Distribution of the 275 kD hair cell antigen and cell surface specialisations on auditory and vestibular hair bundles in the chicken inner ear

Comparative biology of the amniote vestibular utricle

The sensory epithelia of the auditory and vestibular systems of vertebrates have shared developmental and evolutionary histories. However, while the auditory epithelia show great variation across vertebrates, the vestibular sensory epithelia appear seemingly more conserved. An exploration of the current knowledge of the comparative biology of the amniote utricle, a vestibular sensory epithelium that senses linear acceleration, shows interesting instances of variability between birds and mammals. The distribution of sensory hair cell types, the position of the line of hair bundle polarity reversal and the properties of supporting cells show marked differences, likely impacting vestibular function and hair cell regeneration potential.

Critical role of TPRN rings in the stereocilia for hearing

Inner ear hair cells detect sound vibration through the deflection of mechanosensory stereocilia. Cytoplasmic protein TPRN has been shown to localize at the taper region of the stereocilia, and mutations in TPRN cause hereditary hearing loss through an unknown mechanism. Here, using biochemistry and dual stimulated emission depletion microscopy imaging, we show that the TPRN, together with its binding proteins CLIC5 and PTPRQ, forms concentric rings in the taper region of stereocilia. The disruption of TPRN rings, triggered by the competitive inhibition of the interaction of TPRN and CLIC5 or exogenous TPRN overexpression, leads to stereocilia degeneration and severe hearing loss. Most importantly, restoration of the TPRN rings can rescue the damaged auditory function of Tprn knockout mice by exogenously expressing TPRN at an appropriate level in HCs via promoter recombinant adeno-associated virus (AAV). In summary, our results reveal highly structured TPRN rings near the taper region of stereocilia that are crucial for stereocilia function and hearing. Also, TPRN ring restoration in stereocilia by AAV-Tprn effectively repairs damaged hearing, which lays the foundation for the clinical application of AAV-mediated gene therapy in patients with TPRN mutation.

Hair-Bundle Links: Genetics as the Gateway to Function

Up to five distinct cell-surface specializations interconnect the stereocilia and the kinocilium of the mature hair bundle in some species: kinocilial links, tip links, top connectors, shaft connectors, and ankle links. In developing hair bundles, transient lateral links are prominent. Mutations in genes encoding proteins associated with these links cause Usher deafness/blindness syndrome or nonsyndromic (isolated) forms of human hereditary deafness, and mice with constitutive or conditional alleles of these genes have provided considerable insight into the molecular composition and function of the different links. We describe the structure of these links and review evidence showing CDH23 and PCDH15 are components of the tip, kinocilial, and transient-lateral links, that stereocilin (STRC) and protein tyrosine phosphatase (PTPRQ) are associated with top and shaft connectors, respectively, and that USH2A and ADGRV1 are associated with the ankle links. Whereas tip links are required for mechanoelectrical transduction, all link proteins play key roles in the normal development and/or the maintenance of hair bundle structure and function. Recent crystallographic and single-particle analyses of PCDH15 and CDH23 provide insight as to how the structure of tip link may contribute to the elastic element predicted to lie in series with the hair cell's mechanoelectrical transducer channel.

Multiscale modeling of mechanotransduction in the utricle

We review recent progress in using numerical models to relate utricular hair bundle and otoconial membrane (OM) structure to the functional requirements imposed by natural behavior in turtles. The head movements section reviews the evolution of experimental attempts to understand vestibular system function with emphasis on turtles, including data showing that accelerations occurring during natural head movements achieve higher magnitudes and frequencies than previously assumed. The structure section reviews quantitative anatomical data documenting topographical variation in the structures underlying macromechanical and micromechanical responses of the turtle utricle to head movement: hair bundles, OM, and bundle-OM coupling. The macromechanics section reviews macromechanical models that incorporate realistic anatomical and mechanical parameters and reveal that the system is significantly underdamped, contrary to previous assumptions. The micromechanics: hair bundle motion and met currents section reviews work based on micromechanical models, which demonstrates that topographical variation in the structure of hair bundles and OM, and their mode of coupling, result in regional specializations for signaling of low frequency (or static) head position and high frequency head accelerations. We conclude that computational models based on empirical data are especially promising for investigating mechanotransduction in this challenging sensorimotor system.

Tprn is essential for the integrity of stereociliary rootlet in cochlear hair cells in mice

Tprn encodes the taperin protein, which is concentrated in the tapered region of hair cell stereocilia in the inner ear. In humans, TPRN mutations cause autosomal recessive nonsyndromic deafness (DFNB79) by an unknown mechanism. To determine the role of Tprn in hearing, we generated Tprn-null mice by clustered regularly interspaced short palindromic repeat/Cas9 genome-editing technology from a CBA/CaJ background. We observed significant hearing loss and progressive degeneration of stereocilia in the outer hair cells of Tprn-null mice starting from postnatal day 30. Transmission electron microscopy images of stereociliary bundles in the mutant mice showed some stereociliary rootlets with curved shafts. The central cores of the stereociliary rootlets possessed hollow structures with surrounding loose peripheral dense rings. Radixin, a protein expressed at stereocilia tapering, was abnormally dispersed along the stereocilia shafts in Tprn-null mice. The expression levels of radixin and β-actin significantly decreased.We propose that Tprn is critical to the retention of the integrity of the stereociliary rootlet. Loss of Tprn in Tprn-null mice caused the disruption of the stereociliary rootlet, which resulted in damage to stereociliary bundles and hearing impairments. The generated Tprn-null mice are ideal models of human hereditary deafness DFNB79.

PDZD7-MYO7A complex identified in enriched stereocilia membranes

While more than 70 genes have been linked to deafness, most of which are expressed in mechanosensory hair cells of the inner ear, a challenge has been to link these genes into molecular pathways. One example is Myo7a (myosin VIIA), in which deafness mutations affect the development and function of the mechanically sensitive stereocilia of hair cells. We describe here a procedure for the isolation of low-abundance protein complexes from stereocilia membrane fractions. Using this procedure, combined with identification and quantitation of proteins with mass spectrometry, we demonstrate that MYO7A forms a complex with PDZD7, a paralog of USH1C and DFNB31. MYO7A and PDZD7 interact in tissue-culture cells, and co-localize to the ankle-link region of stereocilia in wild-type but not Myo7a mutant mice. Our data thus describe a new paradigm for the interrogation of low-abundance protein complexes in hair cell stereocilia and establish an unanticipated link between MYO7A and PDZD7.

