and 2 (TMC1 and TMC2) have emerged as very promising candidate mechanotransduction channels in hair cells. However, controversy persists because the heterogeneously expressed TMC1/2 in cultured cells lack evidence of mechanical gating, primarily due to their absence from the plasma membrane. By employing domain swapping with OSCA1.1 and subsequent point mutations, we successfully identified membrane-localized mouse TMC1/2 mutants, demonstrating that they are mechanically gated in heterologous cells. Further, whole-genome CRISPRi screening enabled wild-type human TMC1/2 localization in the plasma membrane, where they responded robustly to poking stimuli. In addition, wild-type human TMC1/2 showed stretch-activated currents and clear single-channel current activities. Deafness-related TMC1 mutations altered the reversal potential of TMC1, indicating that TMC1/2 are pore-forming mechanotransduction channels. In summary, our study provides evidence that human TMC1/2 are pore-forming, mechanically activated ion channels, supporting their roles as mechanotransduction channels in hair cells.

Sequence variations and accessory proteins adapt TMC functions to distinct sensory modalities

Transmembrane channel-like (TMC) proteins are expressed throughout the animal kingdom and are thought to encode components of ion channels. Mammals express eight TMCs (mTMC1-8), two of which (mTMC1 and mTMC2) are subunits of mechanotransduction channels. C. elegans expresses two TMCs (TMC-1 and TMC-2), which mediate mechanosensation, egg laying, and alkaline sensing. The mechanisms by which nematode TMCs contribute to such diverse physiological processes and their functional relationship to mammalian mTMCs is unclear. Here, we show that association with accessory proteins tunes nematode TMC-1 to divergent sensory functions. In addition, distinct TMC-1 domains enable touch and alkaline sensing. Strikingly, these domains are segregated in mammals between mTMC1 and mTMC3. Consistent with these findings, mammalian mTMC1 can mediate mechanosensation in nematodes, while mTMC3 can mediate alkaline sensation. We conclude that sequence diversification and association with accessory proteins has led to the emergence of TMC protein complexes with diverse properties and physiological functions.

Stereocilia fusion pathology in the cochlear outer hair cells at the nanoscale level

The hair bundle of cochlear hair cells comprises specialized microvilli, the stereocilia, which fulfil the role of mechanotransduction. Genetic defects and environmental noise challenge the maintenance of hair bundle structure, critically contributing to age-related hearing loss. Stereocilia fusion is a major component of the hair bundle pathology in mature hair cells, but its role in hearing loss and its molecular basis are poorly understood. Here, we utilized super-resolution expansion microscopy to examine the molecular anatomy of outer hair cell stereocilia fusion in mouse models of age-related hearing loss, heightened endoplasmic reticulum stress and prolonged noise exposure. Prominent stereocilia fusion in our model of heightened endoplasmic reticulum stress, Manf (Mesencephalic astrocyte-derived neurotrophic factor)-inactivated mice in a background with Cadherin 23 missense mutation, impaired mechanotransduction and calcium balance in stereocilia. This was indicated by reduced FM1-43 dye uptake through the mechanotransduction channels, reduced neuroplastin/PMCA2 expression and increased expression of the calcium buffer oncomodulin inside stereocilia. Sparse BAIAP2L2 and myosin 7a expression was retained in the fused stereocilia but mislocalized away from their functional sites at the tips. These hair bundle abnormalities preceded cell soma degeneration, suggesting a sequela from stereociliary molecular perturbations to cell death signalling. In the age-related hearing loss and noise-exposure models, stereocilia fusion was more restricted within the bundles, yet both models exhibited oncomodulin upregulation at the fusion sites, implying perturbed calcium homeostasis. We conclude that stereocilia fusion is linked with the failure to maintain cellular proteostasis and with disturbances in stereociliary calcium balance. KEY POINTS: Stereocilia fusion is a hair cell pathology causing hearing loss. Inactivation of Manf, a component of the endoplasmic reticulum proteostasis machinery, has a cell-intrinsic mode of action in triggering outer hair cell stereocilia fusion and the death of these cells. The genetic background with Cadherin 23 missense mutation contributes to the high susceptibility of outer hair cells to stereocilia fusion, evidenced in Manf-inactivated mice and in the mouse models of early-onset hearing loss and noise exposure. Endoplasmic reticulum stress feeds to outer hair cell stereocilia bundle pathology and impairs the molecular anatomy of calcium regulation. The maintenance of the outer hair cell stereocilia bundle cohesion is challenged by intrinsic and extrinsic stressors, and understanding the underlying mechanisms will probably benefit the development of interventions to promote hearing health.

Sensory transduction in auditory hair cells-PIEZOs can't touch this

In this Viewpoint, Holt, Fettiplace, and Müller weigh the evidence supporting a role for PIEZO and TMC channels in mechanosensory transduction in inner ear hair cells.

The Piezo channel is a mechano-sensitive complex component in the mammalian inner ear hair cell

The inner ear is the hub where hair cells (HCs) transduce sound, gravity, and head acceleration stimuli to the brain. Hearing and balance rely on mechanosensation, the fastest sensory signals transmitted to the brain. The mechanoelectrical transducer (MET) channel is the entryway for the sound-balance-brain interface, but the channel-complex composition is not entirely known. Here, we report that the mouse utilizes Piezo1 (Pz1) and Piezo2 (Pz2) isoforms as MET-complex components. The Pz channels, expressed in HC stereocilia, and cell lines are co-localized and co-assembled with MET complex partners. Mice expressing non-functional Pz1 and Pz2 at the ROSA26 locus have impaired auditory and vestibular traits that can only be explained if the Pzs are integral to the MET complex. We suggest that Pz subunits constitute part of the MET complex and that interactions with other MET complex components yield functional MET units to generate HC MET currents.

Nonextensive realizations in interacting ion channels: Implications for mechano-electrical transducer mechanisms

We propose a theoretical model to investigate the thermodynamics of single and coupled two-state ion channels, associated with mechanoelectrical transduction (MET) and hair cell biophysics. The modeling was based on the Tsallis nonextensive statistical mechanics. The choice for a nonextensive framework in modeling ion channels is encouraged on the fact that we take into account the presence of interactions or long-range correlations in the dynamics of single and coupled ion channels. However, the basic assumptions that support Boltzmann-Gibbs statistics, traditionally used to model ion channel dynamics, state that the system is formed by independent or weakly interacting elements. Despite being well studied in many biological systems, the literature has not yet addressed the study of both entropy and mutual information related to isolated or physically interacting pairs of MET channels. Inspired by hair cell biophysics, we show how the presence of nonextensivity, or subadditivity and superadditivity modulates the nonextensive entropy and mutual information as functions of stereocilia displacements. We also observe that the magnitude of the interaction between the two channels, given by a nonextensive parameter, influences the amplitude of the nonextensive joint entropy and mutual information as functions of the hair cell displacements. Finally, we show how nonextensivity regulates the current versus displacement curve for a single and a pair of interacting two-state channels. The present findings shed light on the thermodynamic process involved in the molecular mechanisms of the auditory system.

