Development and properties of stereociliary link types in hair cells of the mouse cochlea

Mutations of the mouse ELMO domain containing 1 gene (Elmod1) link small GTPase signaling to actin cytoskeleton dynamics in hair cell stereocilia

Stereocilia, the modified microvilli projecting from the apical surfaces of the sensory hair cells of the inner ear, are essential to the mechanoelectrical transduction process underlying hearing and balance. The actin-filled stereocilia on each hair cell are tethered together by fibrous links to form a highly patterned hair bundle. Although many structural components of hair bundles have been identified, little is known about the signaling mechanisms that regulate their development, morphology, and maintenance. Here, we describe two naturally occurring, allelic mutations that result in hearing and balance deficits in mice, named roundabout (rda) and roundabout-2J (rda(2J)). Positional cloning identified both as mutations of the mouse ELMO domain containing 1 gene (Elmod1), a poorly characterized gene with no previously reported mutant phenotypes. The rda mutation is a 138 kb deletion that includes exons 1-5 of Elmod1, and rda(2J) is an intragenic duplication of exons 3-8 of Elmod1. The deafness associated with these mutations is caused by cochlear hair cell dysfunction, as indicated by conspicuous elongations and fusions of inner hair cell stereocilia and progressive degeneration of outer hair cell stereocilia. Mammalian ELMO-family proteins are known to be involved in complexes that activate small GTPases to regulate the actin cytoskeleton during phagocytosis and cell migration. ELMOD1 and ELMOD2 recently were shown to function as GTPase-activating proteins (GAPs) for the Arf family of small G proteins. Our finding connecting ELMOD1 deficiencies with stereocilia dysmorphologies thus establishes a link between the Ras superfamily of small regulatory GTPases and the actin cytoskeleton dynamics of hair cell stereocilia.

Current understanding of usher syndrome type II

Usher syndrome is the most common deafness-blindness caused by genetic mutations. To date, three genes have been identified underlying the most prevalent form of Usher syndrome, the type II form (USH2). The proteins encoded by these genes are demonstrated to form a complex in vivo. This complex is localized mainly at the periciliary membrane complex in photoreceptors and the ankle-link of the stereocilia in hair cells. Many proteins have been found to interact with USH2 proteins in vitro, suggesting that they are potential additional components of this USH2 complex and that the genes encoding these proteins may be the candidate USH2 genes. However, further investigations are critical to establish their existence in the USH2 complex in vivo. Based on the predicted functional domains in USH2 proteins, their cellular localizations in photoreceptors and hair cells, the observed phenotypes in USH2 mutant mice, and the known knowledge about diseases similar to USH2, putative biological functions of the USH2 complex have been proposed. Finally, therapeutic approaches for this group of diseases are now being actively explored.

Sensing sound: molecules that orchestrate mechanotransduction by hair cells

Animals use acoustic signals to communicate and to obtain information about their environment. The processing of acoustic signals is initiated at auditory sense organs, where mechanosensory hair cells convert sound-induced vibrations into electrical signals. Although the biophysical principles underlying the mechanotransduction process in hair cells have been characterized in much detail over the past 30 years, the molecular building-blocks of the mechanotransduction machinery have proved to be difficult to determine. We review here recent studies that have both identified some of these molecules and established the mechanisms by which they regulate the activity of the still-elusive mechanotransduction channel.

Whirlin interacts with espin and modulates its actin-regulatory function: an insight into the mechanism of Usher syndrome type II

Whirlin mutations cause retinal degeneration and hearing loss in Usher syndrome type II (USH2) and non-syndromic deafness, DFNB31. Its protein recruits other USH2 causative proteins to form a complex at the periciliary membrane complex in photoreceptors and the ankle link of the stereocilia in hair cells. However, the biological function of this USH2 protein complex is largely unknown. Using a yeast two-hybrid screen, we identified espin, an actin-binding/bundling protein involved in human deafness when defective, as a whirlin-interacting protein. The interaction between these two proteins was confirmed by their coimmunoprecipitation and colocalization in cultured cells. This interaction involves multiple domains of both proteins and only occurs when espin does not bind to actin. Espin was partially colocalized with whirlin in the retina and the inner ear. In whirlin knockout mice, espin expression changed significantly in these two tissues. Further studies found that whirlin increased the mobility of espin and actin at the actin bundles cross-linked by espin and, eventually, affected the dimension of these actin bundles. In whirlin knockout mice, the stereocilia were thickened in inner hair cells. We conclude that the interaction between whirlin and espin and the balance between their expressions are required to maintain the actin bundle network in photoreceptors and hair cells. Disruption of this actin bundle network contributes to the pathogenic mechanism of hearing loss and retinal degeneration caused by whirlin and espin mutations. Espin is a component of the USH2 protein complex and could be a candidate gene for Usher syndrome.

Integrating the biophysical and molecular mechanisms of auditory hair cell mechanotransduction

Mechanosensation is a primitive and somewhat ubiquitous sense. At the inner ear, sensory hair cells are refined to enhance sensitivity, dynamic range and frequency selectivity. Thirty years ago, mechanisms of mechanotransduction and adaptation were well accounted for by simple mechanical models that incorporated physiological and morphological properties of hair cells. Molecular and genetic tools, coupled with new optical techniques, are now identifying and localizing specific components of the mechanotransduction machinery. These new findings challenge long-standing theories, and require modification of old and development of new models. Future advances require the integration of molecular and physiological data to causally test these new hypotheses.

Auditory and vestibular hair cell stereocilia: relationship between functionality and inner ear disease

The stereocilia of the inner ear are unique cellular structures which correlate anatomically with distinct cochlear functions, including mechanoelectrical transduction, cochlear amplification, adaptation, frequency selectivity and tuning. Their function is impaired by inner ear stressors, by various types of hereditary deafness, syndromic hearing loss and inner ear disease (e.g. Ménière's disease). The anatomical and physiological characteristics of stereocilia are discussed in relation to inner ear malfunctions.

Regulation of PCDH15 function in mechanosensory hair cells by alternative splicing of the cytoplasmic domain

Protocadherin 15 (PCDH15) is expressed in hair cells of the inner ear and in photoreceptors of the retina. Mutations in PCDH15 cause Usher Syndrome (deaf-blindness) and recessive deafness. In developing hair cells, PCDH15 localizes to extracellular linkages that connect the stereocilia and kinocilium into a bundle and regulate its morphogenesis. In mature hair cells, PCDH15 is a component of tip links, which gate mechanotransduction channels. PCDH15 is expressed in several isoforms differing in their cytoplasmic domains, suggesting that alternative splicing regulates PCDH15 function in hair cells. To test this model, we generated three mouse lines, each of which lacks one out of three prominent PCDH15 isoforms (CD1, CD2 and CD3). Surprisingly, mice lacking PCDH15-CD1 and PCDH15-CD3 form normal hair bundles and tip links and maintain hearing function. Tip links are also present in mice lacking PCDH15-CD2. However, PCDH15-CD2-deficient mice are deaf, lack kinociliary links and have abnormally polarized hair bundles. Planar cell polarity (PCP) proteins are distributed normally in the sensory epithelia of the mutants, suggesting that PCDH15-CD2 acts downstream of PCP components to control polarity. Despite the absence of kinociliary links, vestibular function is surprisingly intact in the PCDH15-CD2 mutants. Our findings reveal an essential role for PCDH15-CD2 in the formation of kinociliary links and hair bundle polarization, and show that several PCDH15 isoforms can function redundantly at tip links.

