Mechanoelectrical transduction by hair cells

Three-dimensional cultured ampullae from rats as a screening tool for vestibulotoxicity: Proof of concept using styrene

Numerous ototoxic drugs, such as some antibiotics and chemotherapeutics, are both cochleotoxic and vestibulotoxic (causing hearing loss and vestibular disorders). However, the impact of some industrial cochleotoxic compounds on the vestibular receptor, if any, remains unknown. As in vivo studies are long and expensive, there is considerable need for predictive and cost-effective in vitro models to test ototoxicity. Here, we present an organotypic model of cultured ampullae harvested from rat neonates. When cultured in a gelatinous matrix, ampulla explants form an enclosed compartment that progressively fills with a high-potassium (K+) endolymph-like fluid. Morphological analyses confirmed the presence of a number of cell types, sensory epithelium, secretory cells, and canalar cells. Treatments with inhibitors of potassium transporters demonstrated that the potassium homeostasis mechanisms were functional. To assess the potential of this model to reveal the toxic effects of chemicals, explants were exposed for either 2 or 72 h to styrene at a range of concentrations (0.5-1 mM). In the 2-h exposure condition, K+ concentration was significantly reduced, but ATP levels remained stable, and no histological damage was visible. After 72 h exposure, variations in K+ concentration were associated with histological damage and decreased ATP levels. This in vitro 3D neonatal rat ampulla model therefore represents a reliable and rapid means to assess the toxic properties of industrial compounds on this vestibular tissue, and can be used to investigate the specific underlying mechanisms.

Vestibular Deficits in Deafness: Clinical Presentation, Animal Modeling, and Treatment Solutions

The inner ear is responsible for both hearing and balance. These functions are dependent on the correct functioning of mechanosensitive hair cells, which convert sound- and motion-induced stimuli into electrical signals conveyed to the brain. During evolution of the inner ear, the major changes occurred in the hearing organ, whereas the structure of the vestibular organs remained constant in all vertebrates over the same period. Vestibular deficits are highly prevalent in humans, due to multiple intersecting causes: genetics, environmental factors, ototoxic drugs, infections and aging. Studies of deafness genes associated with balance deficits and their corresponding animal models have shed light on the development and function of these two sensory systems. Bilateral vestibular deficits often impair individual postural control, gaze stabilization, locomotion and spatial orientation. The resulting dizziness, vertigo, and/or falls (frequent in elderly populations) greatly affect patient quality of life. In the absence of treatment, prosthetic devices, such as vestibular implants, providing information about the direction, amplitude and velocity of body movements, are being developed and have given promising results in animal models and humans. Novel methods and techniques have led to major progress in gene therapies targeting the inner ear (gene supplementation and gene editing), 3D inner ear organoids and reprograming protocols for generating hair cell-like cells. These rapid advances in multiscale approaches covering basic research, clinical diagnostics and therapies are fostering interdisciplinary research to develop personalized treatments for vestibular disorders.

Differential regulation of mammalian and avian ATOH1 by E2F1 and its implication for hair cell regeneration in the inner ear

The mammalian inner ear has a limited capacity to regenerate its mechanosensory hair cells. This lack of regenerative capacity underlies the high incidence of age-related hearing loss in humans. In contrast, non-mammalian vertebrates can form new hair cells when damage occurs, a mechanism that depends on re-activation of expression of the pro-hair cell transcription factor Atoh1. Here, we show that members of the E2F transcription factor family, known to play a key role in cell cycle progression, regulate the expression of Atoh1. E2F1 activates chicken Atoh1 by directly interacting with a cis-regulatory region distal to the avian Atoh1 gene. E2F does not activate mouse Atoh1 gene expression, since this regulatory element is absent in mammals. We also show that E2F1 expression changes dynamically in the chicken auditory epithelium during ototoxic damage and hair cell regeneration. Therefore, we propose a model in which the mitotic regeneration of non-mammalian hair cells is due to E2F1-mediated activation of Atoh1 expression, a mechanism which has been lost in mammals.

CYLD deficiency causes auditory neuropathy due to reduced neurite outgrowth

BACKGROUND

Auditory neuropathy is a cause of hearing loss that has been studied in a number of animal models. Signal transmission from hair cells to spiral ganglion neurons plays an important role in normal hearing. CYLD is a microtubule-binding protein, and deubiquitinase involved in the regulation of various cellular processes. In this study, we used Cyld knockout (KO) mice and nerve cell lines to examine whether CYLD is associated with auditory neuropathy.

METHODS

Hearing of Cyld KO mice was studied using the TDT RZ6 auditory physiology workstation. The expression and localization of CYLD in mouse cochlea and cell lines were examined by RT-PCR, immunoblotting, and immunofluorescence. CYLD expression was knocked down in SH-SY5Y cells by shRNAs and in PC12 and N2A cells by siRNAs. Nerve growth factor and retinoic acid were used to induce neurite outgrowth, and the occurrence and length of neurites were statistically analyzed between knockdown and control groups.

RESULTS

Cyld KO mice had mild hearing impairment. Moreover, CYLD was widely expressed in mouse cochlear tissues and different nerve cell lines. Knocking down CYLD significantly reduced the length and proportion of neurites growing from nerve cells.

CONCLUSIONS

The abnormal hearing of Cyld KO mice might be caused by a decrease in the length and number of neurites growing from auditory nerve cells in the cochlea, suggesting that CYLD is a key protein affecting hearing.