Mechanosensitive membrane proteins: Usual and unusual suspects in mediating mechanotransduction

This Viewpoint, which accompanies a Special Issue focusing on membrane mechanosensors, discusses unifying and unique features of both established and emerging mechanosensitive (MS) membrane proteins, their distribution across protein families and phyla, and current and future challenges in the study of these important proteins and their partners. MS membrane proteins are essential for tissue development, cellular motion, osmotic homeostasis, and sensing external and self-generated mechanical cues like those responsible for touch and proprioception. Though researchers' attention and this Viewpoint focus on a few famous ion channels that are considered the usual suspects as MS mechanosensors, we also discuss some of the more unusual suspects, such as G-protein coupled receptors. As the field continues to grow, so too will the list of proteins suspected to function as mechanosensors and the diversity of known MS membrane proteins.

Sensing sound: Cellular specializations and molecular force sensors

Organisms of all phyla express mechanosensitive ion channels with a wide range of physiological functions. In recent years, several classes of mechanically gated ion channels have been identified. Some of these ion channels are intrinsically mechanosensitive. Others depend on accessory proteins to regulate their response to mechanical force. The mechanotransduction machinery of cochlear hair cells provides a particularly striking example of a complex force-sensing machine. This molecular ensemble is embedded into a specialized cellular compartment that is crucial for its function. Notably, mechanotransduction channels of cochlear hair cells are not only critical for auditory perception. They also shape their cellular environment and regulate the development of auditory circuitry. Here, we summarize recent discoveries that have shed light on the composition of the mechanotransduction machinery of cochlear hair cells and how this machinery contributes to the development and function of the auditory system.

The conductance and organization of the TMC1-containing mechanotransducer channel complex in auditory hair cells

We studied the role of TMC1 as the central component of the hair cell mechanotransducer (MET) channel by characterizing transduction in mice harboring mutations in the pore region. All Tmc1 mutations reduced the Ca²⁺ influx into the hair bundle. Two mutations (Tmc1 p.D528N or Tmc1 p.E520Q) also decreased channel conductance and two (Tmc1 p. D569N or Tmc1 p.W554L) lowered expression. These mutations endorse TMC1 as the pore of the MET channel. The MET channel also contains accessory subunits, LHFPL5 and TMIE. MET currents were small in Lhfpl5 or Tmie knockout mice. Nevertheless, MET channels could still be activated by hair bundle displacement; single-channel conductance was unaffected in Lhfpl5^−/− but reduced in Tmie^−/−, suggesting TMIE likely contributes to the pore.

Transmembrane channel-like protein 1 (TMC1) is thought to form the ion-conducting pore of the mechanoelectrical transducer (MET) channel in auditory hair cells. Using single-channel analysis and ionic permeability measurements, we characterized six missense mutations in the purported pore region of mouse TMC1. All mutations reduced the Ca²⁺ permeability of the MET channel, triggering hair cell apoptosis and deafness. In addition, Tmc1 p.E520Q and Tmc1 p.D528N reduced channel conductance, whereas Tmc1 p.W554L and Tmc1 p.D569N lowered channel expression without affecting the conductance. Tmc1 p.M412K and Tmc1 p.T416K reduced only the Ca²⁺ permeability. The consequences of these mutations endorse TMC1 as the pore of the MET channel. The accessory subunits, LHFPL5 and TMIE, are thought to be involved in targeting TMC1 to the tips of the stereocilia. We found sufficient expression of TMC1 in outer hair cells of Lhfpl5 and Tmie knockout mice to determine the properties of the channels, which could still be gated by hair bundle displacement. Single-channel conductance was unaffected in Lhfpl5^−/− but was reduced in Tmie^−/−, implying TMIE very likely contributes to the pore. Both the working range and half-saturation point of the residual MET current in Lhfpl5^−/− were substantially increased, suggesting that LHFPL5 is part of the mechanical coupling between the tip-link and the MET channel. Based on counts of numbers of stereocilia per bundle, we estimate that each PCDH15 and LHFPL5 monomer may contact two channels irrespective of location.

Regulation of membrane homeostasis by TMC1 mechanoelectrical transduction channels is essential for hearing

The mechanoelectrical transduction (MET) channel in auditory hair cells converts sound into electrical signals, enabling hearing. Transmembrane-like channel 1 and 2 (TMC1 and TMC2) are implicated in forming the pore of the MET channel. Here, we demonstrate that inhibition of MET channels, breakage of the tip links required for MET, or buffering of intracellular Ca^... induces pronounced phosphatidylserine externalization, membrane blebbing, and ectosome release at the hair cell sensory organelle, culminating in the loss of TMC1. Membrane homeostasis triggered by MET channel inhibition requires Tmc1 but not Tmc2, and three deafness-causing mutations in Tmc1 cause constitutive phosphatidylserine externalization that correlates with deafness phenotype. Our results suggest that, in addition to forming the pore of the MET channel, TMC1 is a critical regulator of membrane homeostasis in hair cells, and that Tmc1-related hearing loss may involve alterations in membrane homeostasis.

Mechanical gating of the auditory transduction channel TMC1 involves the fourth and sixth transmembrane helices

The transmembrane (TM) channel-like 1 (TMC1) and TMC2 proteins play a central role in auditory transduction, forming ion channels that convert sound into electrical signals. However, the molecular mechanism of their gating remains unknown. Here, using predicted structural models as a guide, we probed the effects of 12 mutations on the mechanical gating of the transduction currents in native hair cells of Tmc1/2-null mice expressing virally introduced TMC1 variants. Whole-cell electrophysiological recordings revealed that mutations within the pore-lining TM4 and TM6 helices modified gating, reducing the force sensitivity or shifting the open probability of the channels, or both. For some of the mutants, these changes were accompanied by a change in single-channel conductance. Our observations are in line with a model wherein conformational changes in the TM4 and TM6 helices are involved in the mechanical gating of the transduction channel.

Physiological changes throughout an insect ear due to age and noise - a longitudinal study

Hearing loss is not unique to humans and is experienced by all animals in the face of wild and eclectic differences in ear morphology. Here, we exploited the high throughput and accessible tympanal ear of the desert locust, Schistocerca gregaria to rigorously quantify changes in the auditory system due to noise exposure and age. In this exploratory study, we analyzed tympanal displacements, morphology of the auditory Müller’s organ and measured activity of the auditory nerve, the transduction current, and electrophysiological properties of individual auditory receptors. This work shows that hearing loss manifests as a complex disorder due to differential effects of age and noise on several processes and cell types within the ear. The “middle-aged deafness” pattern of hearing loss found in locusts mirrors that found for humans exposed to noise early in their life suggesting a fundamental interaction of the use of an auditory system (noise) and its aging.

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Locusts routinely exposed to noise follow same pattern of hearing loss as humans

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Parts of the auditory system are affected by noise, age, or both noise and age

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Hearing loss is a multifaceted disorder caused by defects in distinct ear processes

Mechanotransduction in mammalian sensory hair cells

In the inner ear, the auditory and vestibular systems detect and translate sensory information regarding sound and balance. The sensory cells that transform mechanical input into an electrical signal in these systems are called hair cells. A specialized organelle on the apical surface of hair cells called the hair bundle detects mechanical signals. Displacement of the hair bundle causes mechanotransduction channels to open. The morphology and organization of the hair bundle, as well as the properties and characteristics of the mechanotransduction process, differ between the different hair cell types in the auditory and vestibular systems. These differences likely contribute to maximizing the transduction of specific signals in each system. This review will discuss the molecules essential for mechanotransduction and the properties of the mechanotransduction process, focusing our attention on recent data and differences between the auditory and vestibular systems.