Dynamic changes in hair cell stereocilia and cochlear transduction after noise exposure

The structures of cochlear transduction include stereocilia at the apical surface of hair cells and their connection to the tectorial membrane. The transduction site is one of the loci for noise-induced cochlear damage. Although stereocilia are susceptible to noise, it has been found that in the inner ears of avians, this fragile structure is largely self-repairing and is associated with recovery of hearing sensitivity after noise exposure, as observed in the difference between the temporal threshold shift (TTS) and the permanent threshold shift (PTS). In the mammalian cochleae, however, threshold shifts measured in the auditory brainstem responses (ABR) did not parallel the chronological changes in the stereocilia on hair cells. It is unclear how the morphological recovery of the stereocilia on the mammalian hair cells is correlated with the changes in cochlear transduction that can be assessed by measuring receptor potential. In the present study, guinea pigs were exposed to a broadband noise of 110 dB SPL for 2h. Auditory sensitivity was evaluated using ABR and cochlear transduction was assessed using cochlear microphonics (CM). Stereocilia morphology was quantified at different time points after the noise and compared with the control. The noise produced a TTS of 55.69 ± 14.13 dB in frequency-averaged ABR thresholds. The threshold shift was reduced to 9.58 ± 11.75 dB SPL 1 month later with virtually no loss of hair cells. Damage to the stereocilia immediately after noise exposure was found to be associated with depression of CM amplitude. Virtually no abnormal stereocilia were observed 1month after the noise in association with a fully recovered CM.

Evidence for multiple, developmentally regulated isoforms of Ptprq on hair cells of the inner ear

Ptprq is a receptor-like inositol lipid phosphatase associated with the shaft connectors of hair bundles. Three lines of evidence suggest Ptprq is a chondroitin sulfate proteoglycan: (1) chondroitinase ABC treatment causes a loss of the ruthenium-red reactive, electron-dense particles associated with shaft connectors, (2) chondroitinase ABC causes an increase in the electrophoretic mobility of Ptprq, and (3) hair bundles in the developing inner ear of wild-type mice, but not those of Ptprq(-/-) mice, react with monoclonal antibody (mAb) 473-HD, an IgM that recognizes the dermatan-sulfate-dependent epitope DSD1. Two lines of evidence indicate that there may be multiple isoforms of Ptprq expressed in hair bundles. First, although Ptprq is expressed throughout the lifetime of most hair cells, hair bundles in the mouse and chick inner ear only express the DSD1 epitope transiently during development. Second, mAb H10, a novel mAb that recognizes an epitope common to several avian inner-ear proteins including Ptprq, only stains mature hair bundles in the extrastriolar regions of the vestibular maculae. MAb H10 does not stain mature hair bundles in the striolar regions of the maculae or in the basilar papilla, nor does it stain immature hair bundles in any organ. Three distinct, developmentally regulated isoforms of Ptprq may therefore be expressed on hair bundles of the chick inner ear. Hair bundles in the mature chick ear that do not express the H10 epitope have longer shaft connectors than those that do, indicating the presence or absence of the H10 epitope on Ptprq may modulate the spacing of stereocilia.

Expression and vesicular localization of mouse Trpml3 in stria vascularis, hair cells, and vomeronasal and olfactory receptor neurons

TRPML3 is a member of the mucolipin branch of the transient receptor potential cation channel family. A dominant missense mutation in Trpml3 (also known as Mcoln3) causes deafness and vestibular impairment characterized by stereocilia disorganization, hair cell loss, and endocochlear potential reduction. Both marginal cells of the stria vascularis and hair cells express Trpml3 mRNA. Here we used in situ hybridization, quantitative RT-qPCR, and immunohistochemistry with several antisera raised against TRPML3 to determine the expression and subcellular distribution of TRPML3 in the inner ear as well as in other sensory organs. We also use Trpml3 knockout tissues to distinguish TRPML3-specific from nonspecific immunoreactivities. We find that TRPML3 localizes to vesicles of hair cells and strial marginal cells but not to stereociliary ankle links or pillar cells, which nonspecifically react with two antisera raised against TRPML3. Upon cochlear maturation, TRPML3 protein is redistributed to perinuclear vesicles of strial marginal cells and is augmented in inner hair cells vs. outer hair cells. Mouse somatosensory neurons, retinal neurons, and taste receptor cells do not appear to express physiologically relevant levels of TRPML3. Finally, we found that vomeronasal and olfactory sensory receptor cells do express TRPML3 mRNA and protein, which localizes to vesicles in their somas and dendrites as well as at apical dendritic knobs.

Carcinoembryonic antigen-related cell adhesion molecule 16 interacts with alpha-tectorin and is mutated in autosomal dominant hearing loss (DFNA4)

We report on a secreted protein found in mammalian cochlear outer hair cells (OHC) that is a member of the carcinoembryonic antigen-related cell adhesion molecule (CEACAM) family of adhesion proteins. Ceacam16 mRNA is expressed in OHC, and its protein product localizes to the tips of the tallest stereocilia and the tectorial membrane (TM). This specific localization suggests a role in maintaining the integrity of the TM as well as in the connection between the OHC stereocilia and TM, a linkage essential for mechanical amplification. In agreement with this role, CEACAM16 colocalizes and coimmunoprecipitates with the TM protein α-tectorin. In addition, we show that mutation of CEACAM16 leads to autosomal dominant nonsyndromic deafness (ADNSHL) at the autosomal dominant hearing loss (DFNA4) locus. In aggregate, these data identify CEACAM16 as an α-tectorin-interacting protein that concentrates at the point of attachment of the TM to the stereocilia and, when mutated, results in ADNSHL at the DFNA4 locus.

Stereocilin connects outer hair cell stereocilia to one another and to the tectorial membrane

Stereocilin is defective in a recessive form of deafness, DFNB16. We studied the distribution of stereocilin in the developing and mature mouse inner ear and analyzed the consequences of its absence in stereocilin-null (Strc(-/-)) mice that suffer hearing loss starting at postnatal day 15 (P15) and progressing until P60. Using immunofluorescence and immunogold electron microscopy, stereocilin was detected in association with two cell surface specializations specific to outer hair cells (OHCs) in the mature cochlea: the horizontal top connectors that join the apical regions of adjacent stereocilia within the hair bundle, and the attachment links that attach the tallest stereocilia to the overlying tectorial membrane. Stereocilin was also detected around the kinocilium of vestibular hair cells and immature OHCs. In Strc(-/-) mice the OHC hair bundle was structurally and functionally normal until P9. Top connectors, however, did not form and the cohesiveness of the OHC hair bundle progressively deteriorated from P10. The stereocilia were still interconnected by tip links at P14, but these progressively disappeared from P15. By P60 the stereocilia, still arranged in a V-shaped bundle, were fully disconnected from each other. Stereocilia imprints on the lower surface of the tectorial membrane were also not observed in Strc(-/-) mice, thus indicating that the tips of the tallest stereocilia failed to be embedded in this gel. We conclude that stereocilin is essential to the formation of horizontal top connectors. We propose that these links, which maintain the cohesiveness of the mature OHC hair bundle, are required for tip-link turnover.