Utricular afferents: morphology of peripheral terminals

The utricle provides critical information about spatiotemporal properties of head movement. It comprises multiple subdivisions whose functional roles are poorly understood. We previously identified four subdivisions in turtle utricle, based on hair bundle structure and mechanics, otoconial membrane structure and hair bundle coupling, and immunoreactivity to calcium-binding proteins. Here we ask whether these macular subdivisions are innervated by distinctive populations of afferents to help us understand the role each subdivision plays in signaling head movements. We quantified the morphology of 173 afferents and identified six afferent classes, which differ in structure and macular locus. Calyceal and dimorphic afferents innervate one striolar band. Bouton afferents innervate a second striolar band; they have elongated terminals and the thickest processes and axons of all bouton units. Bouton afferents in lateral (LES) and medial (MES) extrastriolae have small-diameter axons but differ in collecting area, bouton number, and hair cell contacts (LES >> MES). A fourth, distinctive population of bouton afferents supplies the juxtastriola. These results, combined with our earlier findings on utricular hair cells and the otoconial membrane, suggest the hypotheses that MES and calyceal afferents encode head movement direction with high spatial resolution and that MES afferents are well suited to signal three-dimensional head orientation and striolar afferents to signal head movement onset.

CLIC5 stabilizes membrane-actin filament linkages at the base of hair cell stereocilia in a molecular complex with radixin, taperin, and myosin VI

Chloride intracellular channel 5 protein (CLIC5) was originally isolated from microvilli in complex with actin binding proteins including ezrin, a member of the Ezrin-Radixin-Moesin (ERM) family of membrane-cytoskeletal linkers. CLIC5 concentrates at the base of hair cell stereocilia and is required for normal hearing and balance in mice, but its functional significance is poorly understood. This study investigated the role of CLIC5 in postnatal development and maintenance of hair bundles. Confocal and scanning electron microscopy of CLIC5-deficient jitterbug (jbg) mice revealed progressive fusion of stereocilia as early as postnatal day 10. Radixin (RDX), protein tyrosine phosphatase receptor Q (PTPRQ), and taperin (TPRN), deafness-associated proteins that also concentrate at the base of stereocilia, were mislocalized in fused stereocilia of jbg mice. TPRQ and RDX were dispersed even prior to stereocilia fusion. Biochemical assays showed interaction of CLIC5 with ERM proteins, TPRN, and possibly myosin VI (MYO6). In addition, CLIC5 and RDX failed to localize normally in fused stereocilia of MYO6 mutant mice. Based on these findings, we propose a model in which these proteins work together as a complex to stabilize linkages between the plasma membrane and subjacent actin cytoskeleton at the base of stereocilia.

Hair bundle defects and loss of function in the vestibular end organs of mice lacking the receptor-like inositol lipid phosphatase PTPRQ

Recent studies have shown that mutations in PTPRQ, a gene encoding a receptor-like inositol lipid phosphatase, cause recessive, nonsyndromic, hereditary hearing loss with associated vestibular dysfunction. Although null mutations in Ptprq cause the loss of high-frequency auditory hair cells and deafness in mice, a loss of vestibular hair cells and overt behavioral defects characteristic of vestibular dysfunction have not been described. Hair bundle structure and vestibular function were therefore examined in Ptprq mutant mice. Between postnatal days 5 and 16, hair bundles in the extrastriolar regions of the utricle in Ptprq(-/-) mice become significantly longer than those in heterozygous controls. This increase in length (up to 50%) is accompanied by the loss and fusion of stereocilia. Loss and fusion of stereocilia also occurs in the striolar region of the utricle in Ptprq(-/-) mice, but is not accompanied by hair bundle elongation. These abnormalities persist until 12 months of age but are not accompanied by significant hair cell loss. Hair bundle defects are also observed in the saccule and ampullae of Ptprq(-/-) mice. At ∼3 months of age, vestibular evoked potentials were absent from the majority (12 of 15) of Ptprq(-/-) mice examined, and could only be detected at high stimulus levels in the other 3 mutants. Subtle but distinct defects in swimming behavior were detected in most (seven of eight) mutants tested. The results reveal a distinct phenotype in the vestibular system of Ptprq(-/-) mice and suggest similar hair bundle defects may underlie the vestibular dysfunction reported in humans with mutations in PTPRQ.

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.

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.

Roles of the espin actin-bundling proteins in the morphogenesis and stabilization of hair cell stereocilia revealed in CBA/CaJ congenic jerker mice

Hearing and vestibular function depend on mechanosensory staircase collections of hair cell stereocilia, which are produced from microvillus-like precursors as their parallel actin bundle scaffolds increase in diameter and elongate or shorten. Hair cell stereocilia contain multiple classes of actin-bundling protein, but little is known about what each class contributes. To investigate the roles of the espin class of actin-bundling protein, we used a genetic approach that benefited from a judicious selection of mouse background strain and an examination of the effects of heterozygosity. A congenic jerker mouse line was prepared by repeated backcrossing into the inbred CBA/CaJ strain, which is known for excellent hearing and minimal age-related hearing loss. We compared stereocilia in wild-type CBA/CaJ mice, jerker homozygotes that lack espin proteins owing to a frameshift mutation in the espin gene, and jerker heterozygotes that contain reduced espin levels. The lack of espins radically impaired stereociliary morphogenesis, resulting in stereocilia that were abnormally thin and short, with reduced differential elongation to form a staircase. Mean stereociliary diameter did not increase beyond ∼0.10–0.14 µm, making stereocilia ∼30%–60% thinner than wild type and suggesting that they contained ∼50%–85% fewer actin filaments. These characteristics indicate a requirement for espins in the appositional growth and differential elongation of the stereociliary parallel actin bundle and fit the known biological activities of espins in vitro and in transfected cells. The stereocilia of jerker heterozygotes showed a transient proximal-distal tapering suggestive of haploinsufficiency and a slowing of morphogenesis that revealed previously unrecognized assembly steps and intermediates. The lack of espins also led to a region-dependent degeneration of stereocilia involving shortening and collapse. We conclude that the espin actin-bundling proteins are required for the assembly and stabilization of the stereociliary parallel actin bundle.