Putting the Pieces Together: the Hair Cell Transduction Complex

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.

Sensory transduction is required for normal development and maturation of cochlear inner hair cell synapses

Acoustic overexposure and aging can damage auditory synapses in the inner ear by a process known as synaptopathy. These insults may also damage hair bundles and the sensory transduction apparatus in auditory hair cells. However, a connection between sensory transduction and synaptopathy has not been established. To evaluate potential contributions of sensory transduction to synapse formation and development, we assessed inner hair cell synapses in several genetic models of dysfunctional sensory transduction, including mice lacking transmembrane channel-like (Tmc) 1, Tmc2, or both, in Beethoven mice which carry a dominant Tmc1 mutation and in Spinner mice which carry a recessive mutation in transmembrane inner ear (Tmie). Our analyses reveal loss of synapses in the absence of sensory transduction and preservation of synapses in Tmc1-null mice following restoration of sensory transduction via Tmc1 gene therapy. These results provide insight into the requirement of sensory transduction for hair cell synapse development and maturation.

The speed of the hair cell mechanotransducer channel revealed by fluctuation analysis

Although mechanoelectrical transducer (MET) channels have been extensively studied, uncertainty persists about their molecular architecture and single-channel conductance. We made electrical measurements from mouse cochlear outer hair cells (OHCs) to reexamine the MET channel conductance comparing two different methods. Analysis of fluctuations in the macroscopic currents showed that the channel conductance in apical OHCs determined from nonstationary noise analysis was about half that of single-channel events recorded after tip link destruction. We hypothesized that this difference reflects a bandwidth limitation in the noise analysis, which we tested by simulations of stochastic fluctuations in modeled channels. Modeling indicated that the unitary conductance depended on the relative values of the channel activation time constant and the applied low-pass filter frequency. The modeling enabled the activation time constant of the channel to be estimated for the first time, yielding a value of only a few microseconds. We found that the channel conductance, assayed with both noise and recording of single-channel events, was reduced by a third in a new deafness mutant, Tmc1 p.D528N. Our results indicate that noise analysis is likely to underestimate MET channel amplitude, which is better characterized from recordings of single-channel events.

Fluid Jet Stimulation of Auditory Hair Bundles Reveal Spatial Non-uniformities and Two Viscoelastic-Like Mechanisms

Hair cell mechanosensitivity resides in the sensory hair bundle, an apical protrusion of actin-filled stereocilia arranged in a staircase pattern. Hair bundle deflection activates mechano-electric transduction (MET) ion channels located near the tops of the shorter rows of stereocilia. The elicited macroscopic current is shaped by the hair bundle motion so that the mode of stimulation greatly influences the cell’s output. We present data quantifying the displacement of the whole outer hair cell bundle using high-speed imaging when stimulated with a fluid jet. We find a spatially non-uniform stimulation that results in splaying, where the hair bundle expands apart. Based on modeling, the splaying is predominantly due to fluid dynamics with a small contribution from hair bundle architecture. Additionally, in response to stimulation, the hair bundle exhibited a rapid motion followed by a slower motion in the same direction (creep) that is described by a double exponential process. The creep is consistent with originating from a linear passive system that can be modeled using two viscoelastic processes. These viscoelastic mechanisms are integral to describing the mechanics of the mammalian hair bundle.

Tmc proteins are essential for zebrafish hearing where Tmc1 is not obligatory

Perception of sound is initiated by mechanically gated ion channels at the tips of stereocilia. Mature mammalian auditory hair cells require transmembrane channel-like 1 (TMC1) for mechanotransduction, and mutations of the cognate genetic sequences result in dominant or recessive heritable deafness forms in humans and mice. In contrast, zebrafish lateral line hair cells, which detect water motion, require Tmc2a and Tmc2b. Here, we use standard and multiplex genome editing in conjunction with functional and behavioral assays to determine the reliance of zebrafish hearing and vestibular organs on Tmc proteins. Surprisingly, our approach using multiple mutant alleles demonstrates that hearing in zebrafish is not dependent on Tmc1, nor is it fully dependent on Tmc2a and Tmc2b. Hearing however is absent in triple-mutant zebrafish that lack Tmc1, Tmc2a and Tmc2b. These outcomes reveal a striking resemblance of Tmc protein reliance in the vestibular sensory epithelia of mammals to the maculae of zebrafish. Moreover, our findings disclose a logic of Tmc use where hearing depends on a complement of Tmc proteins beyond those employed to sense water motion.

CIB2 and CIB3 are auxiliary subunits of the mechanotransduction channel of hair cells

CIB2 is a Ca2+- and Mg2+-binding protein essential for mechanoelectrical transduction (MET) by cochlear hair cells, but not by vestibular hair cells that co-express CIB2 and CIB3. Here, we show that in cochlear hair cells, CIB3 can functionally substitute for CIB2. Using X-ray crystallography, we demonstrate that CIB2 and CIB3 are structurally similar to KChIP proteins, auxiliary subunits of voltage-gated Kv4 channels. CIB2 and CIB3 bind to TMC1/2 through a domain in TMC1/2 flanked by transmembrane domains 2 and 3. The co-crystal structure of the CIB-binding domain in TMC1 with CIB3 reveals that interactions are mediated through a conserved CIB hydrophobic groove, similar to KChIP1 binding of Kv4. Functional studies in mice show that CIB2 regulates TMC1/2 localization and function in hair cells, processes that are affected by deafness-causing CIB2 mutations. We conclude that CIB2 and CIB3 are MET channel auxiliary subunits with striking similarity to Kv4 channel auxiliary subunits.

Mechanisms in cochlear hair cell mechano-electrical transduction for acquisition of sound frequency and intensity

Sound signals are acquired and digitized in the cochlea by the hair cells that further transmit the coded information to the central auditory pathways. Any defect in hair cell function may induce problems in the auditory system and hearing-based brain function. In the past 2 decades, our understanding of auditory transduction has been substantially deepened because of advances in molecular, structural, and functional studies. Results from these experiments can be perfectly embedded in the previously established profile from anatomical, histological, genetic, and biophysical research. This review aims to summarize the progress on the molecular and cellular mechanisms of the mechano-electrical transduction (MET) channel in the cochlear hair cells, which is involved in the acquisition of sound frequency and intensity-the two major parameters of an acoustic cue. We also discuss recent studies on TMC1, the molecule likely to form the MET channel pore.

The Mechanosensory Transduction Machinery in Inner Ear Hair Cells

Sound-induced mechanical stimuli are detected by elaborate mechanosensory transduction (MT) machinery in highly specialized hair cells of the inner ear. Genetic studies of inherited deafness in the past decades have uncovered several molecular constituents of the MT complex, and intense debate has surrounded the molecular identity of the pore-forming subunits. How the MT components function in concert in response to physical stimulation is not fully understood. In this review, we summarize and discuss multiple lines of evidence supporting the hypothesis that transmembrane channel-like 1 is a long-sought MT channel subunit. We also review specific roles of other components of the MT complex, including protocadherin 15, cadherin 23, lipoma HMGIC fusion partner-like 5, transmembrane inner ear, calcium and integrin-binding family member 2, and ankyrins. Based on these recent advances, we propose a unifying theory of hair cell MT that may reconcile most of the functional discoveries obtained to date. Finally, we discuss key questions that need to be addressed for a comprehensive understanding of hair cell MT at molecular and atomic levels.