β-actin and γ-actin are each dispensable for auditory hair cell development but required for Stereocilia maintenance

Hair cell stereocilia structure depends on actin filaments composed of cytoplasmic β-actin and γ-actin isoforms. Mutations in either gene can lead to progressive hearing loss in humans. Since β-actin and γ-actin isoforms are 99% identical at the protein level, it is unclear whether each isoform has distinct cellular roles. Here, we compared the functions of β-actin and γ-actin in stereocilia formation and maintenance by generating mice conditionally knocked out for Actb or Actg1 in hair cells. We found that, although cytoplasmic actin is necessary, neither β-actin nor γ-actin is required for normal stereocilia development or auditory function in young animals. However, aging mice with β-actin– or γ-actin–deficient hair cells develop different patterns of progressive hearing loss and distinct pathogenic changes in stereocilia morphology, despite colocalization of the actin isoforms. These results demonstrate overlapping developmental roles but unique post-developmental functions for β-actin and γ-actin in maintaining hair cell stereocilia.

Genetic mutations that cause progressive hearing loss offer insight into the cellular processes that are required to maintain auditory function. In humans, several such deafness-causing mutations have been identified in the gene encoding γ-actin. This protein, together with the closely-related β-actin protein, comprise the primary structural elements of stereocilia, which are specialized structures on sensory cells in the inner ear that convert mechanical sound energy into neural signals. β-actin and γ-actin are 99% identical, but their slight differences have been exactly conserved through evolution from birds to mammals suggesting that each protein may have important and distinct functions. We tested this idea by knocking out each gene in mouse auditory hair cells. Although the isoforms are completely redundant during stereocilia development, β-actin and γ-actin have different functions during stereocilia maintenance. Both knockout mice had normal hearing at young ages, but developed specific types of progressive hearing loss and stereocilia pathology that differed depending on which isoform was lost. These separate maintenance pathways are likely important for maintaining auditory function during aging and may contribute to future understanding of common forms of age-related hearing loss in humans.

Development and regeneration of sensory transduction in auditory hair cells requires functional interaction between cadherin-23 and protocadherin-15

Tip links are extracellular filaments that connect pairs of hair cell stereocilia and convey tension to mechanosensitive channels. Recent evidence suggests that tip links are formed by calcium-dependent interactions between the N-terminal domains of cadherin-23 (CDH23) and protocadherin-15 (PCDH15). Mutations in either CDH23 or PCDH15 cause deafness in mice and humans, indicating the molecules are required for normal inner ear function. However, there is little physiological evidence to support a direct role for CDH23 and PCDH15 in hair cell mechanotransduction. To investigate the contributions of CDH23 and PCDH15 to mechanotransduction and tip-link formation, we examined outer hair cells of mouse cochleas during development and after chemical disruption of tip links. We found that tip links and mechanotransduction with all the qualitative properties of mature transduction recovered within 24 h after disruption. To probe tip-link formation, we measured transduction currents after extracellular application of recombinant CDH23 and PCDH15 fragments, which included putative interaction domains (EC1). Both fragments inhibited development and regeneration of transduction but did not disrupt transduction in mature cells. PCDH15 fragments that carried a mutation in EC1 that causes deafness in humans did not inhibit transduction development or regeneration. Immunolocalization revealed wild-type fragments bound near the tips of hair cell stereocilia. Scanning electron micrographs revealed that hair bundles exposed to fragments had a reduced number of linkages aligned along the morphological axis of sensitivity of the bundle. Together, the data provide direct evidence implicating CDH23 and PCDH15 proteins in the formation of tip links during development and regeneration of mechanotransduction.

Sliding adhesion confers coherent motion to hair cell stereocilia and parallel gating to transduction channels

When the tip of a hair bundle is deflected by a sensory stimulus, the stereocilia pivot as a unit, producing a shearing displacement between adjacent tips. It is not clear how stereocilia can stick together laterally but still shear. We used dissociated hair cells from the bullfrog saccule and high-speed video imaging to characterize this sliding adhesion. Movement of individual stereocilia was proportional to height, indicating that stereocilia pivot at their basal insertion points. All stereocilia moved by approximately the same angular deflection, and the same motion was observed at 1, 20, and 700 Hz stimulus frequency. Motions were consistent with a geometric model that assumes the stiffness of lateral links holding stereocilia together is >1000 times the pivot stiffness of stereocilia and that these links can slide in the plane of the membrane-in essence, that stereocilia shear without separation. The same motion was observed when bundles were moved perpendicular to the tip links, or when tip links, ankle links, and shaft connectors were cut, ruling out these links as the basis for sliding adhesion. Stereocilia rootlets are angled toward the center of the bundle, tending to push stereocilia tips together for small deflections. However, stereocilia remained cohesive for deflections of up to +/-35 degrees, ruling out rootlet prestressing as the basis for sliding adhesion. These observations suggest that horizontal top connectors mediate a sliding adhesion. They also indicate that all transduction channels of a hair cell are mechanically in parallel, an arrangement that may enhance amplification in the inner ear.

Recovery of mechano-electrical transduction in rat cochlear hair bundles after postnatal destruction of the stereociliar cross-links

Mechano-electrical transduction (MET) in the stereocilia of outer hair cells (OHCs) was studied in newborn Wistar rats using scanning electron microscopy to investigate the stereociliar cross-links, Nomarski laser differential interferometry to investigate stereociliar stiffness and by testing the functionality of the MET channels by recording the entry of fluorescent dye, FM1-43, into stereocilia. Preparations were taken from rats on their day of birth (P0) or 1–4 days later (P1–P4). Hair bundles developed from the base to the apex and from the inner to outer OHC rows. MET channel responses were detected in apical coil OHCs on P1. To study the possible recovery of MET after disrupting the cross-links, the same investigations were performed after the application of Ca²⁺ chelator 1,2-bis(o-aminophenoxy)ethane-N,N,N′,N′-tetraacetic acid (BAPTA) and allowing the treated samples to recover in culture medium for 0–20 h. We found that the structure and function were abolished by BAPTA. In P0–P1 samples, structural recovery was complete and the open probability of MET channels reached control values. In P3–P4 samples, complete recovery only occurred in OHCs of the outermost row. Although our results demonstrate an enormous recovery potential of OHCs in the postnatal period, the structural component restricts the potential for therapy in patients.

Review series: The cell biology of hearing

Mammals have an astonishing ability to sense and discriminate sounds of different frequencies and intensities. Fundamental for this process are mechanosensory hair cells in the inner ear that convert sound-induced vibrations into electrical signals. The study of genes that are linked to deafness has provided insights into the cell biological mechanisms that control hair cell development and their function as mechanosensors.