How Transmembrane Inner Ear (TMIE) plays role in the auditory system: A mystery to us

Different cellular mechanisms contribute to the hearing sense, so it is obvious that any disruption in such processes leads to hearing impairment that greatly influences the global economy and quality of life of the patients and their relatives. In the past two decades, transmembrane inner ear (TMIE) protein has received a great deal of research interest because its impairments cause hereditary deafness in humans. This evolutionarily conserved membrane protein contributes to a fundamental complex that plays role in the maintenance and function of the sensory hair cells. Although the critical roles of the TMIE in mechanoelectrical transduction or hearing procedures have been discussed, there are little to no review papers summarizing the roles of the TMIE in the auditory system. In order to fill this gap, herein, we discuss the important roles of this protein in the auditory system including its role in mechanotransduction, olivocochlear synapse, morphology and different signalling pathways; we also review the genotype-phenotype correlation that can per se show the possible roles of this protein in the auditory system.

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.

CIB2, defective in isolated deafness, is key for auditory hair cell mechanotransduction and survival

Defects of CIB2, calcium‐ and integrin‐binding protein 2, have been reported to cause isolated deafness, DFNB48 and Usher syndrome type‐IJ, characterized by congenital profound deafness, balance defects and blindness. We report here two new nonsense mutations (pGln12* and pTyr110*) in CIB2 patients displaying nonsyndromic profound hearing loss, with no evidence of vestibular or retinal dysfunction. Also, the generated CIB2^−/− mice display an early onset profound deafness and have normal balance and retinal functions. In these mice, the mechanoelectrical transduction currents are totally abolished in the auditory hair cells, whilst they remain unchanged in the vestibular hair cells. The hair bundle morphological abnormalities of CIB2^−/− mice, unlike those of mice defective for the other five known USH1 proteins, begin only after birth and lead to regression of the stereocilia and rapid hair‐cell death. This essential role of CIB2 in mechanotransduction and cell survival that, we show, is restricted to the cochlea, probably accounts for the presence in CIB2^−/− mice and CIB2 patients, unlike in Usher syndrome, of isolated hearing loss without balance and vision deficits.

A mechanical model of stereocilia that demonstrates a shift in the high-sensitivity region due to the interplay of a negative stiffness and an adaptation mechanism

Stereocilia are the basic sensory units of nature's inertial sensors and are highly sensitive over broad dynamic ranges, which is a major challenge in the design of conventional engineering sensors. The high sensitivity that is maintained by stereocilia was hypothesized to exist due to a combination of adaptation and negative stiffness mechanisms, which shift the region of highest sensitivity toward the active operation range of the stereocilia bundle. To examine the adaptation hypothesis in terms of its potential applicability to future applications regarding the design of inertial sensors, we developed a mechanical mimicry of the interplay between negative stiffness and the adaptation of the stereocilia that produces spontaneous oscillation of the hair bundle. The mechanical model consists of an inverted pendulum and a fixed T-bar that mimic the interaction of two adjacent stereocilia. To focus on the interaction of one gating spring and the corresponding adaptation motor without the effect of coupling from the other gating springs attached to the neighboring stereocilia, we fixed one bar that contains the adaptation motor. To emulate the negative resistance of the tip-link due to the transient stiffness softening by the gating ion channel, a magnet pair was attached to the top of the inverted pendulum and the fixed T-bar. Readjustment of the tip-link tension by the 'slipping down and climbing up' motion of the adaptation molecular motors was demonstrated by the side-to-side movement of the magnet by a step motor. The negative stiffness region was observed near the equilibrium position and shifted with the activation of the adaptation motor. The temporal demonstration of the stiffness shift was measured as a spontaneous oscillation. The results showed that the interplay between the negative stiffness and the adaptation mechanism was mechanically produced by the combination of a repulsive force and its continuous readjustment and is better understood through a parameter study of a biomimetic mechanical system.

Caprin-1 is a target of the deafness gene Pou4f3 and is recruited to stress granules in cochlear hair cells in response to ototoxic damage

The POU4 family of transcription factors are required for survival of specific cell types in different sensory systems. Pou4f3 is essential for the survival of auditory sensory hair cells and several mutations in human POU4F3 cause hearing loss. Thus, genes regulated by Pou4f3 are likely to be essential for hair cell survival. We performed a subtractive hybridisation screen in an inner-ear-derived cell line to find genes with differential expression in response to changes in Pou4f3 levels. The screen identified the stress-granule-associated protein Caprin-1 as being downregulated by Pou4f3. We demonstrated that this regulation occurs through the direct interaction of Pou4f3 with binding sites in the Caprin-1 5' flanking sequence, and describe the expression pattern of Caprin-1 mRNA and protein in the cochlea. Moreover, we found Caprin-1-containing stress granules are induced in cochlear hair cells following aminoglycoside-induced damage. This is the first report of stress granule formation in mammalian hair cells and suggests that the formation of Caprin-1-containing stress granules is a key damage response to a clinically relevant ototoxic agent. Our results have implications for the understanding of aminoglycoside-induced hearing loss and provide further evidence that stress granule formation is a fundamental cellular stress response.

Eukaryotic mechanosensitive channels

Mechanosensitive ion channels are gated directly by physical stimuli and transduce these stimuli into electrical signals. Several criteria must apply for a channel to be considered mechanically gated. Mechanosensitive channels from bacterial systems have met these criteria, but few eukaryotic channels have been confirmed by the same standards. Recent work has suggested or confirmed that diverse types of channels, including TRP channels, K(2P) channels, MscS-like proteins, and DEG/ENaC channels, are mechanically gated. Several studies point to the importance of the plasma membrane for channel gating, but intracellular and/or extracellular structures may also be required.