Stereocilia are the fingerlike projections of inner ear hair cells that detect sound and motion. Stereocilia grow to specific lengths and diameters and form staircase-like arrays. The changes in size appear to be driven by matching alterations in the dimensions of an underlying molecular scaffold consisting of a bundle of actin filaments cross-linked by actin-bundling proteins. To elucidate the roles of the espin actin-bundling proteins in hair cell stereocilia, we carry out an in-depth accounting of stereociliary size and shape in the jerker mutant mouse, which lacks the espin proteins because of a mutation in the espin gene. We examine a new and improved jerker mouse with a genetic background known for high-quality lifelong hearing. We find that, in the absence of espins, stereocilia do not increase in diameter or complete their elongation, but instead bend, shorten, and disappear. Although the specifics vary according to inner ear region, the stereociliary defects are profound and can readily account for the deafness and balance problems of jerker mice and humans with certain espin gene mutations. Even reducing espin levels by one-half leads to temporary defects in stereociliary diameter. Thus, espins play crucial roles in the formation and maintenance of hair cell stereocilia.

Asymmetric distribution of cadherin 23 and protocadherin 15 in the kinocilial links of avian sensory hair cells

Cadherin 23 and protocadherin 15 are components of tip links, fine filaments that interlink the stereocilia of hair cells and are believed to gate the hair cell's mechanotransducer channels. Tip links are aligned along the hair bundle's axis of mechanosensitivity, stretching obliquely from the top of one stereocilium to the side of an adjacent, taller stereocilium. In guinea pig auditory hair cells, tip links are polarized with cadherin 23 at the upper end and protocadherin 15 at the lower end, where the transducer channel is located. Double immunogold labeling of avian hair cells was used to study the distribution of these two proteins in kinocilial links, a link type that attaches the tallest stereocilia of the hair bundle to the kinocilium. In the kinocilial links of vestibular hair bundles, cadherin 23 localizes to the stereocilium and protocadherin 15 to the kinocilium. The two cadherins are therefore asymmetrically distributed within the kinocilial links but of a polarity that is, within those links that are aligned along the hair bundle's axis of sensitivity, reversed relative to that of tip links. Conventional transmission electron microscopy of hair bundles fixed in the presence of tannic acid reveals a distinct density in the 120–130 nm long kinocilial links that is located 35–40 nm from the kinociliary membrane. The location of this density is consistent with it being the site at which interactions occur in an in trans configuration between the opposing N-termini of homodimeric forms of cadherin 23 and protocadherin 15. J. Comp. Neurol. 518:4288–4297, 2010. © 2010 Wiley-Liss, Inc.

Identification of the hair cell soma-1 antigen, HCS-1, as otoferlin

Hair cells, the mechanosensitive receptor cells of the inner ear, are critical for our senses of hearing and balance. The small number of these receptor cells in the inner ear has impeded the identification and characterization of proteins important for hair cell function. The binding specificity of monoclonal antibodies provides a means for identifying hair cell-specific proteins and isolating them for further study. We have generated a monoclonal antibody, termed hair cell soma-1 (HCS-1), which specifically immunolabels hair cells in at least five vertebrate classes, including sharks and rays, bony fish, amphibians, birds, and mammals. We used HCS-1 to immunoprecipitate the cognate antigen and identified it as otoferlin, a member of the ferlin protein family. Mutations in otoferlin underlie DFNB9, a recessive, nonsyndromic form of prelingual deafness characterized as an auditory neuropathy. Using immunocytochemistry, we find that otoferlin is associated with the entire basolateral membrane of the hair cells and with vesicular structures distributed throughout most of the hair cell cytoplasm. Biochemical assays indicate that otoferlin is tightly associated with membranes, as it is not solubilized by alterations in calcium or salt concentrations. HCS-1 immunolabeling does not co-localize with ribeye, a constituent of synaptic ribbons, suggesting that otoferlin may, in addition to its proposed function in synaptic vesicle release, play additional roles in hair cells.

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.

The dimensions and structural attachments of tip links in mammalian cochlear hair cells and the effects of exposure to different levels of extracellular calcium

The tip links between stereocilia of acousticolateral hair cells have been suggested to contain cadherin 23 (CDH23) comprising an upper branched portion that is bound to a lower portion composed of protocadherin 15 (PCDH15). The molecular conformation of CDH23, its binding to PCDH15, the tip links, and mechanoelectrical transduction have all been shown previously to be sensitive to exposure to low levels of calcium. The aim of this study was to compare the characteristics of tip links in guinea-pig cochlear hair cells with reported features of the CDH23-PCDH15 complex. Tip links were examined using field emission scanning electron microscopy and transmission electron microscopy in conventional preparations and after treatment with the detergent Triton-X-100 or varying calcium concentrations in the extracellular solution. The results showed that tip links have a twisted double-stranded appearance with a branched upper region. They survived demembranation of the stereocilia by detergent suggesting that they have transmembrane domains at both ends. Their lengths, when fixed in the presence of 2 mM extracellular calcium, were approximately 150 nm. With prior exposure to 1 mM calcium their lengths were approximately 164 nm. The lengths in 50 microM calcium are similar ( approximately 185 nm) to those reported for CDH23-PCDH15 complexes in 100 microM calcium ( approximately 180 nm). Exposure to 1 microM calcium caused loss of tip links and an increased distance between the residual attachment sites. The data indicate that extracellular calcium concentration affects tip-link length. One model compatible with the recently proposed tip-link structure is that the CDH23 double strand undergoes calcium-dependent unfolding, changing the length of the links. The bundle may also tilt in the direction of the tallest row of stereocilia as the tip link lengthens and then is lost. Overall, our data are consistent with a tip link composed of complexes of CDH23 and PCDH15 but do not rule out other possibilities.

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.

Sharpened cochlear tuning in a mouse with a genetically modified tectorial membrane

Frequency tuning in the cochlea is determined by the passive mechanical properties of the basilar membrane and active feedback from the outer hair cells, sensory-effector cells that detect and amplify sound-induced basilar membrane motions. The sensory hair bundles of the outer hair cells are imbedded in the tectorial membrane, a sheet of extracellular matrix that overlies the cochlea's sensory epithelium. The tectorial membrane contains radially organized collagen fibrils that are imbedded in an unusual striated-sheet matrix formed by two glycoproteins, alpha-tectorin (Tecta) and beta-tectorin (Tectb). In Tectb(-/-) mice the structure of the striated-sheet matrix is disrupted. Although these mice have a low-frequency hearing loss, basilar-membrane and neural tuning are both significantly enhanced in the high-frequency regions of the cochlea, with little loss in sensitivity. These findings can be attributed to a reduction in the acting mass of the tectorial membrane and reveal a new function for this structure in controlling interactions along the cochlea.