Cadherin-23, myosin VIIa and harmonin, encoded by Usher syndrome type I genes, form a ternary complex and interact with membrane phospholipids

Cadherin-23 is a component of early transient lateral links of the auditory sensory cells' hair bundle, the mechanoreceptive structure to sound. This protein also makes up the upper part of the tip links that control gating of the mechanoelectrical transduction channels. We addressed the issue of the molecular complex that anchors these links to the hair bundle F-actin core. By using surface plasmon resonance assays, we show that the cytoplasmic regions of the two cadherin-23 isoforms that do or do not contain the exon68-encoded peptide directly interact with harmonin, a submembrane PDZ (post-synaptic density, disc large, zonula occludens) domain-containing protein, with unusually high affinity. This interaction involves the harmonin Nter-PDZ1 supramodule, but not the C-terminal PDZ-binding motif of cadherin-23. We establish that cadherin-23 directly binds to the tail of myosin VIIa. Moreover, cadherin-23, harmonin and myosin VIIa can form a ternary complex, which suggests that myosin VIIa applies tension forces on hair bundle links. We also show that the cadherin-23 cytoplasmic region, harmonin and myosin VIIa interact with phospholipids on synthetic liposomes. Harmonin and the cytoplasmic region of cadherin-23, both independently and as a binary complex, can bind specifically to phosphatidylinositol 4,5-bisphosphate (PI(4,5)P₂), which may account for the role of this phospholipid in the adaptation of mechanoelectrical transduction in the hair bundle. The distributions of cadherin-23, harmonin, myosin VIIa and PI(4,5)P₂ in the growing and mature auditory hair bundles as well as the abnormal locations of harmonin and myosin VIIa in cadherin-23 null mutant mice strongly support the functional relevance of these interactions.

Actin-bundling protein TRIOBP forms resilient rootlets of hair cell stereocilia essential for hearing

Inner ear hair cells detect sound through deflection of mechanosensory stereocilia. Each stereocilium is supported by a paracrystalline array of parallel actin filaments that are packed more densely at the base, forming a rootlet extending into the cell body. The function of rootlets and the molecules responsible for their formation are unknown. We found that TRIOBP, a cytoskeleton-associated protein mutated in human hereditary deafness DFNB28, is localized to rootlets. In vitro, purified TRIOBP isoform 4 protein organizes actin filaments into uniquely dense bundles reminiscent of rootlets but distinct from bundles formed by espin, an actin crosslinker in stereocilia. We generated mutant Triobp mice (Triobp(Deltaex8/Deltaex8)) that are profoundly deaf. Stereocilia of Triobp(Deltaex8/Deltaex8) mice develop normally but fail to form rootlets and are easier to deflect and damage. Thus, F-actin bundling by TRIOBP provides durability and rigidity for normal mechanosensitivity of stereocilia and may contribute to resilient cytoskeletal structures elsewhere.

Ablation of whirlin long isoform disrupts the USH2 protein complex and causes vision and hearing loss

Mutations in whirlin cause either Usher syndrome type II (USH2), a deafness-blindness disorder, or nonsyndromic deafness. The molecular basis for the variable disease expression is unknown. We show here that only the whirlin long isoform, distinct from a short isoform by virtue of having two N-terminal PDZ domains, is expressed in the retina. Both long and short isoforms are expressed in the inner ear. The N-terminal PDZ domains of the long whirlin isoform mediates the formation of a multi-protein complex that includes usherin and VLGR1, both of which are also implicated in USH2. We localized this USH2 protein complex to the periciliary membrane complex (PMC) in mouse photoreceptors that appears analogous to the frog periciliary ridge complex. The latter is proposed to play a role in photoreceptor protein trafficking through the connecting cilium. Mice carrying a targeted disruption near the N-terminus of whirlin manifest retinal and inner ear defects, reproducing the clinical features of human USH2 disease. This is in contrast to mice with mutations affecting the C-terminal portion of whirlin in which the phenotype is restricted to the inner ear. In mice lacking any one of the USH2 proteins, the normal localization of all USH2 proteins is disrupted, and there is evidence of protein destabilization. Taken together, our findings provide new insights into the pathogenic mechanism of Usher syndrome. First, the three USH2 proteins exist as an obligatory functional complex in vivo, and loss of one USH2 protein is functionally close to loss of all three. Second, defects in the three USH2 proteins share a common pathogenic process, i.e., disruption of the PMC. Third, whirlin mutations that ablate the N-terminal PDZ domains lead to Usher syndrome, but non-syndromic hearing loss will result if they are spared.

Cochlear outer hair cells undergo an apical circumference remodeling constrained by the hair bundle shape

Epithelial cells acquire diverse shapes relating to their different functions. This is particularly relevant for the cochlear outer hair cells (OHCs), whose apical and basolateral shapes accommodate the functioning of these cells as mechano-electrical and electromechanical transducers, respectively. We uncovered a circumferential shape transition of the apical junctional complex (AJC) of OHCs, which occurs during the early postnatal period in the mouse, prior to hearing onset. Geometric analysis of the OHC apical circumference using immunostaining of the AJC protein ZO1 and Fourier-interpolated contour detection characterizes this transition as a switch from a rounded-hexagon to a non-convex circumference delineating two lateral lobes at the neural side of the cell, with a negative curvature in between. This shape tightly correlates with the 'V'-configuration of the OHC hair bundle, the apical mechanosensitive organelle that converts sound-evoked vibrations into variations in cell membrane potential. The OHC apical circumference remodeling failed or was incomplete in all the mouse mutants affected in hair bundle morphogenesis that we tested. During the normal shape transition, myosin VIIa and myosin II (A and B isoforms) displayed polarized redistributions into and out of the developing lobes, respectively, while Shroom2 and F-actin transiently accumulated in the lobes. Defects in these redistributions were observed in the mutants, paralleling their apical circumference abnormalities. Our results point to a pivotal role for actomyosin cytoskeleton tensions in the reshaping of the OHC apical circumference. We propose that this remodeling contributes to optimize the mechanical coupling between the basal and apical poles of mature OHCs.

Targeted capture and next-generation sequencing identifies C9orf75, encoding taperin, as the mutated gene in nonsyndromic deafness DFNB79

Targeted genome capture combined with next-generation sequencing was used to analyze 2.9 Mb of the DFNB79 interval on chromosome 9q34.3, which includes 108 candidate genes. Genomic DNA from an affected member of a consanguineous family segregating recessive, nonsyndromic hearing loss was used to make a library of fragments covering the DFNB79 linkage interval defined by genetic analyses of four pedigrees. Homozygosity for eight previously unreported variants in transcribed sequences was detected by evaluating a library of 402,554 sequencing reads and was later confirmed by Sanger sequencing. Of these variants, six were determined to be polymorphisms in the Pakistani population, and one was in a noncoding gene that was subsequently excluded genetically from the DFNB79 linkage interval. The remaining variant was a nonsense mutation in a predicted gene, C9orf75, renamed TPRN. Evaluation of the other three DFNB79-linked families identified three additional frameshift mutations, for a total of four truncating alleles of this gene. Although TPRN is expressed in many tissues, immunolocalization of the protein product in the mouse cochlea shows prominent expression in the taper region of hair cell stereocilia. Consequently, we named the protein taperin.