Expression of a membrane-targeted fluorescent reporter disrupts auditory hair cell mechanoelectrical transduction and causes profound deafness

The reporter mT/mG mice expressing a membrane-targeted fluorescent protein are becoming widely used to study the auditory and vestibular system due to its versatility. Here we show that high expression levels of the fluorescent mtdTomato reporter affect the function of the sensory hair cells and the auditory performance of mT/mG transgenic mice. Auditory brainstem responses and distortion product otoacoustic emissions revealed that adult mT/mG homozygous mice are profoundly deaf, whereas heterozygous mice present high frequency loss. We explore whether this line would be useful for studying and visualizing the membrane of auditory hair cells by airyscan super-resolution confocal microscopy. Membrane localization of the reporter was observed in hair cells of the cochlea, facilitating imaging of both cell bodies and stereocilia bundles without altering cellular architecture or the expression of the integral membrane motor protein prestin. Remarkably, hair cells from mT/mG homozygous mice failed to uptake the FM1-43 dye and to locate TMC1 at the stereocilia, indicating defective mechanotransduction machinery. Our work emphasizes that precautions must be considered when working with reporter mice and highlights the potential role of the cellular membrane in maintaining functional hair cells and ensuring proper hearing.

New Tmc1 Deafness Mutations Impact Mechanotransduction in Auditory Hair Cells

Transmembrane channel-like protein isoform 1 (TMC1) is a major component of the mechano-electrical transducer (MET) channel in cochlear hair cells and is subject to numerous mutations causing deafness. We report a new dominant human deafness mutation, TMC1 p.T422K, and have characterized the homologous mouse mutant, Tmc1 p.T416K, which caused deafness and outer hair cell (OHC) loss by the fourth postnatal week. MET channels showed decreased Ca²⁺ permeability and resting open probability, but no change in single-channel conductance or expression. Three adjacent deafness mutations are TMC1 p.L416R, p.G417R, and p.M418K, the last homologous to the mouse Beethoven that exhibits similar channel effects. All substitute a positive for a neutral residue, which could produce charge screening in the channel pore or influence binding of an accessory subunit. Channel properties were compared in mice of both sexes between dominant (Tmc1 p.T416K, Tmc1 p.D569N) and recessive (Tmc1 p.W554L, Tmc1 p.D528N) mutations of residues near the putative pore of the channel. Tmc1 p.W554L and p.D569N exhibit reduced maximum current with no effect on single-channel conductance, implying a smaller number of channels transported to the stereociliary tips; this may stem from impaired TMC1 binding to LHFPL5. Tmc1 p.D528N, located in the pore's narrowest region, uniquely caused large reductions in MET channel conductance and block by dihydrostreptomycin (DHS). For Tmc1 p.T416K and Tmc1 p.D528N, transduction loss occurred between P15 and P20. We propose two mechanisms linking channel mutations and deafness: decreased Ca²⁺ permeability, common to all mutants, and decreased resting open probability in low Ca²⁺, confined to dominant mutations.

SIGNIFICANCE STATEMENT Transmembrane channel-like protein isoform 1 (TMC1) is thought to be a major component of the mechanotransducer channel in auditory hair cells, but the protein organization and channel structure are still uncertain. We made four mouse lines harboring Tmc1 point mutations that alter channel properties, causing hair cell degeneration and deafness. These include a mouse homolog of a new human deafness mutation pT416K that decreased channel Ca²⁺ permeability by introducing a positively-charged amino acid in the putative pore. All mutations are consistent with the channel structure predicted from modeling, but only one, p.D528N near the external face of the pore, substantially reduced channel conductance and Ca²⁺ permeability and virtually abolished block by dihydrostreptomycin (DHS), strongly endorsing its siting within the pore.

Genome and base editing for genetic hearing loss

Genome editing opens up a new frontier in developing personalized therapeutic solutions. With the unprecedented advance in the discovery and engineering of gene editing nucleases, it has now become potentially feasible to therapeutically influence up to 90% of all human genetic mutations. Hearing loss is one of the most well studied fields from the genetics perspective, with more than one hundred identified deafness genes. Novel viral and non-viral vectors have been established as safe and efficient modalities to deliver transgenes into cells of the cochlea and to the vestibular system in animal models. Recent studies demonstrated proof-of-concept for therapeutic genome and base editing in the mammalian inner ear and preclinical development is ongoing. This review summarizes important advances and future challenges for this transformative therapeutic modality for genetic and non-genetic hearing loss.

TMIE Defines Pore and Gating Properties of the Mechanotransduction Channel of Mammalian Cochlear Hair Cells

TMC1 and TMC2 (TMC1/2) have been proposed to form the pore of the mechanotransduction channel of cochlear hair cells. Here, we show that TMC1/2 cannot form mechanotransduction channels in cochlear hair cells without TMIE. TMIE binds to TMC1/2, and a TMIE mutation that perturbs TMC1/2 binding abolishes mechanotransduction. N-terminal TMIE deletions affect the response of the mechanotransduction channel to mechanical force. Similar to mechanically gated TREK channels, the C-terminal cytoplasmic TMIE domain contains charged amino acids that mediate binding to phospholipids, including PIP2. TMIE point mutations in the C terminus that are linked to deafness disrupt phospholipid binding, sensitize the channel to PIP2 depletion from hair cells, and alter the channel's unitary conductance and ion selectivity. We conclude that TMIE is a subunit of the cochlear mechanotransduction channel and that channel function is regulated by a phospholipid-sensing domain in TMIE with similarity to those in other mechanically gated ion channels.

Disruption of tmc1/2a/2b Genes in Zebrafish Reveals Subunit Requirements in Subtypes of Inner Ear Hair Cells

Detection of sound and head movement requires mechanoelectrical transduction (MET) channels at tips of hair-cell stereocilia. In vertebrates, the transmembrane channel-like (TMC) proteins TMC1 and TMC2 fulfill critical roles in MET, and substantial evidence implicates these TMCs as subunits of the MET channel. To identify developmental and functional roles of this Tmc subfamily in the zebrafish inner ear, we tested the effects of truncating mutations in tmc1, tmc2a, and tmc2b on in vivo mechanosensation at the onset of hearing and balance, before gender differentiation. We find that tmc1/2a/2b triple-mutant larvae cannot detect sound or orient with respect to gravity. They lack acoustic-evoked behavioral responses, vestibular-induced eye movements, and hair-cell activity as assessed with FM dye labeling and microphonic potentials. Despite complete loss of hair-cell function, tmc triple-mutant larvae retain normal gross morphology of hair bundles and proper trafficking of known MET components Protocadherin 15a (Pcdh15a), Lipoma HMGIC fusion partner-like 5 (Lhfpl5), and Transmembrane inner ear protein (Tmie). Transgenic, hair cell-specific expression of Tmc2b-mEGFP rescues the behavioral and physiological deficits in tmc triple mutants. Results from tmc single and double mutants evince a principle role for Tmc2a and Tmc2b in hearing and balance, respectively, whereas Tmc1 has lower overall impact. Our experiments reveal that, in developing cristae, hair cells stratify into an upper, Tmc2a-dependent layer of teardrop-shaped cells and a lower, Tmc1/2b-dependent tier of gourd-shaped cells. Collectively, our genetic evidence indicates that auditory/vestibular end organs and subsets of hair cells therein rely on distinct combinations of Tmc1/2a/2b.SIGNIFICANCE STATEMENT We assessed the effects of tmc1/2a/2b truncation mutations on mechanoelectrical transduction (MET) in the inner-ear hair cells of larval zebrafish. tmc triple mutants lacked behavioral responses to sound and head movements, while further assays demonstrated no observable mechanosensitivity in the tmc1/2a/2b triple mutant inner ear. Examination of tmc double mutants revealed major contributions from Tmc2a and Tmc2b to macular function; however, Tmc1 had less overall impact. FM labeling of lateral cristae in tmc double mutants revealed the presence of two distinct cell types, an upper layer of teardrop-shaped cells that rely on Tmc2a, and a lower layer of gourd-shaped cells that rely on Tmc1/2b.