Genetics and pathological mechanisms of Usher syndrome

Usher syndrome (USH) comprises a group of autosomal recessively inherited disorders characterized by a dual sensory impairment of the audiovestibular and visual systems. Three major clinical subtypes (USH type I, USH type II and USH type III) are distinguished on the basis of the severity of the hearing loss, the presence or absence of vestibular dysfunction and the age of onset of retinitis pigmentosa (RP). Since the cloning of the first USH gene (MYO7A) in 1995, there have been remarkable advances in elucidating the genetic basis for this disorder, as evidence for 11 distinct loci have been obtained and genes for 9 of them have been identified. The USH genes encode proteins of different classes and families, including motor proteins, scaffold proteins, cell adhesion molecules and transmembrane receptor proteins. Extensive information has emerged from mouse models and molecular studies regarding pathogenesis of this disorder and the wide phenotypic variation in both audiovestibular and/or visual function. A unifying hypothesis is that the USH proteins are integrated into a protein network that regulates hair bundle morphogenesis in the inner ear. This review addresses genetics and pathological mechanisms of USH. Understanding the molecular basis of phenotypic variation and pathogenesis of USH is important toward discovery of new molecular targets for diagnosis, prevention and treatment of this debilitating disorder.

Distribution of frequencies of spontaneous oscillations in hair cells of the bullfrog sacculus

Under in vitro conditions, free-standing hair bundles of the bullfrog (Rana catesbeiana) sacculus have exhibited spontaneous oscillations. We used a high-speed complementary metal oxide semiconductor camera to track the active movements of multiple hair cells in a single field of view. Our techniques enabled us to probe for correlations between pairs of cells, and to acquire records on over 100 actively oscillating bundles per epithelium. We measured the statistical distribution of oscillation periods of cells from different areas within the sacculus, and on different epithelia. Spontaneous oscillations exhibited a peak period of 33 ms (+29 ms, -14 ms) and uniform spatial distribution across the sacculus.

Does BAPTA leave outer hair cell transduction channels closed?

The calcium chelator BAPTA was iontophoresed into the scala media of the second turn of the guinea pig cochlea. This produced a reduction in low frequency cochlear microphonic (CM) measured in scala media and an elevation of the cochlear action potential (CAP) threshold that lasted for the duration of the experiment. Using two pipettes, one filled with KCl and the other KCl and BAPTA (50, 20 and 5 mM) it was possible to observe the effect of passing current through one electrode while measuring the endolymphatic potential (EP) with the other. The results demonstrated that current passed via the BAPTA pipette caused a sustained increase in EP of 8.2, 12.9 and 7.8 mV in the three animals used. This increase coincided with the decrease in low frequency CM that indicated a causal connection between the two. In a second series of experiments, pipettes with larger tips were inserted into scala media in the first cochlear turn and BAPTA was allowed to diffuse from the pipette. The results confirmed the relationship between EP increase and the fall of scala media CM. One interpretation of these results is that lowering the Ca2+ concentration of endolymph with BAPTA inhibits mechano-electrical transduction in outer hair cells (OHCs) and leaves the hair cell transduction channels in a closed state, thus increasing the resistance across OHCs and increasing the EP. These findings are consistent with a model of hair cell transduction in which tension on stereo cilia opens the transduction channels.

Mechanosensitive Ion Channels in Caenorhabditis elegans

Caenorhabditis elegans depends critically on mechanosensory perception to negotiate its natural habitat, the soil. The worm displays a rich repertoire of mechanosensitive behaviors, which can be easily examined in the laboratory. This, coupled with the availability of sophisticated genetic and molecular biology tools, renders C. elegans a particularly attractive model organism to study the transduction of mechanical stimuli to biological responses. Systematic genetic analysis has facilitated the dissection of the molecular mechanisms that underlie mechanosensation in the nematode. Studies of various worm mechanosensitive behaviors have converged to identify highly specialized plasma membrane ion channels that are required for the conversion of mechanical energy to cellular signals. Strikingly, similar mechanosensitive ion channels appear to function at the core of the mechanotransduction apparatus in higher organisms, including humans. Thus, the mechanisms responsible for the detection of mechanical stimuli are likely conserved across metazoans. The nematode offers a powerful platform for elucidating the fundamental principles that govern the function of metazoan mechanotransducers. This chapter evaluates the current understanding of mechanotransduction in C. elegans and focuses on the role of mechanosensitive ion channels in specific mechanosensory behavioral responses. The chapter also outlines potential unifying themes, common to mechanosensory transduction in diverse species.

Usher syndrome: molecular links of pathogenesis, proteins and pathways

Usher syndrome is the most common form of deaf-blindness. The syndrome is both clinically and genetically heterogeneous, and to date, eight causative genes have been identified. The proteins encoded by these genes are part of a dynamic protein complex that is present in hair cells of the inner ear and in photoreceptor cells of the retina. The localization of the Usher proteins and the phenotype in animal models indicate that the Usher protein complex is essential in the morphogenesis of the stereocilia bundle in hair cells and in the calycal processes of photoreceptor cells. In addition, the Usher proteins are important in the synaptic processes of both cell types. The association of other proteins with the complex indicates functional links to a number of basic cell-biological processes. Prominently present is the connection to the dynamics of the actin cytoskeleton, involved in cellular morphology, cell polarity and cell-cell interactions. The Usher protein complex can also be linked to the cadherins/catenins in the adherens junction-associated protein complexes, suggesting a role in cell polarity and tissue organization. A third link can be established to the integrin transmembrane signaling network. The Usher interactome, as outlined in this review, participates in pathways common in inner ear and retina that are disrupted in the Usher syndrome.