Mutations in Grxcr1 are the basis for inner ear dysfunction in the pirouette mouse

Recessive mutations at the mouse pirouette (pi) locus result in hearing loss and vestibular dysfunction due to neuroepithelial defects in the inner ear. Using a positional cloning strategy, we have identified mutations in the gene Grxcr1 (glutaredoxin cysteine-rich 1) in five independent allelic strains of pirouette mice. We also provide sequence data of GRXCR1 from humans with profound hearing loss suggesting that pirouette is a model for studying the mechanism of nonsyndromic deafness DFNB25. Grxcr1 encodes a 290 amino acid protein that contains a region of similarity to glutaredoxin proteins and a cysteine-rich region at its C terminus. Grxcr1 is expressed in sensory epithelia of the inner ear, and its encoded protein is localized along the length of stereocilia, the actin-filament-rich mechanosensory structures at the apical surface of auditory and vestibular hair cells. The precise architecture of hair cell stereocilia is essential for normal hearing. Loss of function of Grxcr1 in homozygous pirouette mice results in abnormally thin and slightly shortened stereocilia. When overexpressed in transfected cells, GRXCR1 localizes along the length of actin-filament-rich structures at the dorsal-apical surface and induces structures with greater actin filament content and/or increased lengths in a subset of cells. Our results suggest that deafness in pirouette mutants is associated with loss of GRXCR1 function in modulating actin cytoskeletal architecture in the developing stereocilia of sensory hair cells.

Studies on the very large G protein-coupled receptor: from initial discovery to determining its role in sensorineural deafness in higher animals

The very large G protein-coupled receptor 1 (VLGRI), also known as MASS1 or GPR98, is most notable among the family of adhesion-GPCR for its size. Encoded by an 18.9 kb open reading frame, the approximately 700 kDa primary translation product is by far the largest GPCR and additionally, the largest cell surface protein known to date. The large ectodomain of the protein contains several repeated motifs, including some 35 calcium binding, Calx-beta repeats and seven copies of an epitempin repeat thought to be associated with the development of epilepsy. The extreme carboxy-terminus contains a consensus PDZ ligand sequence, suggesting interactions with other cytosolic or cytoskeletal proteins. At least two spontaneous and two targeted mutant mouse lines are currently known. The mutant mice present with sensitivity to audiogenic seizures but also have cochlear defects and significant, progressive hearing impairment. Although its ligand is currently unknown, VLGR1 is one of the few adhesion-GPCR family members in which mutations have been shown to be responsible for a human malady. Mutations in VLGRI in humans result in one form (2C) of Usher syndrome, the most common genetic cause of combined blindness and deafness.

Fate of mammalian cochlear hair cells and stereocilia after loss of the stereocilia

Cochlear hair cells transduce mechanical stimuli into electrical activity. The site of hair cell transduction is the hair bundle, an array of stereocilia with different height arranged in a staircase. Tip links connect the apex of each stereocilium to the side of its taller neighbor. The hair bundle and tip links of hair cells are susceptible to acoustic trauma and ototoxic drugs. It has been shown that hair cells in lower vertebrates and in the mammalian vestibular system may survive bundle loss and undergo self-repair of the stereocilia. Our goals were to determine whether cochlear hair cells could survive the trauma and whether the tip link and/or the hair bundle could be regenerated. We simulated the acoustic trauma-induced tip link damage or stereociliary loss by disrupting tip links or ablating the hair bundles in the cultured organ of Corti from neonatal gerbils. Hair-cell fate and stereociliary morphology and function were examined using confocal and scanning electron microscopies and electrophysiology. Most bundleless hair cells survived and developed for approximately 2 weeks. However, no spontaneous hair-bundle regeneration was observed. When tip links were ruptured, repair of tip links and restoration of mechanotransduction were observed in <24 h. Our study suggests that the dynamic nature of the hair cell's transduction apparatus is retained despite the fact that regeneration of the hair bundle is lost in mammalian cochlear hair cells.

Tip links in hair cells: molecular composition and role in hearing loss

PURPOSE OF REVIEW

Tip links are thought to be an essential element of the mechanoelectrical transduction (MET) apparatus in sensory hair cells of the inner ear. The molecules that form tip links have recently been identified, and the analysis of their properties has not only changed our view of MET but also suggests that tip-link defects can cause hearing loss.

RECENT FINDINGS

Structural, histological and biochemical studies show that the extracellular domains of two deafness-associated cadherins, cadherin 23 (CDH23) and protocadherin 15 (PCDH15), interact in trans to form the upper and lower part of each tip link, respectively. High-speed Ca imaging suggests that MET channels are localized exclusively at the lower end of each tip link. Biochemical and genetic studies provide evidence that defects in tip links cause hearing impairment in humans.

SUMMARY

The identification of the proteins that form tip links have shed new light on the molecular basis of MET and the mechanisms causing hereditary deafness, noise-induced hearing loss and presbycusis.

A mechanical model of the gating spring mechanism of stereocilia

The stereocilium is the basic sensory unit of nature's mechanotransducers, which include the cochlear and vestibular organs. In noisy environments, stereocilia display high sensitivity to miniscule stimuli, effectively dealing with a situation that is a design challenge in micro systems. The gating spring hypothesis suggests that the mechanical stiffness of stereocilia bundle is softened by tip-link gating in combination with active bundle movement, contributing to the nonlinear amplification of miniscule stimuli. To demonstrate that the amplification is induced mechanically by the gating as hypothesized, we developed a biomimetic model of stereocilia and fabricated the model at the macro scale. The model consists of an inverted pendulum array with bistable buckled springs at its tips, which represent the mechanically gated ion channel. Model simulations showed that at the moment of gating, instantaneous stiffness softening generates an increase in response magnitude, which then sequentially occurs as the number of gating increases. This amplification mechanism appeared to be robust to the change of model parameters. Experimental data from the fabricated macro model also showed a significant increase in the open probability and pendulum deflection at the region having a smaller input magnitude. The results demonstrate that the nonlinear amplification of miniscule stimuli is mechanically produced by stiffness softening from channel gating.

Linking genes underlying deafness to hair-bundle development and function

The identification of genes underlying monogenic, early-onset forms of deafness in humans has provided unprecedented insight into the molecular mechanisms of hearing in the peripheral auditory system. The molecules involved in the development and function of the cochlea eluded characterization until recently due to the paucity of the principle cell types present in cochlear hair cells, yet a genetic approach has circumvented this problem and succeeded in identifying proteins and deciphering some of the molecular complexes that operate in these cells . In combination with mouse models, the genetic approach is now revealing some of the principles underlying the development and physiology of the cochlea. The review centers on this facet of the genetics of hearing. Focusing on the hair bundle, the mechanosensory device of the sensory hair cell, we highlight recent advances in understanding the way in which the hair bundle is formed, how it operates as a mechanotransducer and how it processes sound. In particular, we discuss how this work highlights the roles played by various hair-bundle link types.