Dextran Labeling and Uptake in Live and Functional Murine Cochlear Hair Cells

The hair cell mechanotransduction (MET) channel plays an important role in hearing. However, the molecular identity and structural information of MET remain unknown. Electrophysiological studies of hair cells revealed that the MET channel has a large conductance and is permeable to relatively large fluorescent cationic molecules, including some styryl dyes and Texas Red-labeled aminoglycoside antibiotics. In this protocol, we describe a method to visualize and evaluate the uptake of fluorescent dextrans in hair cells of the organ of Corti explants that can be used to assay for functional MET channels. We found that 3 kDa Texas Red-labeled dextran specifically labels functional auditory hair cells after 1-2 h incubation. In particular, 3 kDa dextran labels the two shorter stereocilia rows and accumulates in the cell body in a diffuse pattern when functional MET channels are present. An additional vesicle-like pattern of labeling was observed in the cell body of hair cells and surrounding supporting cells. Our data suggest that 3 kDa Texas-Red dextran can be used to visualize and study two pathways for cellular dye uptake; a hair cell-specific entry route through functional MET channels and endocytosis, a pattern also available to larger dextran.

The contribution of TMC1 to adaptation of mechanoelectrical transduction channels in cochlear outer hair cells

KEY POINTS

Hair cell mechanoelectrical transducer channels are opened by deflections of the hair bundle about a resting position set by incompletely understood adaptation mechanisms. We used three characteristics to define adaptation in hair cell mutants of transmembrane channel-like proteins, TMC1 and TMC2, which are considered to be channel constituents. The results obtained demonstrate that the three characteristics are not equivalent, and raise doubts about simple models in which intracellular Ca²⁺ regulates adaptation. Adaptation is faster and more effective in TMC1-containing than in TMC2-containing transducer channels. This result ties adaptation to the channel complex, and suggests that TMC1 is a better isoform for use in cochlear hair cells. We describe a TMC1 point mutation, D569N, that reduces the resting open probability and Ca²⁺ permeability of the transducer channels, comprising properties that may contribute to the deafness phenotype.

ABSTRACT

Recordings of mechanoelectrical transducer (MET) currents in cochlear hair cells were made in mice with mutations of transmembrane channel-like (TMC) protein to examine the effects on fast transducer adaptation. Adaptation was faster and more complete in Tmc2^-/- than in Tmc1^-/- , although this disparity was not explained by differences in Ca²⁺ permeability or Ca²⁺ influx between the two isoforms, with TMC2 having the larger permeability. We made a mouse mutation, Tmc1 p.D569N, homologous to a human DFNA36 deafness mutation, which also had MET channels with lower Ca²⁺ -permeability but showed better fast adaptation than wild-type Tmc1^+/+ channels. Consistent with the more effective adaptation in Tmc1 p.D569N, the resting probability of MET channel opening was smaller. The three TMC variants studied have comparable single-channel conductances, although the lack of correlation between channel Ca²⁺ permeability and adaptation opposes the hypothesis that adaptation is controlled simply by Ca²⁺ influx through the channels. During the first postnatal week of mouse development, the MET currents amplitude grew, and transducer adaptation became faster and more effective. We attribute changes in adaptation partly to a developmental switch from TMC2- to TMC1- containing channels and partly to an increase in channel expression. More complete and faster adaptation, coupled with larger MET currents, may account for the sole use of TMC1 in the adult cochlear hair cells.

Distinct functions of TMC channels: a comparative overview

In the past two decades, transmembrane channel-like (TMC) proteins have attracted a significant amount of research interest, because mutations of Tmc1 lead to hereditary deafness. As evolutionarily conserved membrane proteins, TMC proteins are widely involved in diverse sensorimotor functions of many species, such as hearing, chemosensation, egg laying, and food texture detection. Interestingly, recent structural and physiological studies suggest that TMC channels may share a similar membrane topology with the Ca2+-activated Cl- channel TMEM16 and the mechanically activated OSCA1.2/TMEM63 channel. Namely, these channels form dimers and each subunit consists of ten transmembrane segments. Despite this important structural insight, a key question remains: what is the gating mechanism of TMC channels? The major technical hurdle to answer this question is that the reconstitution of TMC proteins as functional ion channels has been challenging in mammalian heterologous systems. Since TMC channels are conserved across taxa, genetic studies of TMC channels in model organisms such as C. elegans, Drosophila, and zebrafish may provide us critical information on the physiological function and regulation of TMCs. Here, we present a comparative overview on the diverse functions of TMC channels in different species.

TMC1 is an essential component of a leak channel that modulates tonotopy and excitability of auditory hair cells in mice

Hearing sensation relies on the mechano-electrical transducer (MET) channel of cochlear hair cells, in which transmembrane channel-like 1 (TMC1) and transmembrane channel-like 2 (TMC2) have been proposed to be the pore-forming subunits in mammals. TMCs were also found to regulate biological processes other than MET in invertebrates, ranging from sensations to motor function. However, whether TMCs have a non-MET role remains elusive in mammals. Here, we report that in mouse hair cells, TMC1, but not TMC2, provides a background leak conductance, with properties distinct from those of the MET channels. By cysteine substitutions in TMC1, we characterized four amino acids that are required for the leak conductance. The leak conductance is graded in a frequency-dependent manner along the length of the cochlea and is indispensable for action potential firing. Taken together, our results show that TMC1 confers a background leak conductance in cochlear hair cells, which may be critical for the acquisition of sound-frequency and -intensity.