Autocorrelation Analysis of Hair Bundle Structure in the Utricle

The ability of hair bundles to signal head movements and sounds depends significantly on their structure, but a quantitative picture of bundle structure has proved elusive. The problem is acute for vestibular organs because their hair bundles exhibit complex morphologies that vary with endorgan, hair cell type, and epithelial locus. Here we use autocorrelation analysis to quantify stereociliary arrays (the number, spacing, and distribution of stereocilia) on hair cells of the turtle utricle. Our first goal was to characterize zonal variation across the macula, from medial extrastriola, through striola, to lateral extrastriola. This is important because it may help explain zonal variation in response dynamics of utricular hair cells and afferents. We also use known differences in type I and II bundles to estimate array characteristics of these two hair cell types. Our second goal was to quantify variation in array orientation at single macular loci and use this to estimate directional tuning in utricular afferents. Our major findings are that, of the features measured, array width is the most distinctive feature of striolar bundles, and within the striola there are significant, negatively correlated gradients in stereocilia number and spacing that parallel gradients in bundle heights. Together with previous results on stereocilia number and bundle heights, our results support the hypothesis that striolar hair cells are specialized to signal high-frequency/acceleration head movements. Finally, there is substantial variation in bundle orientation at single macular loci that may help explain why utricular afferents respond to stimuli orthogonal to their preferred directions.

Evolutionary changes in the cochlea and labyrinth: Solving the problem of sound transmission to the balance organs of the inner ear

This review article examines the evolutionary adaptations in the vertebrate inner ear that allow selective activation of auditory or vestibular hair cells, although both are housed in the same bony capsule. The problem of separating acoustic stimuli from the vestibular end organs in the inner ear has recently reemerged with the recognition of clinical conditions such as superior canal dehiscence syndrome and enlarged vestibular aqueduct syndrome. In these syndromes, anatomical defects in the otic capsule alter the functional separation of auditory and vestibular stimuli and lead to pathological activation of vestibular reflexes in response to sound. This review demonstrates that while the pars superior of the labyrinth (utricle and semicircular canals) has remained fairly constant throughout evolution, the pars inferior (saccule and other otolith, macular, and auditory end organs) has seen considerable change as many adaptations were made for the development of auditory function. Among these were a relatively rigid membranous labyrinth wall, a variably rigid otic capsule, immersion of the membranous labyrinth in perilymph, a perilymphatic duct to channel acoustic pressure changes away from the vestibular organs, and different operating frequencies for vestibular versus auditory epithelia. Even in normal human ears, acoustic sensitivity of the labyrinth to loud clicks or tones is retained enough to be measured in a standard clinical test, the vestibular-evoked myogenic potential test.

Cellular mechanotransduction: putting all the pieces together again

Analysis of cellular mechanotransduction, the mechanism by which cells convert mechanical signals into biochemical responses, has focused on identification of critical mechanosensitive molecules and cellular components. Stretch-activated ion channels, caveolae, integrins, cadherins, growth factor receptors, myosin motors, cytoskeletal filaments, nuclei, extracellular matrix, and numerous other structures and signaling molecules have all been shown to contribute to the mechanotransduction response. However, little is known about how these different molecules function within the structural context of living cells, tissues, and organs to produce the orchestrated cellular behaviors required for mechanosensation, embryogenesis, and physiological control. Recent work from a wide range of fields reveals that organ, tissue, and cell anatomy are as important for mechanotransduction as individual mechanosensitive proteins and that our bodies use structural hierarchies (systems within systems) composed of interconnected networks that span from the macroscale to the nanoscale in order to focus stresses on specific mechanotransducer molecules. The presence of isometric tension (prestress) at all levels of these multiscale networks ensures that various molecular scale mechanochemical transduction mechanisms proceed simultaneously and produce a concerted response. Future research in this area will therefore require analysis, understanding, and modeling of tensionally integrated (tensegrity) systems of mechanochemical control.

Mechano-electrical transduction: new insights into old ideas

The gating-spring theory of hair cell mechanotransduction channel activation was first postulated over twenty years ago. The basic tenets of this hypothesis have been reaffirmed in hair cells from both auditory and vestibular systems and across species. In fact, the basic findings have been reproduced in every hair cell type tested. A great deal of information regarding the structural, mechanical, molecular and biophysical properties of the sensory hair bundle and the mechanotransducer channel has accumulated over the past twenty years. The goal of this review is to investigate new data, using the gating spring hypothesis as the framework for discussion. Mechanisms of channel gating are presented in reference to the need for a molecular gating spring or for tethering to the intra- or extracellular compartments. Dynamics of the sensory hair bundle and the presence of motor proteins are discussed in reference to passive contributions of the hair bundle to gating compliance. And finally, the molecular identity of the channel is discussed in reference to known intrinsic properties of the native transducer channel.

Nociceptor and hair cell transducer properties of TRPA1, a channel for pain and hearing

Mechanosensory channels of sensory cells mediate the sensations of hearing, touch, and some forms of pain. The TRPA1 (a member of the TRP family of ion channel proteins) channel is activated by pain-producing chemicals, and its inhibition impairs hair cell mechanotransduction. As shown here and previously, TRPA1 is expressed by hair cells as well as by most nociceptors (small neurons of dorsal root, trigeminal, and nodose ganglia) and localizes to their sensory terminals (mechanosensory stereocilia and peripheral free nerves, respectively). Thus, TRPA1 channels are proposed to mediate transduction in both hair cells and nociceptors. Accordingly, we find that heterologously expressed TRPA1 display channel behaviors expected for both auditory and nociceptive transducers. First, TRPA1 and the hair cell transducer share a unique set of pore properties not described for any other channel (block by gadolinium, amiloride, gentamicin, and ruthenium red, a ranging conductance of approximately 100 pS that is reduced to 54% by calcium, permeating calcium-induced potentiation followed by closure, and reopening by depolarization), supporting a direct role of TRPA1 as a pore-forming subunit of the hair cell transducer. Second, TRPA1 channels inactivate in hyperpolarized cells but remain open in depolarized cells. This property provides a mechanism for the lack of desensitization, coincidence detection, and allodynia that characterize pain by allowing a sensory neuron to respond constantly to sustained stimulation that is suprathreshold (i.e., noxious) and yet permitting the same cell to ignore sustained stimulation that is subthreshold (i.e., innocuous). Our results support a TRPA1 role in both nociceptor and hair cell transduction.