Fast adaptation and Ca2+ sensitivity of the mechanotransducer require myosin-XVa in inner but not outer cochlear hair cells

In inner ear hair cells, activation of mechanotransduction channels is followed by extremely rapid deactivation that depends on the influx of Ca(2+) through these channels. Although the molecular mechanisms of this "fast" adaptation are largely unknown, the predominant models assume Ca(2+) sensitivity as an intrinsic property of yet unidentified mechanotransduction channels. Here, we examined mechanotransduction in the hair cells of young postnatal shaker 2 mice (Myo15(sh2/sh2)). These mice have no functional myosin-XVa, which is critical for normal growth of mechanosensory stereocilia of hair cells. Although stereocilia of both inner and outer hair cells of Myo15(sh2/sh2) mice lack myosin-XVa and are abnormally short, these cells have dramatically different hair bundle morphology. Myo15(sh2/sh2) outer hair cells retain a staircase arrangement of the abnormally short stereocilia and prominent tip links. Myo15(sh2/sh2) inner hair cells do not have obliquely oriented tip links, and their mechanosensitivity is mediated exclusively by "top-to-top" links between equally short stereocilia. In both inner and outer hair cells of Myo15(sh2/sh2) mice, we found mechanotransduction responses with a normal "wild-type" amplitude and speed of activation. Surprisingly, only outer hair cells exhibit fast adaptation and sensitivity to extracellular Ca(2+). In Myo15(sh2/sh2) inner hair cells, fast adaptation is disrupted and the transduction current is insensitive to extracellular Ca(2+). We conclude that the Ca(2+) sensitivity of the mechanotransduction channels and the fast adaptation require a structural environment that is dependent on myosin-XVa and is disrupted in Myo15(sh2/sh2) inner hair cells, but not in Myo15(sh2/sh2) outer hair cells.

Tonotopic gradient in the developmental acquisition of sensory transduction in outer hair cells of the mouse cochlea

Inner ear hair cells are exquisite mechanosensors that transduce nanometer scale deflections of their sensory hair bundles into electrical signals. Several essential elements must be precisely assembled during development to confer the unique structure and function of the mechanotransduction apparatus. Here we investigated the functional development of the transduction complex in outer hair cells along the length of mouse cochlea acutely excised between embryonic day 17 (E17) and postnatal day 8 (P8). We charted development of the stereociliary bundle using scanning electron microscopy; FM1-43 uptake, which permeates hair cell transduction channels, mechanotransduction currents evoked by rapid hair bundle deflections, and mRNA expression of possible components of the transduction complex. We demonstrated that uptake of FM1-43 first occurred in the basal portion of the cochlea at P0 and progressed toward the apex over the subsequent week. Electrophysiological recordings obtained from 234 outer hair cells between E17 and P8 from four cochlear regions revealed a correlation between the pattern of FM1-43 uptake and the acquisition of mechanotransduction. We found a spatiotemporal gradient in the properties of transduction including onset, amplitude, operating range, time course, and extent of adaptation. We used quantitative RT-PCR to examine relative mRNA expression of several hair cell myosins and candidate tip-link molecules. We found spatiotemporal expression patterns for mRNA that encodes cadherin 23, protocadherin 15, myosins 3a, 7a, 15a, and PMCA2 that preceded the acquisition of transduction. The spatiotemporal expression patterns of myosin 1c and PMCA2 mRNA were correlated with developmental changes in several properties of mechanotransduction.

Stereocilin-deficient mice reveal the origin of cochlear waveform distortions

Although the cochlea is an amplifier and a remarkably sensitive and finely tuned detector of sounds, it also produces conspicuous mechanical and electrical waveform distortions. These distortions reflect nonlinear mechanical interactions within the cochlea. By allowing one tone to suppress another (masking effect), they contribute to speech intelligibility. Tones can also combine to produce sounds with frequencies not present in the acoustic stimulus. These sounds compose the otoacoustic emissions that are extensively used to screen hearing in newborns. Because both cochlear amplification and distortion originate from the outer hair cells-one of the two types of sensory receptor cells-it has been speculated that they stem from a common mechanism. Here we show that the nonlinearity underlying cochlear waveform distortions relies on the presence of stereocilin, a protein defective in a recessive form of human deafness. Stereocilin was detected in association with horizontal top connectors, lateral links that join adjacent stereocilia within the outer hair cell's hair bundle. These links were absent in stereocilin-null mutant mice, which became progressively deaf. At the onset of hearing, however, their cochlear sensitivity and frequency tuning were almost normal, although masking was much reduced and both acoustic and electrical waveform distortions were completely lacking. From this unique functional situation, we conclude that the main source of cochlear waveform distortions is a deflection-dependent hair bundle stiffness resulting from constraints imposed by the horizontal top connectors, and not from the intrinsic nonlinear behaviour of the mechanoelectrical transducer channel.

Presence of interstereocilial links in waltzer mutants suggests Cdh23 is not essential for tip link formation

Cadherin23 has been proposed to form the upper part of the tip link, an interstereocilial link believed to control opening of transducer channels of sensory hair cells. However, we detect tip link-like links in mouse mutants with null alleles of Cdh23, suggesting the presence of other components that permit formation of a link between the tip of one stereocilium and the side of the adjacent taller stereocilium.

A Myo6 mutation destroys coordination between the myosin heads, revealing new functions of myosin VI in the stereocilia of mammalian inner ear hair cells

Myosin VI, found in organisms from Caenorhabditis elegans to humans, is essential for auditory and vestibular function in mammals, since genetic mutations lead to hearing impairment and vestibular dysfunction in both humans and mice. Here, we show that a missense mutation in this molecular motor in an ENU-generated mouse model, Tailchaser, disrupts myosin VI function. Structural changes in the Tailchaser hair bundles include mislocalization of the kinocilia and branching of stereocilia. Transfection of GFP-labeled myosin VI into epithelial cells and delivery of endocytic vesicles to the early endosome revealed that the mutant phenotype displays disrupted motor function. The actin-activated ATPase rates measured for the D179Y mutation are decreased, and indicate loss of coordination of the myosin VI heads or ‘gating’ in the dimer form. Proper coordination is required for walking processively along, or anchoring to, actin filaments, and is apparently destroyed by the proximity of the mutation to the nucleotide-binding pocket. This loss of myosin VI function may not allow myosin VI to transport its cargoes appropriately at the base and within the stereocilia, or to anchor the membrane of stereocilia to actin filaments via its cargos, both of which lead to structural changes in the stereocilia of myosin VI–impaired hair cells, and ultimately leading to deafness.

Human deafness is extremely heterogeneous, with mutations in over 50 genes known to be associated with this common form of sensory loss. Among them, mutations in five myosins are associated with human hereditary hearing impairment, demonstrating that this family of proteins is essential for the proper function of the inner ear. Myosins, motor proteins found in eukaryotic cells, are responsible for actin-based motility. Composed of a motor domain and a tail, the former binds filamentous actin and uses ATP hydrolysis to generate force and move along the filaments, while the latter binds to cargos in the cell. Myosin VI is unique among myosins due to its movement along actin towards the minus or pointed end, rather than the positive or barbed end. Mutations in this myosin are associated with human deafness. Much of our information regarding myosin VI comes from studies in cell culture or mouse mutants with mutations leading to deafness. Here, we describe a deaf mouse mutant, Tailchaser, with a mutation in myosin VI. Our data describe new functions for myosin VI in the hair cells of the inner ear, showing how alterations in this motor can lead to a human sensory disorder.