Dan is required for normal morphogenesis and patterning in the developing chick inner ear

During vertebrate inner ear development, compartmentalization of the auditory and vestibular apparatuses along two axes depends on the patterning of transcription factors expressed in a region-specific manner. Although most of the patterning is regulated by extrinsic signals, it is not known how Nkx5.1 and Msx1 are patterned. We focus on Dan, the founding member of the Cerberus/Dan gene family that encodes BMP antagonists, and describe its function in morphogenesis and patterning. First, we confirmed that Dan is expressed in the dorso-medial region of the otic vesicle that corresponds to the presumptive endolymphatic duct and sac (ed/es). Second, we used siRNA knockdown to demonstrate that depletion of Dan induced both a severe reduction in the size of the ed/es and moderate deformities of the semicircular canals and cochlear duct. Depletion of Dan also caused suppression of Nkx5.1 in the dorso-lateral region, suppression of Msx1 in the dorso-medial region, and ectopic induction of Nkx5.1 and Msx1 in the ventro-medial region. Most of these phenotypes also appeared following misexpression of the constitutively active form of BMP receptor type Ib. Thus, Dan is required for the normal morphogenesis of the inner ear and, by inhibiting BMP signaling, for the patterning of the transcription factors Nkx5.1 and Msx1.

The tip-link antigen, a protein associated with the transduction complex of sensory hair cells, is protocadherin-15

Sound and acceleration are detected by hair bundles, mechanosensory structures located at the apical pole of hair cells in the inner ear. The different elements of the hair bundle, the stereocilia and a kinocilium, are interconnected by a variety of link types. One of these links, the tip link, connects the top of a shorter stereocilium with the lateral membrane of an adjacent taller stereocilium and may gate the mechanotransducer channel of the hair cell. Mass spectrometric and Western blot analyses identify the tip-link antigen, a hitherto unidentified antigen specifically associated with the tip and kinocilial links of sensory hair bundles in the inner ear and the ciliary calyx of photoreceptors in the eye, as an avian ortholog of human protocadherin-15, a product of the gene for the deaf/blindness Usher syndrome type 1F/DFNB23 locus. Multiple protocadherin-15 transcripts are shown to be expressed in the mouse inner ear, and these define four major isoform classes, two with entirely novel, previously unidentified cytoplasmic domains. Antibodies to the three cytoplasmic domain-containing isoform classes reveal that each has a different spatiotemporal expression pattern in the developing and mature inner ear. Two isoforms are distributed in a manner compatible for association with the tip-link complex. An isoform located at the tips of stereocilia is sensitive to calcium chelation and proteolysis with subtilisin and reappears at the tips of stereocilia as transduction recovers after the removal of calcium chelators. Protocadherin-15 is therefore associated with the tip-link complex and may be an integral component of this structure and/or required for its formation.

The very large G-protein-coupled receptor VLGR1: a component of the ankle link complex required for the normal development of auditory hair bundles

Sensory hair bundles in the inner ear are composed of stereocilia that can be interconnected by a variety of different link types, including tip links, horizontal top connectors, shaft connectors, and ankle links. The ankle link antigen is an epitope specifically associated with ankle links and the calycal processes of photoreceptors in chicks. Mass spectrometry and immunoblotting were used to identify this antigen as the avian ortholog of the very large G-protein-coupled receptor VLGR1, the product of the Usher syndrome USH2C (Mass1) locus. Like ankle links, Vlgr1 is expressed transiently around the base of developing hair bundles in mice. Ankle links fail to form in the cochleae of mice carrying a targeted mutation in Vlgr1 (Vlgr1/del7TM), and the bundles become disorganized just after birth. FM1-43 [N-(3-triethylammonium)propyl)-4-(4-(dibutylamino)styryl) pyridinium dibromide] dye loading and whole-cell recordings indicate mechanotransduction is impaired in cochlear, but not vestibular, hair cells of early postnatal Vlgr1/del7TM mutant mice. Auditory brainstem recordings and distortion product measurements indicate that these mice are severely deaf by the third week of life. Hair cells from the basal half of the cochlea are lost in 2-month-old Vlgr1/del7TM mice, and retinal function is mildly abnormal in aged mutants. Our results indicate that Vlgr1 is required for formation of the ankle link complex and the normal development of cochlear hair bundles.

Genetics of hearing loss: Allelism and modifier genes produce a phenotypic continuum

Recent genetic and genomic studies have greatly advanced our knowledge of the structure and function of genes involved in hearing loss. We are starting to recognize, however, that many of these genes do not appear to follow traditional Mendelian expression patterns and are subject to the effects of allelism and modifier genes. This review presents two genes illustrative of this concept that have varied expression pattern such that they may produce either syndromic or nonsyndromic hearing loss. One of these genes, cadherin 23, produces a spectrum of phenotypic traits, including presbycusis, nonsyndromic prelingual hearing loss (DFNB12), and syndromic hearing loss as part of Usher syndrome (Usher 1D). Missense mutations in CDH23 have been associated with presbycusis and DFNB12, whereas null alleles cause the majority of Usher 1D. Modifier gene products that interact with cadherin 23 also affect the phenotypic spectrum. Similarly, allelsim in the gene encoding wolframin (WFS1) causes either a nonsyndromic dominant low-frequency hearing loss (DFNA6/14/38) or Wolfram syndrome. Missense mutations within a defined region are associated with DFNA6/14/38, while more severe mutations spanning WFS1 are found in Wolfram syndrome patients. The phenotypic spectrum of Wolfram syndrome is also hypothesized to be influenced by modifier genes products. These studies provide increasing evidence for the importance of modifier genes in elucidating the functional pathways of primary hearing loss genes. Characterizing modifier genes may result in better treatment options for patients with hearing loss and define new diagnostic and therapeutic targets.