The structure of tip links and kinocilial links in avian sensory hair bundles

Recent studies have indicated that the tip links and kinocilial links of sensory hair bundles in the inner ear have similar properties and share a common epitope, and that cadherin 23 may also be a component of each link type. Transmission electron microscopy was therefore used to study and compare the fine structure of the tip links and kinocilial links in avian sensory hair bundles. Tannic acid treatment revealed a thin strand, 150-200 nm long and 8-11 nm thick, present in both link types. Fourier analysis of link images showed that the strand of both link types is formed from two filaments coiled in a helix-like arrangement with an axial period of 20-25 nm, with each filament composed of globular structures that are approximately 4 nm in diameter. Differences in the radius and period of the helix-like structure may underlie the observed variation in the length of tip and kinocilial links. The similar helix-like structure of the tip links and kinocilial links is in accord with the presence of a common cell-surface antigen (TLA antigen) and similarities in the physical and chemical properties of the two link types. The spacing of the globular structures comprising each filament of the two link types is similar to the 4.3 nm center-to-center spacing reported for the globular cadherin repeat, and is consistent with the suggestion that cadherin 23 is the tip link.

Computational models of hair cell bundle mechanics: I. Single stereocilium

A distributed parameter model for describing the response of a stereocilium to an applied force is presented. This model is based on elasticity theory, plus the geometry and material properties of the stereocilium. The stereocilia shaft above the taper is not assumed to be perfectly rigid. It is assumed to be deformable and that two separate mechanisms are involved in its deformation: bending and shear. The influence of each mode of deformation is explored in parametric studies. Results show that the magnitude of tip deflection depends on the shear compliance of the stereocilium material, the degree of base taper, and stereocilium height. Furthermore, the deformation profiles observed experimentally will occur only if there are constraints on the geometry and material properties of the stereocilium.

Genetic Models of Mechanotransduction: The NematodeCaenorhabditis elegans

Mechanotransduction, the conversion of a mechanical stimulus into a biological response, constitutes the basis for a plethora of fundamental biological processes such as the senses of touch, balance, and hearing and contributes critically to development and homeostasis in all organisms. Despite this profound importance in biology, we know remarkably little about how mechanical input forces delivered to a cell are interpreted to an extensive repertoire of output physiological responses. Recent, elegant genetic and electrophysiological studies have shown that specialized macromolecular complexes, encompassing mechanically gated ion channels, play a central role in the transformation of mechanical forces into a cellular signal, which takes place in mechanosensory organs of diverse organisms. These complexes are highly efficient sensors, closely entangled with their surrounding environment. Such association appears essential for proper channel gating and provides proximity of the mechanosensory apparatus to the source of triggering mechanical energy. Genetic and molecular evidence collected in model organisms such as the nematode worm Caenorhabditis elegans, the fruit fly Drosophila melanogaster, and the mouse highlight two distinct classes of mechanically gated ion channels: the degenerin (DEG)/epithelial Na+channel (ENaC) family and the transient receptor potential (TRP) family of ion channels. In addition to the core channel proteins, several other potentially interacting molecules have in some cases been identified, which are likely parts of the mechanotransducing apparatus. Based on cumulative data, a model of the sensory mechanotransducer has emerged that encompasses our current understanding of the process and fulfills the structural requirements dictated by its dedicated function. It remains to be seen how general this model is and whether it will withstand the impiteous test of time.

Radixin deficiency causes deafness associated with progressive degeneration of cochlear stereocilia

Ezrin/radixin/moesin (ERM) proteins cross-link actin filaments to plasma membranes to integrate the function of cortical layers, especially microvilli. We found that in cochlear and vestibular sensory hair cells of adult wild-type mice, radixin was specifically enriched in stereocilia, specially developed giant microvilli, and that radixin-deficient (Rdx^−/−) adult mice exhibited deafness but no obvious vestibular dysfunction. Before the age of hearing onset (∼2 wk), in the cochlea and vestibule of Rdx^−/− mice, stereocilia developed normally in which ezrin was concentrated. As these Rdx^−/− mice grew, ezrin-based cochlear stereocilia progressively degenerated, causing deafness, whereas ezrin-based vestibular stereocilia were maintained normally in adult Rdx^−/− mice. Thus, we concluded that radixin is indispensable for the hearing ability in mice through the maintenance of cochlear stereocilia, once developed. In Rdx^−/− mice, ezrin appeared to compensate for radixin deficiency in terms of the development of cochlear stereocilia and the development/maintenance of vestibular stereocilia. These findings indicated the existence of complicate functional redundancy in situ among ERM proteins.