Hair Bundle Stimulation Mode Modifies Manifestations of Mechanotransduction Adaptation

Sound detection in auditory sensory hair cells depends on the deflection of the stereocilia hair bundle which opens mechano-electric transduction (MET) channels. Adaptation is hypothesized to be a critical property of MET that contributes to the auditory system's wide dynamic range and sharp frequency selectivity. Our recent work using a stiff probe to displace hair bundles showed that the fastest adaptation mechanism (fast adaptation) does not require calcium entry. Using fluid-jet stimuli, others obtained data showing only a calcium-dependent fast adaptation response. Because cochlear stereocilia do not move coherently and the hair cell response depends critically on the magnitude and time course of the hair bundle deflection, we developed a high-speed imaging technique to quantify this deflection in rat cochlear hair cells. The fluid jet delivers a force stimulus, and force steps lead to a complex time course of hair bundle displacement (mechanical creep), which affects the hair cell's macroscopic MET current response by masking the time course of the fast adaptation response. Modifying the fluid-jet stimulus to generate a hair bundle displacement step produced rapidly adapting currents that did not depend on membrane potential, confirming that fast adaptation does not depend on calcium entry. MET current responses differ with stimulus modality and will shape receptor potentials of different hair cell types based on their in vivo stimulus mode. These transformations will directly affect how stimuli are encoded.SIGNIFICANCE STATEMENT Mechanotransduction by sensory hair cells represents a key first step for the sound sensing ability in vertebrates. The sharp frequency tuning and wide dynamic range of sound sensation are hypothesized to require a mechanotransduction adaptation mechanism. Recent work indicated that the apparent calcium dependence of the fastest adaptation differs with the method of cochlear hair cell stimulation. Here, we reconcile existing data and show that calcium entry does not drive the fastest adaptation process, independent of the stimulation method. With force stimulation of the hair bundle, adaptation manifests differently than with displacement stimulation, indicating that the stimulation mode of the hair bundle will affect the hair cell receptor current and stimulus coding.

Function and Dysfunction of TMC Channels in Inner Ear Hair Cells

The TMC1 channel was identified as a protein essential for hearing in mouse and human, and recognized as one of a family of eight such proteins in mammals. The TMC family is part of a superfamily of seven branches, which includes the TMEM16s. Vertebrate hair cells express both TMC1 and TMC2. They are located at the tips of stereocilia and are required for hair cell mechanotransduction. TMC1 assembles as a dimer and its similarity to the TMEM16s has enabled a predicted tertiary structure with an ion conduction pore in each subunit of the dimer. Cysteine mutagenesis of the pore supports the role of TMC1 and TMC2 as the core channel proteins of a larger mechanotransduction complex that includes PCDH15 and LHFPL5, and perhaps TMIE, CIB2 and others.

A Tmc1 mutation reduces calcium permeability and expression of mechanoelectrical transduction channels in cochlear hair cells

Cochlear hair cells transduce sound into electrical signals by activation of mechanically sensitive ion channels thought to be formed by TMC1. We generated a single aspartate/asparagine substitution in mouse TMC1 which is homologous to a human genetic deafness mutation. The main consequence was reduction in the Ca²⁺ permeability of the mechanically sensitive channel with little change in its unitary conductance. Nevertheless, there was a much reduced expression of the ion channel, which led within 4 wk to death of the outer hair cells culminating in deafness. The mouse mutant accounts for the human deafness and implies that TMC1, in addition to forming the mechanically sensitive ion channel, regulates its own expression.

Mechanoelectrical transducer (MET) currents were recorded from cochlear hair cells in mice with mutations of transmembrane channel-like protein TMC1 to study the effects on MET channel properties. We characterized a Tmc1 mouse with a single-amino-acid mutation (D569N), homologous to a dominant human deafness mutation. Measurements were made in both Tmc2 wild-type and Tmc2 knockout mice. By 30 d, Tmc1 pD569N heterozygote mice were profoundly deaf, and there was substantial loss of outer hair cells (OHCs). MET current in OHCs of Tmc1 pD569N mutants developed over the first neonatal week to attain a maximum amplitude one-third the size of that in Tmc1 wild-type mice, similar at apex and base, and lacking the tonotopic size gradient seen in wild type. The MET-channel Ca²⁺ permeability was reduced 3-fold in Tmc1 pD569N homozygotes, intermediate deficits being seen in heterozygotes. Reduced Ca²⁺ permeability resembled that of the Tmc1 pM412K Beethoven mutant, a previously studied semidominant mouse mutation. The MET channel unitary conductance, assayed by single-channel recordings and by measurements of current noise, was unaffected in mutant apical OHCs. We show that, in contrast to the Tmc1 M412K mutant, there was reduced expression of the TMC1 D569N channel at the transduction site assessed by immunolabeling, despite the persistence of tip links. The reduction in MET channel Ca²⁺ permeability seen in both mutants may be the proximate cause of hair-cell apoptosis, but changes in bundle shape and protein expression in Tmc1 D569N suggest another role for TMC1 apart from forming the channel.

Molecular Structure of the Hair Cell Mechanoelectrical Transduction Complex

Cochlear hair cells employ mechanically gated ion channels located in stereocilia that open in response to sound wave-induced motion of the basilar membrane, converting mechanical stimulation to graded changes in hair cell membrane potential. Membrane potential changes in hair cells cause neurotransmitter release from hair cells that initiate electrical signals in the nerve terminals of afferent fibers from spiral ganglion neurons. These signals are then propagated within the central nervous system (CNS) to mediate the sensation of hearing. Recent studies show that the mechanoelectrical transduction (MET) machinery of hair cells is formed by an ensemble of proteins. Candidate components forming the MET channel have been identified, but none alone fulfills all criteria necessary to define them as pore-forming subunits of the MET channel. We will review here recent findings on the identification and function of proteins that are components of the MET machinery in hair cells and consider remaining open questions.

Beyond Cell-Cell Adhesion: Sensational Cadherins for Hearing and Balance

Cadherins form a large family of proteins often involved in calcium-dependent cellular adhesion. Although classical members of the family can provide a physical bond between cells, a subset of special cadherins use their extracellular domains to interlink apical specializations of single epithelial sensory cells. Two of these cadherins, cadherin-23 (CDH23) and protocadherin-15 (PCDH15), form extracellular "tip link" filaments that connect apical bundles of stereocilia on hair cells essential for inner-ear mechanotransduction. As these bundles deflect in response to mechanical stimuli from sound or head movements, tip links gate hair-cell mechanosensitive channels to initiate sensory perception. Here, we review the unusual and diverse structural properties of these tip-link cadherins and the functional significance of their deafness-related missense mutations. Based on the structural features of CDH23 and PCDH15, we discuss the elasticity of tip links and models that bridge the gap between the nanomechanics of cadherins and the micromechanics of hair-cell bundles during inner-ear mechanotransduction.

Structural relationship between the putative hair cell mechanotransduction channel TMC1 and TMEM16 proteins

The hair cell mechanotransduction (MET) channel complex is essential for hearing, yet it's molecular identity and structure remain elusive. The transmembrane channel-like 1 (TMC1) protein localizes to the site of the MET channel, interacts with the tip-link responsible for mechanical gating, and genetic alterations in TMC1 alter MET channel properties and cause deafness, supporting the hypothesis that TMC1 forms the MET channel. We generated a model of TMC1 based on X-ray and cryo-EM structures of TMEM16 proteins, revealing the presence of a large cavity near the protein-lipid interface that also harbors the Beethoven mutation, suggesting that it could function as a permeation pathway. We also find that hair cells are permeable to 3 kDa dextrans, and that dextran permeation requires TMC1/2 proteins and functional MET channels, supporting the presence of a large permeation pathway and the hypothesis that TMC1 is a pore forming subunit of the MET channel complex.