Mechanical properties and consequences of stereocilia and extracellular links in vestibular hair bundles

Although knowledge of the fine structure of vestibular hair bundles is increasing, the mechanical properties and functional significance of those structures remain unclear. In 2004, Bashtanov and colleagues reported the contribution of different extracellular links to bundle stiffness. We simulated Bashtanov's experimental protocol using a three-dimensional finite element bundle model with geometry measured from a typical striolar hair cell. Unlike any previous models, we separately consider two types of horizontal links: shaft links and upper lateral links. Our most important results are as follows. First, we identified the material properties required to match Bashtanov's experiment: stereocilia Young's modulus of 0.74 GPa, tip link assembly (gating spring) stiffness of 5,300 pN/microm, and the combined stiffness of shaft links binding two adjacent stereocilia of 750 approximately 2,250 pN/microm. Second, we conclude that upper lateral links are likely to have nonlinear mechanical properties: they have minimal stiffness during small bundle deformations but stiffen as the bundle deflects further. Third, we estimated the stiffness of the gating spring based on our realistic three-dimensional bundle model rather than a conventional model relying on the parallel arrangement assumption. Our predicted stiffness of the gating spring was greater than the previous estimation.

Cadherin 23 is a component of the transient lateral links in the developing hair bundles of cochlear sensory cells

Cadherin 23 is required for normal development of the sensory hair bundle, and recent evidence suggests it is a component of the tip links, filamentous structures thought to gate the hair cells' mechano-electrical transducer channels. Antibodies against unique peptide epitopes were used to study the properties of cadherin 23 and its spatio-temporal expression patterns in developing cochlear hair cells. In the rat, intra- and extracellular domain epitopes are readily detected in the developing hair bundle between E18 and P5, and become progressively restricted to the distal tip of the hair bundle. From P13 onwards, these epitopes are no longer detected in hair bundles, but immunoreactivity is observed in the apical, vesicle-rich, pericuticular region of the hair cell. In the P2-P3 mouse cochlea, immunogold labeling reveals cadherin 23 is associated with kinocilial links and transient lateral links located between and within stereociliary rows. At this stage, the cadherin 23 ectodomain epitope remains on the hair bundle following BAPTA or La(3+) treatment, but is lost following exposure to the protease subtilisin. In contrast, mechano-electrical transduction is abolished by BAPTA but unaffected by subtilisin. These results suggest cadherin 23 is associated with transient lateral links that have properties distinct from those of the tip-link.

Spatiotemporal pattern and isoforms of cadherin 23 in wild type and waltzer mice during inner ear hair cell development

Mutant alleles of the gene encoding cadherin 23 are associated with Usher syndrome type 1 (USH1D), isolated deafness (DFNB12) in humans, and deafness and circling behavior in waltzer (v) mice. Stereocilia of waltzer mice are disorganized and the kinocilia misplaced, indicating the importance of cadherin 23 for hair bundle development. Cadherin 23 was localized to developing stereocilia and proposed as a component of the tip link. We show that, during development of the inner ear, cadherin 23 is initially detected in centrosomes at E14.5, then along the length of emerging stereocilia, and later becomes concentrated at and subsequently disappears from the tops of stereocilia. In mature vestibular hair bundles, cadherin 23 is present along the kinocilium and in the region of stereocilia-kinocilium bonds, a pattern conserved in mammals, chicks, and frogs. Cadherin 23 is also present in Reissner's membrane (RM) throughout development. In homozygous v(6J) mice, a reported null allele, cadherin 23 was absent from stereocilia, but present in kinocilia, RM, and centrosomes. We reconciled these results by identifying two novel isoforms of Cdh23 unaffected in sequence and expression by the v(6J) allele. Our results suggest that Cdh23 participation in stereocilia links may be restricted to developing hair bundles.

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

The hair bundles of outer hair cells in the mature mouse cochlea possess three distinct cell-surface specializations: tip links, horizontal top connectors, and tectorial membrane attachment crowns. Electron microscopy was used to study the appearance and maturation of these link types and examine additional structures transiently associated with the developing hair bundle. At embryonic day 17.5 (E17.5), the stereocilia are interconnected by fine lateral links and have punctate elements distributed over their surface. Oblique tip links are also seen at this stage. By postnatal day 2 (P2), outer hair cell bundles have a dense cell coat, but have lost many of the lateral links seen at E17.5. At P2, ankle links appear around the base of the bundle and tectorial membrane attachment crowns are seen at the stereociliary tips. Ankle links become less apparent by P9 and are completely lost by P12. The appearance of horizontal top connectors, which persist into adulthood, occurs concomitant with this loss of ankle links. Treatment with the calcium chelator BAPTA or the protease subtilisin enabled these links to be further distinguished. Ankle links are susceptible to both treatments, tip links are only sensitive to BAPTA, and tectorial membrane attachment crowns are removed by subtilisin but not BAPTA. The cell-coat material is partially sensitive to subtilisin alone, while horizontal top connectors resist both treatments. These results indicate there is a rich, rapidly changing array of different links covering the developing hair bundle that becomes progressively refined to generate the mature complement by P19.

Hair cells require phosphatidylinositol 4,5-bisphosphate for mechanical transduction and adaptation

After opening in response to mechanical stimuli, hair cell transduction channels adapt with fast and slow mechanisms that each depend on Ca(2+). We demonstrate here that transduction and adaptation require phosphatidylinositol 4,5-bisphosphate (PIP(2)) for normal kinetics. PIP(2) has a striking distribution in hair cells, being excluded from the basal region of hair bundles and apical surfaces of frog saccular hair cells. Localization of a phosphatidylinositol lipid phosphatase, Ptprq, to these PIP(2)-free domains suggests that Ptprq maintains low PIP(2) levels there. Depletion of PIP(2) by inhibition of phosphatidylinositol 4-kinase or sequestration by aminoglycosides reduces the rates of fast and slow adaptation. PIP(2) and other anionic phospholipids bind directly to the IQ domains of myosin-1c, the motor that mediates slow adaptation, permitting a strong interaction with membranes and likely regulating the motor's activity. PIP(2) depletion also causes a loss in transduction current. PIP(2) therefore plays an essential role in hair cell adaptation and 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.