TRPML3 mutations cause impaired mechano-electrical transduction and depolarization by an inward-rectifier cation current in auditory hair cells of varitint-waddler mice

TRPML3 (mucolipin-3) belongs to one of the transient-receptor-potential (TRP) ion channel families. Mutations in the Trpml3 gene cause disorganization of the stereociliary hair bundle, structural aberrations in outer and inner hair cells and stria vascularis defects, leading to deafness in the varitint-waddler (Va) mouse. Here we refined the stereociliary localization of TRPML3 and investigated cochlear hair cell function in varitint-waddler (Va(J)) mice carrying the TRPML3<I362T/A419P> mutations. Using a TRPML3-specific antibody we detected a approximately 68 kDa protein with near-equal expression levels in cochlea and vestibule of wild-type and Va(J) mutants. At postnatal days 3 and 5, we observed abundant localization of TRPML3 at the base of stereocilia near the position of the ankle links. This stereociliary localization domain was absent in Va(J) heterozygotes and homozygotes. Electrophysiological recordings revealed reduced mechano-electrical transducer currents in hair cells from Va(J)/+ and Va(J)/Va(J) mice. Furthermore, FM1-43 uptake and [(3)H]gentamicin accumulation were decreased in hair cells in cultured organs of Corti from Va(J)/+ and Va(J)/Va(J) mice. We propose that TRPML3 plays a critical role at the ankle-link region during hair-bundle growth and that an adverse effect of mutant TRPML3 on bundle development and mechano-electrical transduction is the main cause of hearing loss in Va(J)/+ mutant mice. Outer hair cells of Va(J)/Va(J) mice additionally had depolarized resting potentials due to an inwardly rectifying leak conductance formed by the mutant channels, leading over time to hair-cell degeneration and contributing to their deafness. Our findings argue against TRPML3 being a component of the hair-cell transducer channel.

Theoretical conditions for high-frequency hair bundle oscillations in auditory hair cells

Substantial evidence exists for spontaneous oscillations of hair cell stereociliary bundles in the lower vertebrate inner ear. Since the oscillations are larger than expected from Brownian motion, they must result from an active process in the stereociliary bundle suggested to underlie amplification of the sensory input as well as spontaneous otoacoustic emissions. However, their low frequency (<100 Hz) makes them unsuitable for amplification in birds and mammals that hear up to 5 kHz or higher. To examine the possibility of high-frequency oscillations, we used a finite-element model of the outer hair cell bundle incorporating previously measured mechanical parameters. Bundle motion was assumed to activate mechanotransducer channels according to the gating spring hypothesis, and the channels were regulated adaptively by Ca(2+) binding. The model generated oscillations of freestanding bundles at 4 kHz whose sharpness of tuning depended on the mechanotransducer channel number and location, and the Ca(2+) concentration. Entrainment of the oscillations by external stimuli was used to demonstrate nonlinear amplification. The oscillation frequency depended on channel parameters and was increased to 23 kHz principally by accelerating Ca(2+) binding kinetics. Spontaneous oscillations persisted, becoming very narrow-band, when the hair bundle was loaded with a tectorial membrane mass.

Cadherins and mechanotransduction by hair cells

Mechanotransduction, the conversion of a mechanical stimulus into an electrical signal is crucial for our ability to hear and to maintain balance. Recent findings indicate that two members of the cadherin superfamily are components of the mechanotransduction machinery in sensory hair cells of the vertebrate inner ear. These studies show that cadherin 23 (CDH23) and protocadherin 15 (PCDH15) form several of the extracellular filaments that connect the stereocilia and kinocilium of a hair cell into a bundle. One of these filaments is the tip link that has been proposed to gate the mechanotransduction channel in hair cells. The extracellular domains of CDH23 and PCDH15 differ in their structure from classical cadherins and their cytoplasmic domains bind to distinct effectors, suggesting that evolutionary pressures have shaped the two cadherins for their function in mechanotransduction.

A core cochlear phenotype in USH1 mouse mutants implicates fibrous links of the hair bundle in its cohesion, orientation and differential growth

The planar polarity and staircase-like pattern of the hair bundle are essential to the mechanoelectrical transduction function of inner ear sensory cells. Mutations in genes encoding myosin VIIa, harmonin, cadherin 23, protocadherin 15 or sans cause Usher syndrome type I (USH1, characterized by congenital deafness, vestibular dysfunction and retinitis pigmentosa leading to blindness) in humans and hair bundle disorganization in mice. Whether the USH1 proteins are involved in common hair bundle morphogenetic processes is unknown. Here, we show that mouse models for the five USH1 genetic forms share hair bundle morphological defects. Hair bundle fragmentation and misorientation (25-52 degrees mean kinociliary deviation, depending on the mutant) were detected as early as embryonic day 17. Abnormal differential elongation of stereocilia rows occurred in the first postnatal days. In the emerging hair bundles, myosin VIIa, the actin-binding submembrane protein harmonin-b, and the interstereocilia-kinocilium lateral link components cadherin 23 and protocadherin 15, all concentrated at stereocilia tips, in accordance with their known in vitro interactions. Soon after birth, harmonin-b switched from the tip of the stereocilia to the upper end of the tip link, which also comprises cadherin 23 and protocadherin 15. This positional change did not occur in mice deficient for cadherin 23 or protocadherin 15. We suggest that tension forces applied to the early lateral links and to the tip link, both of which can be anchored to actin filaments via harmonin-b, play a key role in hair bundle cohesion and proper orientation for the former, and in stereociliary elongation for the latter.

Quiet as a mouse: dissecting the molecular and genetic basis of hearing

Mouse genetics has made crucial contributions to the understanding of the molecular mechanisms of hearing. With the help of a plethora of mouse mutants, many of the key genes that are involved in the development and functioning of the auditory system have been elucidated. Mouse mutants continue to shed light on the genetic and physiological bases of human hearing impairment, including both early- and late-onset deafness. A combination of genetic and physiological studies of mouse mutant lines, allied to investigations into the protein networks of the stereocilia bundle in the inner ear, are identifying key complexes that are crucial for auditory function and for providing profound insights into the underlying causes of hearing loss.

The actions of calcium on hair bundle mechanics in mammalian cochlear hair cells

Sound stimuli excite cochlear hair cells by vibration of each hair bundle, which opens mechanotransducer (MT) channels. We have measured hair-bundle mechanics in isolated rat cochleas by stimulation with flexible glass fibers and simultaneous recording of the MT current. Both inner and outer hair-cell bundles exhibited force-displacement relationships with a nonlinearity that reflects a time-dependent reduction in stiffness. The nonlinearity was abolished, and hair-bundle stiffness increased, by maneuvers that diminished calcium influx through the MT channels: lowering extracellular calcium, blocking the MT current with dihydrostreptomycin, or depolarizing to positive potentials. To simulate the effects of Ca(2+), we constructed a finite-element model of the outer hair cell bundle that incorporates the gating-spring hypothesis for MT channel activation. Four calcium ions were assumed to bind to the MT channel, making it harder to open, and, in addition, Ca(2+) was posited to cause either a channel release or a decrease in the gating-spring stiffness. Both mechanisms produced Ca(2+) effects on adaptation and bundle mechanics comparable to those measured experimentally. We suggest that fast adaptation and force generation by the hair bundle may stem from the action of Ca(2+) on the channel complex and do not necessarily require the direct involvement of a myosin motor. The significance of these results for cochlear transduction and amplification are discussed.