Espins are multifunctional actin cytoskeletal regulatory proteins in the microvilli of chemosensory and mechanosensory cells

Espins are associated with the parallel actin bundles of hair cell stereocilia and are the target of mutations that cause deafness and vestibular dysfunction in mice and humans. Here, we report that espins are also concentrated in the microvilli of a number of other sensory cells: vomeronasal organ sensory neurons, solitary chemoreceptor cells, taste cells, and Merkel cells. Moreover, we show that hair cells and these other sensory cells contain novel espin isoforms that arise from a different transcriptional start site and differ significantly from other espin isoforms in their complement of ligand-binding activities and their effects on actin polymerization. The novel espin isoforms of sensory cells bundled actin filaments with high affinity in a Ca(2+)-resistant manner, bound actin monomer via a WASP (Wiskott-Aldrich syndrome protein) homology 2 domain, bound profilin via a single proline-rich peptide, and caused a dramatic elongation of microvillus-type parallel actin bundles in transfected epithelial cells. In addition, the novel espin isoforms of sensory cells differed from other espin isoforms in that they potently inhibited actin polymerization in vitro, did not bind the Src homology 3 domain of the adapter protein insulin receptor substrate p53, and did not bind the acidic, signaling phospholipid phosphatidylinositol 4,5-bisphosphate. Thus, the espins constitute a family of multifunctional actin cytoskeletal regulatory proteins with the potential to differentially influence the organization, dimensions, dynamics, and signaling capabilities of the actin filament-rich, microvillus-type specializations that mediate sensory transduction in various mechanosensory and chemosensory cells.

Expression of prestin, a membrane motor protein, in the mammalian auditory and vestibular periphery

Hair cells are specialized mechanoreceptors common to auditory and vestibular sensory organs of mammalian and non-mammalian species. Different hair cells are believed to share common features related to their mechanosensory function. It has been shown that hair cells possess various forms of motile properties that enhance their receptor function. Membrane-based electromotility is a form of hair cell motility observed in isolated outer hair cells (OHCs) of the cochlea. A novel membrane protein, prestin, recently cloned from gerbil and rat tissues, is presumably responsible for electromotility. We cloned prestin from mouse organ of Corti and confirmed strong homology of this protein among different rodent species. We explored whether or not prestin is present in hair cells of the vestibular system. Using reverse transcription-polymerase chain reaction, we demonstrated that prestin is expressed in mouse and rat auditory and vestibular organs, but not in chicken auditory periphery. In situ hybridization and immunolocalization studies confirmed the presence of prestin in OHCs as well as in vestibular hair cells (VHCs) of rodent saccule, utricle and crista ampullaris. However, in the VHCs, staining of varying intensity with anti-prestin antibodies was observed in the cytoplasm, but not in the lateral plasma membrane or in the stereociliary membrane. Whole-cell patch-clamp recordings showed that VHCs do not possess the voltage-dependent capacitance associated with membrane-based electromotility. We conclude that although prestin is expressed in VHCs, it is unlikely that it supports the form of somatic motility observed in OHCs.

Mutations in cadherin 23 affect tip links in zebrafish sensory hair cells

Hair cells have highly organized bundles of apical projections, or stereocilia, that are deflected by sound and movement. Displacement of stereocilia stretches linkages at the tips of stereocilia that are thought to gate mechanosensory channels. To identify the molecular machinery that mediates mechanotransduction in hair cells, zebrafish mutants were identified with defects in balance and hearing. In sputnik mutants, stereociliary bundles are splayed to various degrees, with individuals displaying reduced or absent mechanotransduction. Here we show that the defects in sputnik mutants are caused by mutations in cadherin 23 (cdh23). Mutations in Cdh23 also cause deafness and vestibular defects in mice and humans, and the protein is present in hair bundles. We show that zebrafish Cdh23 protein is concentrated near the tips of hair bundles, and that tip links are absent in homozygous sputnik(tc317e) larvae. Moreover, tip links are absent in larvae carrying weak alleles of cdh23 that affect mechanotransduction but not hair bundle integrity. We conclude that Cdh23 is an essential tip link component required for hair-cell mechanotransduction.

Sensation is painless

Emily Dickinson declared: 'After great pain, a formal feeling comes'. This formal feeling begins when sensory neurons are activated by noxious stimuli, such as stepping on a tack. Recently, Seymour Benzer's group identified sensory neurons in Drosophila larvae that mediate aversive responses to noxious heat and mechanical stimuli. Thresholds for behavioral and nerve responses are elevated by mutations in the painless gene, which encodes a TRP ion channel protein. Painless thus joins an elite group of TRPs implicated in sensory transduction in insects, nematodes, mammals and fish.

Espin cross-links cause the elongation of microvillus-type parallel actin bundles in vivo

The espin actin-bundling proteins, which are the target of the jerker deafness mutation, caused a dramatic, concentration-dependent lengthening of LLC-PK1-CL4 cell microvilli and their parallel actin bundles. Espin level was also positively correlated with stereocilium length in hair cells. Villin, but not fascin or fimbrin, also produced noticeable lengthening. The espin COOH-terminal peptide, which contains the actin-bundling module, was necessary and sufficient for lengthening. Lengthening was blocked by 100 nM cytochalasin D. Espin cross-links slowed actin depolymerization in vitro less than twofold. Elimination of an actin monomer-binding WASP homology 2 domain and a profilin-binding proline-rich domain from espin did not decrease lengthening, but made it possible to demonstrate that actin incorporation was restricted to the microvillar tip and that bundles continued to undergo actin treadmilling at ∼1.5 s⁻¹ during and after lengthening. Thus, through relatively subtle effects on actin polymerization/depolymerization reactions in a treadmilling parallel actin bundle, espin cross-links cause pronounced barbed-end elongation and, thereby, make a longer bundle without joining shorter modules.

Genetics of sensory mechanotransduction

The molecular mechanisms for the transduction of light and chemical signals in animals are fairly well understood. In contrast, the processes by which the senses of touch, balance, hearing, and proprioception are transduced are still largely unknown. Biochemical approaches to identify transduction components are difficult to use with mechanosensory systems, but genetic approaches are proving more successful. Genetic research in several organisms has demonstrated the importance of cytoskeletal, extracellular, and membrane components for sensory mechanotransduction. In particular, researchers have identified channel proteins in the DEG/ENaC and TRP families that are necessary for signaling in a variety of mechanosensory cells. Proof that these proteins are components of the transduction channel, however, is incomplete.