Digenic inheritance of deafness caused by mutations in genes encoding cadherin 23 and protocadherin 15 in mice and humans

Mutations in genes coding for cadherin 23 and protocadherin 15 cause deafness in both mice and humans. Here, we provide evidence that mutations at these two cadherin loci can interact to cause hearing loss in digenic heterozygotes of both species. Using a classical genetic approach, we generated mice that were heterozygous for both Cdh23 and Pcdh15 mutations on a uniform C57BL/6J background. Significant levels of hearing loss were detected in these mice when compared to age-matched single heterozygous animals or normal controls. Cytoarchitectural defects in the cochlea of digenic heterozygotes, including degeneration of the stereocilia and a base-apex loss of hair cells and spiral ganglion cells, were consistent with the observed age-related hearing loss of these mice beginning with the high frequencies. In humans, we also have obtained evidence for a digenic inheritance of a USH1 phenotype in three unrelated families with mutations in CDH23 and PCDH15. Altogether, our data indicate that CDH23 and PCDH15 play an essential long-term role in maintaining the normal organization of the stereocilia bundle.

Computational models of hair cell bundle mechanics: III. 3-D utricular bundles

Six utricular hair bundles from a red-eared turtle are modeled using 3-D finite element analysis. The mechanical model includes shear deformable stereocilia, realignment of all forces during force load increments, and tip and lateral link inter-stereocilia connections. Results show that there are two distinct bundle types that can be separated by mechanical bundle stiffness. The more compliant group has fewer total stereocilia and short stereocilia relative to kinocilium height; these cells are located in the medial and lateral extrastriola. The stiff group are located in the striola. They have more stereocilia and long stereocilia relative to kinocilia heights. Tip link tensions show parallel behavior in peripheral columns of the bundle and serial behavior in central columns when the tip link modulus is near or above that of collagen (1x10(9) N/m(2)). This analysis shows that lumped parameter models of single stereocilia columns can show some aspects of bundle mechanics; however, a distributed, 3-D model is needed to explore overall bundle behavior.

DAN directs endolymphatic sac and duct outgrowth in the avian inner ear

Bone morphogenetic proteins (BMPs) are expressed in the developing vertebrate inner ear and participate in inner ear axial patterning and the development of its sensory epithelium. BMP antagonists, such as noggin, chordin, gremlin, cerberus, and DAN (differential screening-selected gene aberrative in neuroblastoma) inhibit BMP activity and establish morphogenetic gradients during the patterning of many developing tissues and organs. In this study, the role of the BMP antagonist DAN in inner ear development was investigated. DAN-expressing cell pellets were implanted into the otocyst and the periotic mesenchyme to determine the effects of exogenous DAN on otic development. Similar to the effects on the inner ear seen after exposure of otocysts to the BMP4 antagonist noggin, semicircular canals were truncated or eliminated based upon the site of pellet implantation. Unique to the DAN implantations, however, were effects on the developing endolymphatic duct and sac. In DAN-treated inner ears, endolymphatic ducts and sacs were merged with the crus or grew into the superior semicircular canal. Both the canal and endolymphatic duct and sac effects were rescued by joint implantation of BMP4-expressing cells. Electroporation of DAN antisense morpholinos into the epithelium of stage 15-17 otocysts, blocking DAN protein synthesis, resulted in enlarged endolymphatic ducts and sacs as well as smaller semicircular canals in some cases. Taken together, these data suggest a role for DAN both in helping to regulate BMP activity spatially and temporally and in patterning and partitioning of the medial otic tissue between the endolymphatic duct/sac and medially derived inner ear structures.

The mechanical properties of chick (Gallus domesticus) sensory hair bundles: relative contributions of structures sensitive to calcium chelation and subtilisin treatment

Up to four link types are found between the stereocilia of chick vestibular hair bundles: tip links, horizontal top connectors, shaft connectors and ankle links. A fifth type, the kinocilial link, couples the hair bundle to the kinocilium. Brownian-motion microinterferometry was used to study the mechanical properties of the hair bundle and investigate changes caused by removing different links with the calcium chelator BAPTA or the protease subtilisin. Immunofluorescence with an antibody to the hair-cell antigen (HCA) and electron microscopy were used to verify destruction of the links. The root mean square displacement and the corresponding absolute stiffness of untreated hair bundles were 4.3 nm and 0.9 mN m(-1), respectively. The ratio of Brownian-motion spectra before and after treatment was calculated and processed using a single oscillator model to obtain relative stiffness. Treatment with BAPTA, which cleaves tip, kinocilial and ankle links, reduces hair-bundle stiffness by 43%, whilst subtilisin treatment, which breaks ankle links and shaft connectors, reduces stiffness by 48%. No changes were detected in viscous damping following either treatment. The time course of the subtilisin-induced stiffness change was close to that of HCA loss, but not to the disappearance of the ankle links, suggesting that shaft connectors make a more significant contribution to hair-bundle stiffness. Sequential treatments of the hair bundles with BAPTA and subtilisin show that the effects are additive. The implication of complete additivity is that structures resistant to both agents (e.g. top connectors and stereocilia pivots) are responsible for approximately 9% of the overall bundle stiffness.

Quantitative analysis of stereociliary arrays on vestibular hair cells

We have developed a method for quantifying the number, spacing, and distribution of stereocilia on the apical surface of hair cells using spatial autocorrelation analysis and statistics for directional data. Here, we illustrate the method using idealized hair bundles, and we apply it to scanning micrographs of turtle hair cells from the utricle and posterior canal, and to freeze-fracture preparations of bullfrog saccule. The analysis suggests three common features of stereociliary bundles. First, bundle geometries form a continuum from 'loose' to 'tight' rather than two distinct groups. Second, interciliary spacing along the three hexagon axes is not equal; spacing is usually widest along the hexagon axis closest to the bundle's axis of bilateral symmetry (the presumptive excitatory axis). Third, spacing between stereocilia changes with distance from the kinocilium. All three features will influence predictions of the tip link tensions that accompany bundle deflection.

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.