Mechanically Gated Ion Channels in Mammalian Hair Cells

Hair cells in the inner ear convert mechanical stimuli provided by sound waves and head movements into electrical signal. Several mechanically evoked ionic currents with different properties have been recorded in hair cells. The search for the proteins that form the underlying ion channels is still in progress. The mechanoelectrical transduction (MET) channel near the tips of stereociliary in hair cells, which is responsible for sensory transduction, has been studied most extensively. Several components of the sensory mechanotransduction machinery in stereocilia have been identified, including the multi-transmembrane proteins tetraspan membrane protein in hair cell stereocilia (TMHS)/LHFPL5, transmembrane inner ear (TMIE) and transmembrane channel-like proteins 1 and 2 (TMC1/2). However, there remains considerable uncertainty regarding the molecules that form the channel pore. In addition to the sensory MET channel, hair cells express the mechanically gated ion channel PIEZO2, which is localized near the base of stereocilia and not essential for sensory transduction. The function of PIEZO2 in hair cells is not entirely clear but it might have a role in damage sensing and repair processes. Additional stretch-activated channels of unknown molecular identity and function have been found to localize at the basolateral membrane of hair cells. Here, we review current knowledge regarding the different mechanically gated ion channels in hair cells and discuss open questions concerning their molecular composition and function.

PIEZO2 as the anomalous mechanotransducer channel in auditory hair cells

Throughout postnatal maturation of the mouse inner ear, cochlear hair cells display at least two types of mechanically gated ion channel: normal mechanotransducer (MT) channels at the tips of the stereocilia, activated by tension in interciliary tip links, and anomalous mechanosensitive (MS) channels on the top surface of the cells. The anomalous MS channels are responsible for the reverse‐polarity current that appears in mutants in which normal transduction is lost. They are also seen in wild‐type hair cells around birth, appearing 2 days earlier than normal MT channels, and being down‐regulated with the emergence of the normal channels. We review the evidence that the normal and anomalous channels are distinct channel types, which includes differences in localization, susceptibility to pharmacological agents, single‐channel conductance and Ca²⁺ permeability. The dichotomy is reinforced by the observation that the anomalous current is absent in cochlear cells of Piezo2‐null mice, even though the normal MT current persists. The anomalous current is suppressed by high intracellular Ca²⁺, suggesting that influx of the divalent ion via more Ca²⁺‐permeable normal MT channels inhibits the anomalous channels, thus explaining the temporal relationship between the two. Piezo2‐null mice have largely normal hearing, exhibiting up to 20 dB elevation in threshold in the acoustic brainstem response, so raising questions about the significance of PIEZO2 in the cochlea. Since the anomalous current declines with postnatal age, PIEZO2 may contribute to hair cell development, but it does not underlie the normal MT current. Its role in the development of hearing is not understood.

TMC2 Modifies Permeation Properties of the Mechanoelectrical Transducer Channel in Early Postnatal Mouse Cochlear Outer Hair Cells

The ability of cochlear hair cells to convert sound into receptor potentials relies on the mechanoelectrical transducer (MET) channels present in their stereociliary bundles. There is strong evidence implying that transmembrane channel-like protein (TMC) 1 contributes to the pore-forming subunit of the mature MET channel, yet its expression is delayed (~>P5 in apical outer hair cells, OHCs) compared to the onset of mechanotransduction (~P1). Instead, the temporal expression of TMC2 coincides with this onset, indicating that it could be part of the immature MET channel. We investigated MET channel properties from OHCs of homo- and heterozygous Tmc2 knockout mice. In the presence of TMC2, the MET channel blocker dihydrostreptomycin (DHS) had a lower affinity for the channel, when the aminoglycoside was applied extracellularly or intracellularly, with the latter effect being more pronounced. In Tmc2 knockout mice OHCs were protected from aminoglycoside ototoxicity during the first postnatal week, most likely due to their small MET current and the lower saturation level for aminoglycoside entry into the individual MET channels. DHS entry through the MET channels of Tmc2 knockout OHCs was lower during the first than in the second postnatal week, suggestive of a developmental change in the channel pore properties independent of TMC2. However, the ability of TMC2 to modify the MET channel properties strongly suggests it contributes to the pore-forming subunit of the neonatal channel. Nevertheless, we found that TMC2, different from TMC1, is not necessary for OHC development. While TMC2 is required for mechanotransduction in mature vestibular hair cells, its expression in the immature cochlea may be an evolutionary remnant.

The enduring legacy of the "constant-field equation" in membrane ion transport

In 1943, David Goldman published a seminal paper in The Journal of General Physiology that reported a concise expression for the membrane current as a function of ion concentrations and voltage. This body of work was, and still is, the theoretical pillar used to interpret the relationship between a cell's membrane potential and its external and/or internal ionic composition. Here, we describe from an historical perspective the theory underlying the constant-field equation and its application to membrane ion transport.

Hair Cell Transduction, Tuning, and Synaptic Transmission in the Mammalian Cochlea

Sound pressure fluctuations striking the ear are conveyed to the cochlea, where they vibrate the basilar membrane on which sit hair cells, the mechanoreceptors of the inner ear. Recordings of hair cell electrical responses have shown that they transduce sound via submicrometer deflections of their hair bundles, which are arrays of interconnected stereocilia containing the mechanoelectrical transducer (MET) channels. MET channels are activated by tension in extracellular tip links bridging adjacent stereocilia, and they can respond within microseconds to nanometer displacements of the bundle, facilitated by multiple processes of Ca2+-dependent adaptation. Studies of mouse mutants have produced much detail about the molecular organization of the stereocilia, the tip links and their attachment sites, and the MET channels localized to the lower end of each tip link. The mammalian cochlea contains two categories of hair cells. Inner hair cells relay acoustic information via multiple ribbon synapses that transmit rapidly without rundown. Outer hair cells are important for amplifying sound-evoked vibrations. The amplification mechanism primarily involves contractions of the outer hair cells, which are driven by changes in membrane potential and mediated by prestin, a motor protein in the outer hair cell lateral membrane. Different sound frequencies are separated along the cochlea, with each hair cell being tuned to a narrow frequency range; amplification sharpens the frequency resolution and augments sensitivity 100-fold around the cell's characteristic frequency. Genetic mutations and environmental factors such as acoustic overstimulation cause hearing loss through irreversible damage to the hair cells or degeneration of inner hair cell synapses. © 2017 American Physiological Society. Compr Physiol 7:1197-1227, 2017.

Are TMCs the Mechanotransduction Channels of Vertebrate Hair Cells?

Sensory transduction in vertebrate hair cells and the molecules that mediate it have long been of great interest. Some components of the mechanotransduction apparatus have been identified, most as deafness gene products. Although prior candidates for the mechanotransduction channel have been proposed, each has faded with new evidence. Now, two strong candidates, TMC1 and TMC2 (transmembrane channel-like), have emerged from discovery of deafness genes in humans and mice. They are expressed at the right time during development: exactly at the onset of mechanosensitivity. They are expressed in the right place: in hair cells but not surrounding cells. Fluorescently tagged TMCs localize to the tips of stereocilia, the site of the transduction channels. TMCs bind other proteins essential for mechanosensation, suggesting a larger transduction complex. Although TMC1 and TMC2 can substitute for each other, genetic deletion of both renders mouse hair cells mechanically insensitive. Finally, the conductance and Ca²⁺ selectivity of the transduction channels depend on the TMC proteins, differing when hair cells express one or the other TMC, and differing if TMC1 harbors a point mutation. Some contrary evidence has emerged: a current activated in hair cells by negative pressure, with some similarity to the transduction current, persists in TMC knock-outs. But it is not clear that this anomalous current is carried by the same proteins. Further evidence is desired, such as production of a mechanically gated conductance by pure TMCs. But the great majority of evidence is consistent with these TMCs as pore-forming subunits of the long-sought hair-cell transduction channel.