Architecture of the mouse utricle: macular organization and hair bundle heights

Hair bundles are critical to mechanotransduction by vestibular hair cells, but quantitative data are lacking on vestibular bundles in mice or other mammals. Here we quantify bundle heights and their variation with macular locus and hair cell type in adult mouse utricular macula. We also determined that macular organization differs from previous reports. The utricle has approximately 3,600 hair cells, half on each side of the line of polarity reversal (LPR). A band of low hair cell density corresponds to a band of calretinin-positive calyces, i.e., the striola. The relation between the LPR and the striola differs from previous reports in two ways. First, the LPR lies lateral to the striola instead of bisecting it. Second, the LPR follows the striolar trajectory anteriorly, but posteriorly it veers from the edge of the striola to reach the posterior margin of the macula. Consequently, more utricular bundles are oriented mediolaterally than previously supposed. Three hair cell classes are distinguished in calretinin-stained material: type II hair cells, type ID hair cells contacting calretinin-negative (dimorphic) afferents, and type IC hair cells contacting calretinin-positive (calyceal) afferents. They differ significantly on most bundle measures. Type II bundles have short stereocilia. Type IC bundles have kinocilia and stereocilia of similar heights, i.e., KS ratios (ratio of kinocilium to stereocilia heights) approximately 1, unlike other receptor classes. In contrast to these class-specific differences, bundles show little regional variation except that KS ratios are lowest in the striola. These low KS ratios suggest that bundle stiffness is greater in the striola than in the extrastriola.

The micromachinery of mechanotransduction in hair cells

Mechanical stimuli generated by head movements and changes in sound pressure are detected by hair cells with amazing speed and sensitivity. The mechanosensitive organelle, the hair bundle, is a highly elaborated structure of actin-based stereocilia arranged in precise rows of increasing height. Extracellular linkages contribute to its cohesion and convey forces to mechanically gated channels. Channel opening is nearly instantaneous and is followed by a process of sensory adaptation that keeps the channels poised in their most sensitive range. This process is served by motors, scaffolds, and homeostatic mechanisms. The molecular constituents of this process are rapidly being elucidated, especially by the discovery of deafness genes and antibody targets.

The deaf mouse mutant whirler suggests a role for whirlin in actin filament dynamics and stereocilia development

Stereocilia, finger-like projections forming the hair bundle on the apical surface of sensory hair cells in the cochlea, are responsible for mechanosensation and ultimately the perception of sound. The actin cytoskeleton of the stereocilia contains hundreds of tightly cross-linked parallel actin filaments in a paracrystalline array and it is vital for their function. Although several genes have been identified and associated with stereocilia development, the molecular mechanisms responsible for stereocilia growth, maintenance and organisation of the hair bundle have not been fully resolved. Here we provide further characterisation of the stereocilia of the whirler mouse mutant. We found that a lack of whirlin protein in whirler mutants results in short stereocilia with larger diameters without a corresponding increase in the number of actin filaments in inner hair cells. However, a decrease in the actin filament packing density was evident in the whirler mutant. The electron-density at the tip of each stereocilium was markedly patchy and irregular in the whirler mutants compared with a uniform band in controls. The outer hair cell stereocilia of the whirler homozygote also showed an increase in diameter and variable heights within bundles. The number of outer hair cell stereocilia was significantly reduced and the centre-to-centre spacing between the stereocilia was greater than in the wildtype. Our findings suggest that whirlin plays an important role in actin filament packing and dynamics during postnatal stereocilium elongation.

Molecular characterization of the ankle-link complex in cochlear hair cells and its role in the hair bundle functioning

Several lines of evidence indicate that very large G-protein-coupled receptor 1 (Vlgr1) makes up the ankle links that connect the stereocilia of hair cells at their base. Here, we show that the transmembrane protein usherin, the putative transmembrane protein vezatin, and the PDZ (postsynaptic density-95/Discs large/zona occludens-1) domain-containing submembrane protein whirlin are colocalized with Vlgr1 at the stereocilia base in developing cochlear hair cells and are absent in Vlgr1-/- mice that lack the ankle links. Direct in vitro interactions between these four proteins further support their involvement in a molecular complex associated with the ankle links and scaffolded by whirlin. In addition, the delocalization of these proteins in myosin VIIa defective mutant mice as well as the myosin VIIa tail direct interactions with vezatin, whirlin, and, we show, Vlgr1 and usherin, suggest that myosin VIIa conveys proteins of the ankle-link complex to the stereocilia. Adenylyl cyclase 6, which was found at the base of stereocilia, was both overexpressed and mislocated in Vlgr1-/- mice. In postnatal day 7 Vlgr1-/- mice, mechanoelectrical transduction currents evoked by displacements of the hair bundle toward the tallest stereocilia (i.e., in the excitatory direction) were reduced in outer but not inner hair cells. In both cell types, stimulation of the hair bundle in the opposite direction paradoxically resulted in significant transduction currents. The absence of ankle-link-mediated cohesive forces within hair bundles lacking Vlgr1 may account for the electrophysiological results. However, because some long cadherin-23 isoforms could no longer be detected in Vlgr1-/- mice shortly after birth, the loss of some apical links could be involved too. The premature disappearance of these cadherin isoforms in the Vlgr1-/- mutant argues in favor of a signaling function of the ankle links in hair bundle differentiation.

Long term effects of BAPTA in scala media on cochlear function

BAPTA was iontophoresed or allowed to diffuse into the scala media of the first turn of the guinea pig cochlea via pipettes inserted through the round window and basilar membrane. Cochlear action potential (CAP) thresholds for basal turn frequencies were elevated, scala media cochlear microphonic in response to a 207Hz tone were drastically reduced and the distortion products 2f1-f2 and f2-f2 in response to primaries set at 18 and 21.6kHz were eliminated or severely reduced. The animals were recovered and the above measurements repeated between 24 and 240h after the application of BAPTA. In all animals thresholds for basal turn frequencies remained elevated, and the distortion components were severely reduced. The endolymphatic potential (EP), measured through the basilar membrane on recovery, was not significantly different from the values measured before BAPTA was applied. If the effect of BAPTA, in lowering endolymphatic Ca(2+) concentration, is restricted to the destruction of tip links, as has been shown in many other preparations, then these results suggest that this effect has permanent consequences, either because the tip links failed to regenerate or because their destruction precipitated the degeneration of OHCs. These results may have a bearing on the mechanisms behind permanent threshold shift.

Vlgr1 is required for proper stereocilia maturation of cochlear hair cells

Very large G-protein coupled receptor (Vlgr1b) is the largest known G-protein coupled receptor. Its function is unknown, although mice with deletion of Vlgr1 (Vlgr1b together with other splicing variants, Vlgr1c, Vlgr1d and Vlgr1e) are known to exhibit audiogenic seizure susceptibility and VLGR1 is reported to be the gene responsible for Usher type 2C syndrome. We demonstrated here that Vlgr1-mutated mice suffered from a hearing defect because of inner ear dysfunction, as indicated by auditory brainstem response (ABR) and distortion product oto-acoustic emissions (DPOAE). The expression of Vlgr1 was identified in the developing hair cells perinatally, and the translated products were seen to be localized in the base of stereocilia on hair cells using confocal microscopy. This Vlgr1 localization was limited to the base of stereocilia within approximately 200-400 nm from the apical surface of hair cells, as shown by immunoelectron microscopy. The Vlgr1-mutated mice exhibited malformation of the stereocilia; the cochlear hair bundles were apparently normal at birth but then became disarranged at postnatal day 8. Furthermore, the stereocilia in the mutant mice became slanted and disarranged thereafter. These results indicate that loss of Vlgr1 resulted in abnormal development of stereocilia formation.

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.