A receptor-like inositol lipid phosphatase is required for the maturation of developing cochlear hair bundles

A screen for protein tyrosine phosphatases (PTPs) expressed in the chick inner ear yielded a high proportion of clones encoding an avian ortholog of protein tyrosine phosphatase receptor Q (Ptprq), a receptor-like PTP. Ptprq was first identified as a transcript upregulated in rat kidney in response to glomerular nephritis and has recently been shown to be active against inositol phospholipids. An antibody to the intracellular domain of Ptprq, anti-Ptprq, stains hair bundles in mice and chicks. In the chick ear, the distribution of Ptprq is almost identical to that of the 275 kDa hair-cell antigen (HCA), a component of hair-bundle shaft connectors recognized by a monoclonal antibody (mAb) that stains inner-ear hair bundles and kidney glomeruli. Furthermore, anti-Ptprq immunoblots a 275 kDa polypeptide immunoprecipitated by the anti-HCA mAb from the avian inner ear, indicating that the HCA and Ptprq are likely to be the same molecule. In two transgenic mouse strains with different mutations in Ptprq, anti-Ptprq immunoreactivity cannot be detected in the ear. Shaft connectors are absent from mutant vestibular hair bundles, but the stereocilia forming the hair bundle are not splayed, indicating that shaft connectors are not necessary to hold the stereocilia together; however, the mice show rapid postnatal deterioration in cochlear hair-bundle structure, associated with smaller than normal transducer currents with otherwise normal adaptation properties, a progressive loss of basal-coil cochlear hair cells, and deafness. These results reveal that Ptprq is required for formation of the shaft connectors of the hair bundle, the normal maturation of cochlear hair bundles, and the long-term survival of high-frequency auditory hair cells.

A novel antigen sensitive to calcium chelation that is associated with the tip links and kinocilial links of sensory hair bundles

Tip links are extracellular, cell-surface-associated filaments of unknown molecular composition that are thought to gate the mechanotransducer channel of the sensory hair cell. They disappear from the hair bundle in response to calcium chelation and lanthanum treatment and resist degradation by the protease subtilisin. A monoclonal antibody derived from a hybridoma screen identified a novel antigen associated with tip links, the tip-link antigen. The tip-link antigen is also associated with kinocilial links, subtilisin-resistant filaments that are sensitive to calcium chelation and connect the kinocilium to the tallest stereocilia of the hair bundle. Furthermore, the tip-link antigen is expressed in the retina, where it is associated with the ciliary calyx, a ring of microvilli that surrounds the outer segment of the photoreceptor. The tip-link antigen rapidly disappears from the surface of the hair bundle in response to calcium chelation. It is also subtilisin resistant, relative to the ankle-link antigen, an antigen associated with another type of hair bundle link. The tip-link antigen is lanthanum sensitive and, like tip links, reappears on the surface of the hair bundle after calcium chelation. The monoclonal antibody to the tip-link antigen immunoprecipitates two concanavalin A-reactive polypeptides with apparent molecular masses of 200 and 250 kDa from detergent extracts of the retina. These results provide the first identification of a cell surface antigen associated with tip links, indicate that tip links share properties in common with kinocilial links, and reveal a second epitope that, along with the ankle-link antigen, is common to both sensory hair bundles and the ciliary calyx of photoreceptors.

Extracellular matrices associated with the apical surfaces of sensory epithelia in the inner ear: molecular and structural diversity

The ultrastructure and molecular composition of the extracellular matrices that are associated with the apical surfaces of the mechanosensory epithelia in the mouse inner ear are compared. A progressive increase in molecular and structural organization is observed, with the cupula being the simplest, the otoconial membrane exhibiting an intermediate degree of complexity, and the tectorial membrane being the most elaborate of the three matrices. These differences may reflect changes that occurred in the acellular membranes of the inner ear as a mammalian hearing organ arose during evolution from a simple equilibrium receptor. A comparison of the molecular composition of the acellular membranes in the chick inner ear suggests the auditory epithelium and the striolar region of the maculae are homologous, indicating the basilar papilla may have evolved from the striolar region of an otolithic organ. A comparison of the tectorial membranes in the chick cochlear duct and the mouse cochlea reveals differences in the structure of the noncollagenous matrix in the two species that may result from differences in the stochiometry of alpha- and beta-tectorin and/or differences in the post-translational modification of alpha-tectorin. This comparison also indicates that the appearance of collagen in the mammalian tectorial membrane may have been a major step in the evolution of an electromechanically tuned vertebrate hearing organ that operates over an extended frequency range.

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.

Hair cell development in vivo and in vitro: analysis by using a monoclonal antibody specific to hair cells in the chick inner ear

The purpose of this study was to establish a hair cell-specific marker and a convenient explant culture system for developing chick otocysts to facilitate in vivo and in vitro studies focusing on hair cell genesis in the inner ear. To achieve this, a hair cell-specific monoclonal antibody, 2A7, was generated by immunizing chick inner ear tissues to a mouse. Through the use of immunofluorescence and immunoelectron microscopy, it was shown that 2A7 immunoreactivity (2A7-IR) was primarily restricted to the apical region of inner ear hair cells, including stereocilia, kinocilia, apical membrane amongst the extending cilia, and superficial layer of the cuticular plate. Although the 2A7 antibody immunolabeled basically all of the hair cells in the posthatch chick inner ear, two different patterns of 2A7-IR were observed; hair cells located in the striolar region of the utricular macula, which consist of two distinct cell types identifiable on the basis of the type of nerve ending, Type I and II hair cells, showed labeling restricted to the basal end of the hair bundles. On the other hand, hair cells in the extrastriolar region, which are exclusively of Type II, showed labeling extending over virtually the entire length of the bundles. These findings raised the possibility that chick vestibular Type II hair cells, characterized by their bouton-type afferent nerve endings, can be divided into two subpopulations. Analysis of developing inner ear by using the 2A7 antibody revealed that this antibody also recognizes newly differentiated immature hair cells. Thus, the 2A7 antibody is able to recognize both immature and mature hair cells in vivo. The developmental potential of embryonic otocysts in vitro was then assessed by using explant cultures as a model. In this study, conventional otocyst explant cultures were modified by placing the tissues on floating polycarbonate filters on culture media, thereby allowing the easy manipulation of explants. In these cultures, 2A7-positive hair cells were differentiated from dividing precursor cells in vitro on the same schedule as in vivo. Furthermore, it was found that hair cells with both types of 2A7-IR were generated in culture as in vivo, indicating that a maturational process of hair cells also occurred. All these results as presented here suggest that the 2A7 monoclonal antibody as a hair cell-specific marker together with the culture system could be a potential tool in analysis of mechanisms underlying hair cell development.

Mechanisms that regulate mechanosensory hair cell differentiation

Hair cells of the vertebrate inner ear are mechanosensors that detect sound, gravity and acceleration. They have a specialized cytoskeleton optimized for the transmission of mechanical force. Hair cell defects are a major cause of deafness. The cloning of disease genes and studies of model organisms have provided insights into the mechanisms that regulate the differentiation of hair cells and their cytoskeleton. The studies have also provided new insights into the function of receptors such as integrins and protocadherins, and cytoplasmic proteins such as Rho-type GTPases and unconventional myosins, in organizing the actin cytoskeleton.