Molecular Identity of the Mechanotransduction Channel in Hair Cells: Not Quiet There Yet

Hair cells in the mammalian cochlea are specialized mechanosensory cells that convert sound-induced vibrations into electrochemical signals. The molecular composition of the mechanotransduction channel underlying auditory perception has been difficult to define. The study of genes that are linked to inherited forms of deafness has recently provided tantalizing clues. Current findings indicate that the mechanotransduction channel in hair cells is a complex molecular machine. Four different proteins (TMHS/LHFPL5, TMIE, TMC1, and TMC2) have so far been linked to the transduction channel, but which proteins contribute to the channel pore still needs to be determined. Current evidence also suggests that the channel complex may contain additional, yet to be identified components.

Is TMC1 the Hair Cell Mechanotransducer Channel?

Transmembrane channel-like protein isoform-1 (TMC1) has emerged over the past five years as a prime contender for the mechano-electrical transducer (MET) channel in hair cells of the inner ear. TMC1 is thought to have a six-transmembrane domain structure reminiscent of some other ion-channel subunits, and is targeted to the tips of the stereocilia in the sensory hair bundle, where the MET channel is located. Moreover, there are TMC1 mutations linked to human deafness causing loss of conventional MET currents, hair cell degeneration, and deafness in mice. Finally, mutations of Tmc1 can alter the conductance and Ca(2+) selectivity of the MET channels. For several reasons though, it is unclear that TMC1 is indeed the MET channel pore: 1) in other animals or tissues, mutations of TMC family members do not directly affect cellular mechanosensitivity; 2) there are residual manifestations of mechanosensitivity in hair cells of mouse Tmc1:Tmc2 double knockouts; 3) there is so far no evidence that expression of mammalian Tmc1 generates a mechanically sensitive ion channel in the plasma membrane when expressed in heterologous cells; and 4) there are other proteins, such as TMIE and LHFPL5, which behave similarly to TMC1, their mutation also leading to loss of MET current and deafness. This review will present these disparate lines of evidence and describes recent work that addresses the role of TMC1.

Mechanosensory hair cells express two molecularly distinct mechanotransduction channels

Auditory hair cells contain mechanotransduction channels that rapidly open in response to sound-induced vibrations. Surprisingly, we report here that auditory hair cells contain two molecularly distinct mechanotransduction channels. One ion channel is activated by sound and is responsible for sensory transduction. This sensory transduction channel is expressed in hair-cell stereocilia and previous studies show that its activity is affected by mutations in the genes encoding the transmembrane proteins TMHS/LHFPL5, TMIE and TMC1/2. We show here that the second ion channel is expressed at the apical surface of hair cells and contains the Piezo2 protein. The activity of the Piezo2-dependent channel is controlled by the intracellular Ca²⁺ concentration and can be recorded following disruption of the sensory transduction machinery or more generally by disruption of the sensory epithelium. We thus conclude that hair cells express two molecularly and functionally distinct mechanotransduction channels with different subcellular distribution.

Adaptation Independent Modulation of Auditory Hair Cell Mechanotransduction Channel Open Probability Implicates a Role for the Lipid Bilayer

The auditory system is able to detect movement down to atomic dimensions. This sensitivity comes in part from mechanisms associated with gating of hair cell mechanoelectric transduction (MET) channels. MET channels, located at the tops of stereocilia, are poised to detect tension induced by hair bundle deflection. Hair bundle deflection generates a force by pulling on tip-link proteins connecting adjacent stereocilia. The resting open probability (P(open)) of MET channels determines the linearity and sensitivity to mechanical stimulation. Classically, P(open) is regulated by a calcium-sensitive adaptation mechanism in which lowering extracellular calcium or depolarization increases P(open). Recent data demonstrated that the fast component of adaptation is independent of both calcium and voltage, thus requiring an alternative explanation for the sensitivity of P(open) to calcium and voltage. Using rat auditory hair cells, we characterize a mechanism, separate from fast adaptation, whereby divalent ions interacting with the local lipid environment modulate resting P(open). The specificity of this effect for different divalent ions suggests binding sites that are not an EF-hand or calmodulin model. GsMTx4, a lipid-mediated modifier of cationic stretch-activated channels, eliminated the voltage and divalent sensitivity with minimal effects on adaptation. We hypothesize that the dual mechanisms (lipid modulation and adaptation) extend the dynamic range of the system while maintaining adaptation kinetics at their maximal rates.

Tmc1 Point Mutation Affects Ca2+ Sensitivity and Block by Dihydrostreptomycin of the Mechanoelectrical Transducer Current of Mouse Outer Hair Cells

The transduction of sound into electrical signals depends on mechanically sensitive ion channels in the stereociliary bundle. The molecular composition of this mechanoelectrical transducer (MET) channel is not yet known. Transmembrane channel-like protein isoforms 1 (TMC1) and 2 (TMC2) have been proposed to form part of the MET channel, although their exact roles are still unclear. Using Beethoven (Tmc1(Bth/Bth)) mice, which have an M412K point mutation in TMC1 that adds a positive charge, we found that Ca(2+) permeability and conductance of the MET channel of outer hair cells (OHCs) were reduced. Tmc1(Bth/Bth) OHCs were also less sensitive to block by the permeant MET channel blocker dihydrostreptomycin, whether applied extracellularly or intracellularly. These findings suggest that the amino acid that is mutated in Bth is situated at or near the negatively charged binding site for dihydrostreptomycin within the permeation pore of the channel. We also found that the Ca(2+) dependence of the operating range of the MET channel was altered by the M412K mutation. Depolarization did not increase the resting open probability of the MET current of Tmc1(Bth/Bth) OHCs, whereas raising the intracellular concentration of the Ca(2+) chelator BAPTA caused smaller increases in resting open probability in Bth mutant OHCs than in wild-type control cells. We propose that these observations can be explained by the reduced Ca(2+) permeability of the mutated MET channel indirectly causing the Ca(2+) sensor for adaptation, at or near the intracellular face of the MET channel, to become more sensitive to Ca(2+) influx as a compensatory mechanism.

SIGNIFICANCE STATEMENT

In the auditory system, the hair cells convert sound-induced mechanical movement of the hair bundles atop these cells into electrical signals through the opening of mechanically gated ion channels at the tips of the bundles. Although the nature of these mechanoelectrical transducer (MET) channels is still unclear, recent studies implicate transmembrane channel-like protein isoform 1 (TMC1) channels in the mammalian cochlea. Using a mutant mouse model (Beethoven) for progressive hearing loss in humans (DFNA36), which harbors a point mutation in the Tmc1 gene, we show that this mutation affects the MET channel pore, reducing its Ca(2+) permeability and its affinity for the permeant blocker dihydrostreptomycin. A number of phenomena that we ascribe to Ca(2+)-dependent adaptation appear stronger, in compensation for the reduced Ca(2+) entry.