Structure of outer hair cell stereocilia side and attachment links in the chinchilla cochlea

The structure and symmetry of chinchilla outer hair cell (OHC) stereocilia side and attachment links were investigated by transmission electron microscopy using tannic acid and Cuprolinic blue histochemical procedures. The side links run laterally between and across the rows of the stereocilia and connect the stereocilia together within the bundle. Attachment links form a crown-like array around the tips of only the tallest OHC stereocilia and attach these stereocilia to the Type B fibrils of the tectorial membrane. Computer averaging of the side links from tannic acid-treated tissues showed a central dense region of the link between adjacent stereocilia and a smaller dense portion at the plasma membrane end of the link. Computer averaging of Cuprolinic blue-treated tissues showed low electron density of the central region of the link, and the plasma membrane ends of the link were electron dense. After tannic acid treatment, the attachment links showed a diffused radial distribution around the tips of the tallest OHC stereocilia. After Cuprolinic blue treatment, the attachment links appeared as electron-dense, membrane-bound granular structures arranged with radial symmetry. The central regions of the side links are reactive to tannic acid. These regions appear to contain neutral and basic residues of proteins and participate in side-by-side association of the side links in regular aggregates. Cuprolinic blue-reactive regions of the side and attachment links appear to contain acidic sulfated residues of glycoproteins or proteoglycans, which may be involved in the attachment of these links to the stereocilium membrane.

Sensory cell damage in two-phase endolymphatic hydrops: a morphologic evaluation of a new experimental model by low-voltage scanning techniques

HYPOTHESIS

The aim of this study was to create a more dynamic animal model of Ménière's disease combining multiple causes, such as the role of endocrine factors and endolymphatic sac dysfunction, that may mimic the fluctuant characteristics of Ménière's disease.

BACKGROUND

Endolymphatic hydrops remains to be considered a pathologic substrate in the etiology of Ménière's disease. The classic guinea pig model of inducing hydrops by total destruction of the endolymphatic sac is a nonphysiologic rigid model of Ménière's disease.

METHODS

The authors developed the two-phase endolymphatic hydrops model by inducing hydrops by mild chronic endolymphatic sac dysfunction, in combination with increased endolymph production by aldosterone. Sensory cell damage was evaluated by low-voltage field emission scanning microscopy.

RESULTS

This study describes a wide spectrum of morphologic effects of the outer hair cells in radial gradients, in which most effects were observed in the third to second row of outer hair cells, and longitudinal gradients in which the most severe effects were observed in the apical turns. Most affected were the ears that underwent distal endolymphatic sac dissection followed by the administration of aldosterone. Damaging effects proceeded from degeneration and absence of short stereocilia of outer hair cells and even some inner hair cells in the apical turns, to stereociliary disarrangement and atrophy, followed by degeneration and absence of outer hair cells, which were replaced by supporting cells.

CONCLUSION

The two-phase endolymphatic hydrops model seems to represent a functional model that may mimic the fluctuant characteristics of Ménière's disease and emphasizes the influence of multiple and coexisting hydrops-inducing influences.

Mechanics of the mammalian cochlea

In mammals, environmental sounds stimulate the auditory receptor, the cochlea, via vibrations of the stapes, the innermost of the middle ear ossicles. These vibrations produce displacement waves that travel on the elongated and spirally wound basilar membrane (BM). As they travel, waves grow in amplitude, reaching a maximum and then dying out. The location of maximum BM motion is a function of stimulus frequency, with high-frequency waves being localized to the "base" of the cochlea (near the stapes) and low-frequency waves approaching the "apex" of the cochlea. Thus each cochlear site has a characteristic frequency (CF), to which it responds maximally. BM vibrations produce motion of hair cell stereocilia, which gates stereociliar transduction channels leading to the generation of hair cell receptor potentials and the excitation of afferent auditory nerve fibers. At the base of the cochlea, BM motion exhibits a CF-specific and level-dependent compressive nonlinearity such that responses to low-level, near-CF stimuli are sensitive and sharply frequency-tuned and responses to intense stimuli are insensitive and poorly tuned. The high sensitivity and sharp-frequency tuning, as well as compression and other nonlinearities (two-tone suppression and intermodulation distortion), are highly labile, indicating the presence in normal cochleae of a positive feedback from the organ of Corti, the "cochlear amplifier." This mechanism involves forces generated by the outer hair cells and controlled, directly or indirectly, by their transduction currents. At the apex of the cochlea, nonlinearities appear to be less prominent than at the base, perhaps implying that the cochlear amplifier plays a lesser role in determining apical mechanical responses to sound. Whether at the base or the apex, the properties of BM vibration adequately account for most frequency-specific properties of the responses to sound of auditory nerve fibers.

Rate influences on tone burst summating potential amplitude in electrocochleography: clinical(a) and experimental(b) data

Electrocochleographic recordings of action and summating potentials are widely used in the electrophysiological assessment of endolymphatic hydrops (ELH). Increased amplitudes of the summating potential (SP) in response to tone burst stimuli are indicative of positive ELH. This study reports the effect of repetition rate of tone burst stimulation on the SP amplitude. Using transtympanic electrocochleography (ECochG), the SP in response to 1 kHz tone bursts was recorded in both a Ménière and a non-Ménière population. Absolute values of the SP were systematically higher in the Ménière group. Moreover, in the Ménière and non-Ménière groups, the response amplitudes of the SP at a repetition rate of 8.4 tone bursts/s were only 66 and 32%, respectively, of the maximal response amplitude which was obtained at the rate of 37.4 tone bursts/s. Additionally, in normal guinea pigs chronically implanted with a round window electrode, the SP was recorded to 0.5-16 kHz tone burst stimulations presented at 100 dB SPL with the same different repetition rates. Similar enhancement of the SP amplitude was observed from 8.4 to 37.4 stimuli/s, whatever the frequency. This effect is interpreted as an increased asymmetry of vibration of the cochlear partition, whose mechanical operating point would not return to the normal resting position at high repetition rates, since it is permanently shifted in ELH.