Electrostatic interaction between stereocilia: II. Influence on the mechanical properties of the hair bundle

This paper is based on our model [Dolgobrodov et al., 2000. Hear. Res., submitted for publication] in which we examine the significance of the polyanionic surface layers of stereocilia for electrostatic interaction between them. We analyse how electrostatic forces modify the mechanical properties of the sensory hair bundle. Different charge distribution profiles within the glycocalyx are considered. When modelling a typical experiment on bundle stiffness measurements, applying an external force to the tallest row of stereocilia shows that the asymptotic stiffness of the hair bundle for negative displacements is always larger than the asymptotic stiffness for positive displacements. This increase in stiffness is monotonic for even charge distribution and shows local minima when the negative charge is concentrated in a thinner layer within the cell coat. The minima can also originate from the co-operative effect of electrostatic repulsion and inter-ciliary links with non-linear mechanical properties. Existing experimental observations are compared with the predictions of the model. We conclude that the forces of electrostatic interaction between stereocilia may influence the mechanical properties of the hair bundle and, being strongly non-linear, contribute to the non-linear phenomena, which have been recorded from the auditory periphery.

Electrostatic interaction between stereocilia: I. Its role in supporting the structure of the hair bundle

This paper provides theoretical estimates for the forces of electrostatic interaction between adjacent stereocilia in auditory and vestibular hair cells. Estimates are given for parameters within the measured physiological range using constraints appropriate for the known geometry of the hair bundle. Stereocilia are assumed to possess an extended, negatively charged surface coat, the glycocalyx. Different charge distribution profiles within the glycocalyx are analysed. It is shown that charged glycocalices on the apical surface of the hair cells can support spatial separation between adjacent stereocilia in the hair bundles through electrostatic repulsion between stereocilia. The charge density profile within the glycocalyx is a crucial parameter. In fact, attraction instead of repulsion between adjacent stereocilia will be observed if the charge of the glycocalyx is concentrated near the membrane of the stereocilia, thereby making this type of charge distribution unlikely. The forces of electrostatic interaction between stereocilia may influence the mechanical properties of the hair bundle and, being strongly non-linear, contribute to the non-linear phenomena that have been recorded from the periphery of the auditory and vestibular systems.

A targeted deletion in alpha-tectorin reveals that the tectorial membrane is required for the gain and timing of cochlear feedback

alpha-tectorin is an extracellular matrix molecule of the inner ear. Mice homozygous for a targeted deletion in a-tectorin have tectorial membranes that are detached from the cochlear epithelium and lack all noncollagenous matrix, but the architecture of the organ of Corti is otherwise normal. The basilar membranes of wild-type and alpha-tectorin mutant mice are tuned, but the alpha-tectorin mutants are 35 dB less sensitive. Basilar membrane responses of wild-type mice exhibit a second resonance, indicating that the tectorial membrane provides an inertial mass against which outer hair cells can exert forces. Cochlear microphonics recorded in alpha-tectorin mutants differ in both phase and symmetry relative to those of wild-type mice. Thus, the tectorial membrane ensures that outer hair cells can effectively respond to basilar membrane motion and that feedback is delivered with the appropriate gain and timing required for amplification.

Addition of the BMP4 antagonist, noggin, disrupts avian inner ear development

Bone morphogenetic protein 4 (BMP4) is known to regulate dorsoventral patterning, limb bud formation and axis specification in many organisms, including the chicken. In the chick developing inner ear, BMP4 expression becomes localized in two cell clusters at the anterior and posterior edges of the otic epithelium beginning at stage 16/17 and is expressed in presumptive sensory tissue at later stages. This restricted spatiotemporal pattern of expression occurs just prior to the otocyst's transition to a more complex three-dimensional structure. To further analyze the role of BMP4 in avian otic morphogenesis, cells expressing BMP4 or its antagonist, noggin, were grown on agarose beads and implanted into the periotic mesenchyme surrounding the chick otocyst. Although the BMP4-producing cells had no effect on the mature inner ear structure when implanted alone, noggin-producing cells implanted adjacent to the BMP4 cell foci prevented normal semicircular canal development. Beads implanted at the anterior BMP4 focus eliminated the anterior and/or the horizontal canals. Noggin cells implanted at the posterior focus eliminated the posterior canal. Canal loss was prevented by co-implantation of BMP4 cell beads next to noggin beads. An antibody to the chick hair cell antigen (HCA) was used to examine sensory cell distribution, which was abnormal only in the affected tissues of noggin-exposed inner ears. These data suggest a role for BMP4 in the accurate and complete morphological development of the semicircular canals.

The ankle-link antigen: an epitope sensitive to calcium chelation associated with the hair-cell surface and the calycal processes of photoreceptors

A monoclonal antibody, mAb E40, that specifically recognizes hair cells and photoreceptors was derived from a mouse immunized with a membrane fraction prepared from the sensory maculae of the chick inner ear. In the mature chick inner ear, punctate labeling is observed along each stereocilium, but staining is mostly concentrated around the basal end of the sensory hair bundles, where it is closely associated with surface specializations known as ankle links. The epitope recognized by mAb E40 is therefore referred to as the ankle-link antigen (ALA). During early embryogenesis, the ALA is initially distributed evenly over the surface of the hair bundle. As development proceeds, it becomes more restricted to the base of the hair bundle, although a spot of the ALA remains associated with the bundle tip until just before hatching. In the eye, mAb E40 stains the calycal processes of photoreceptors. When maculae and retinae are treated with the calcium chelator BAPTA at room temperature, the ALA disappears. BAPTA-induced loss of the ALA from the hair-bundle surface is substantially reduced by lowering the temperature to 2 degrees C. The ALA and ankle links reappear on the hair-bundle surface when cells are cultured for 20 hr after BAPTA treatment. BAPTA sensitivity and recovery after BAPTA-induced loss are properties similar to those described for the tip link, a surface structure thought to gate the mechanotransducer channel. However, unlike the tip link, the ALA and ankle links are sensitive to subtilisin treatment. The results define a new component of the hair-bundle surface, with properties both common to and distinct from those of the tip link.