Hair bundle profiles along the chick basilar papilla

Cochlear hair cells play a central role in the transduction of sound into neural output. Anatomical descriptions of these cells, and their protruding hair bundles, are of fundamental interest since hair cell transduction is dependent on hair bundle micromechanics and hair bundle micromechanics depends on hair bundle morphology. In this paper, we describe quantitatively changes in the staircase profile of the hair bundle along the apical portion of the chick's basilar papilla. Images of hair cells from 8 discretely dissected segments of the apical 3rd of the basilar papilla were archived, and the profile contour outlined by the tips of the stereocilia was digitised and curves were fitted by linear and power equations. The hair bundles of tall hair cells exhibited both linear and curvilinear profiles, which were equally distributed along the papilla. All short hair cells in our sample had straight contours. The differences in hair bundle shape among the tall hair cells may lead to differential susceptibility to injury and some variance in the current-displacement transduction curves due to differences in the translation of forces throughout the hair bundle.

Neuronal KCNQ potassium channels: physiology and role in disease

Humans have over 70 potassium channel genes, but only some of these have been linked to disease. In this respect, the KCNQ family of potassium channels is exceptional: mutations in four out of five KCNQ genes underlie diseases including cardiac arrhythmias, deafness and epilepsy. These disorders illustrate the different physiological functions of KCNQ channels, and provide a model for the study of the 'safety margin' that separates normal from pathological levels of channel expression. In addition, several KCNQ isoforms can associate to form heteromeric channels that underlie the M-current, an important regulator of neuronal excitability.

The deaf jerker mouse has a mutation in the gene encoding the espin actin-bundling proteins of hair cell stereocilia and lacks espins

The espins are actin-bundling proteins of brush border microvilli and Sertoli cell-spermatid junctions. We have determined that espins are also present in hair cell stereocilia and have uncovered a connection between the espin gene and jerker, a recessive mutation that causes hair cell degeneration, deafness, and vestibular dysfunction. The espin gene maps to the same region of mouse chromosome 4 as jerker. The tissues of jerker mice do not accumulate espin proteins but contain normal levels of espin mRNAs. The espin gene of jerker mice has a frameshift mutation that affects the espin C-terminal actin-bundling module. These data suggest that jerker mice are, in effect, espin null and that the jerker phenotype results from a mutation in the espin gene.

Modifier genes of hereditary hearing loss

Phenotypic variation between individuals with the same disease alleles may be attributable to the genotype at another locus, which is referred to as a modifier gene. Recent functional studies of modifier genes of hearing-loss loci have begun to refine our understanding of hearing processes and will guide the rational design of medical therapies for hearing loss.

Physiological effects of extracellular nucleotides in the inner ear

1. Electrochemical homeostasis, sound transduction and auditory neurotransmission in the cochlea are influenced by extracellular purines and pyrimidines. 2. Evidence that ATP and related nucleotides influence inner ear function arises from a considerable number of cellular, molecular and physiological studies in vitro and in vivo. 3. With a full understanding of these processes, which include ionotropic (P2X receptor) and metabotropic (P2Y receptor) signal transduction pathways, signal termination involving ecto-nucleotidases and recycling via nucleoside transporters, exciting possibilities emerge for treating hearing disorders, such as Meniere's disease, tinnitus and sensorineural deafness.

Effects of shaker-1 mutations on myosin-VIIa protein and mRNA expression

Numerous mammalian diseases have been found to be due to mutations in components of the actin cytoskeleton. Recently, mutations in the gene for an unconventional myosin, myosin-VIIa, were found to be the basis for the deafness and vestibular dysfunction observed in shaker-1 (sh1) mice and for a human deafness-blindness syndrome, Usher syndrome type 1B. Seven alleles of sh1 mice were analyzed to assess the affects of different myosin-VIIa mutations on both gene expression and tissue function. Myosin-VIIa is expressed in the inner ear and the retina, as well as the kidney, lung, and testis. Northern blot analysis indicated that myosin-VIIa mRNA expression, size, and stability were unaffected in the seven sh1 alleles. Immunoblot analysis showed that all seven alleles expressed some full-length myosin-VIIa protein. The range of expression, however, ran from sh1 [original], which expressed wild-type levels of protein, to two strains, sh1(4494SB) and sh1(4626SB), which expressed less than 1% of the normal level of myosin-VIIa protein. For the three alleles of sh1 that have been characterized and that have mutations in the motor domain, sh1 [original], sh1(816SB) and sh1(6J), the level of protein expression observed in these sh1 alleles correlated well with the predicted effects of the mutations on motor function. No change in retinal or testicular structure was observed at the light microscopic level during the life span of the seven sh1 alleles. Myosin-VIIa protein, when detectable, was observed to locate properly in the sh1 mice. On the basis of these results, we propose that the mutations in myosin-VIIa in the sh1 alleles leads to both motor dysfunction and to a protein destabilization phenotype.

Insect mechanoreception: what a long, strange TRP it's been

Insect bristles are model mechanosensory organs. An ion channel of the TRP superfamily has recently been identified which is required for production of mechanoreceptor currents by insect bristles, and seems likely to represent a new kind of mechanically gated channel.