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

PKHD1L1 is required for stereocilia bundle maintenance, durable hearing function and resilience to noise exposure

Sensory hair cells of the cochlea are essential for hearing, relying on the mechanosensitive stereocilia bundle at their apical pole for their function. Polycystic Kidney and Hepatic Disease 1-Like 1 (PKHD1L1) is a stereocilia protein required for normal hearing in mice, and for the formation of the transient stereocilia surface coat, expressed during early postnatal development. While the function of the stereocilia coat remains unclear, growing evidence supports PKHD1L1 as a human deafness gene. In this study we carry out in depth characterization of PKHD1L1 expression in mice during development and adulthood, analyze hair-cell bundle morphology and hearing function in aging PKHD1L1-defficient mouse lines, and assess their susceptibility to noise damage. Our findings reveal that PKHD1L1-deficient mice display no disruption to bundle cohesion or tectorial membrane attachment-crown formation during development. However, starting from 6 weeks of age, PKHD1L1-defficient mice display missing stereocilia and disruptions to bundle coherence. Both conditional and constitutive PKHD1L1 knock-out mice develop high-frequency hearing loss progressing to lower frequencies with age. Furthermore, PKHD1L1-deficient mice are susceptible to permanent hearing loss following moderate acoustic overexposure, which induces only temporary hearing threshold shifts in wild-type mice. These results suggest a role for PKHD1L1 in establishing robust sensory hair bundles during development, necessary for maintaining bundle cohesion and function in response to acoustic trauma and aging.

Review and research gap identification in genetics causes of syndromic and nonsyndromic hearing loss in Saudi Arabia

Congenital hearing loss is one of the most common sensory disabilities worldwide. The genetic causes of hearing loss account for 50% of hearing loss. Genetic causes of hearing loss can be classified as nonsyndromic hearing loss (NSHL) or syndromic hearing loss (SHL). NSHL is defined as a partial or complete hearing loss without additional phenotypes; however, SHL, known as hearing loss, is associated with other phenotypes. Both types follow a simple Mendelian inheritance fashion. Several studies have been conducted to uncover the genetic factors contributing to NSHL and SHL in Saudi patients. However, these studies have encountered certain limitations. This review assesses and discusses the genetic factors underpinning NSHL and SHL globally, with a specific emphasis on the Saudi Arabian context. It also explores the prevalence of the most observed genetic causes of NSHL and SHL in Saudi Arabia. It also sheds light on areas where further research is needed to fully understand the genetic foundations of hearing loss in the Saudi population. This review identifies several gaps in research in NSHL and SHL and provides insights into potential research to be conducted.

Disruption of Cdh23 exon 68 splicing leads to progressive hearing loss in mice by affecting tip-link stability

Inner ear hair cells are characterized by the F-actin-based stereocilia that are arranged into a staircase-like pattern on the apical surface of each hair cell. The tips of shorter-row stereocilia are connected with the shafts of their neighboring taller-row stereocilia through extracellular links named tip links, which gate mechano-electrical transduction (MET) channels in hair cells. Cadherin 23 (CDH23) forms the upper part of tip links, and its cytoplasmic tail is inserted into the so-called upper tip-link density (UTLD) that contains other proteins such as harmonin. The Cdh23 gene is composed of 69 exons, and we show here that exon 68 is subjected to hair cell-specific alternative splicing. Tip-link formation is not affected in genetically modified mutant mice lacking Cdh23 exon 68. Instead, the stability of tip links is compromised in the mutants, which also suffer from progressive and noise-induced hearing loss. Moreover, we show that the cytoplasmic tail of CDH23(+68) but not CDH23(-68) cooperates with harmonin in phase separation-mediated condensate formation. In conclusion, our work provides evidence that inclusion of Cdh23 exon 68 is critical for the stability of tip links through regulating condensate formation of UTLD components.

The adhesion G protein-coupled receptor VLGR1/ADGRV1 controls autophagy

VLGR1/ADGRV1 (very large G protein-coupled receptor-1) is the largest known adhesion G protein-coupled receptor. Mutations in VLGR1/ADGRV1 cause Usher syndrome (USH), the most common form of hereditary deaf-blindness, and have been additionally linked to epilepsy. Although VLGR1/ADGRV1 is almost ubiquitously expressed, little is known about the subcellular function and signalling of the VLGR1 protein and thus about mechanisms underlying the development of diseases. Using affinity proteomics, we identified key components of autophagosomes as putative interacting proteins of VLGR1. In addition, whole transcriptome sequencing of the retinae of the Vlgr1/del7TM mouse model revealed altered expression profiles of gene-related autophagy. Monitoring autophagy by immunoblotting and immunocytochemistry of the LC3 and p62 as autophagy marker proteins revealed evoked autophagy in VLGR1-deficient hTERT-RPE1 cells and USH2C patient-derived fibroblasts. Our data demonstrate the molecular and functional interaction of VLGR1 with key components of the autophagy process and point to an essential role of VLGR1 in the regulation of autophagy at internal membranes. The close association of VLGR1 with autophagy helps to explain the pathomechanisms underlying human USH and epilepsy related to VLGR1 defects.

Monitoring paxillin in astrocytes reveals the significance of the adhesion G protein coupled receptor VLGR1/ADGRV1 for focal adhesion assembly

VLGR1/ADGRV1 (very large G protein-coupled receptor-1) is the largest adhesion G protein-coupled receptor (aGPCR). Mutations in VLGR1/ADGRV1 are associated with human Usher syndrome, the most common form of deaf-blindness, and also with epilepsy in humans and mice. VLGR1 is expressed almost ubiquitously but is mainly found in the CNS and in the sensory cells of the eye and inner ear. Little is known about the pathogenesis of the diseases related to VLGR1. We previously identified VLGR1 as a vital component of focal adhesions (FAs) serving as a metabotropic mechanoreceptor controls cell spreading and migration. FAs are highly dynamic and turnover in response to internal and external signals. Here, we aimed to elucidate how VLGR1 participates in FA turnover. Nocodazole washouts and live cell imaging of paxillin-DsRed2 consistently showed that FA disassembly was not altered, but de novo assembly of FA was significantly delayed in Vlgr1-deficient astrocytes, indicating that VLGR1 is enrolled in FA assembly. In FRAP experiments, recovery rates were significantly reduced in Vlgr1-deficient FAs, indicating reduced turnover kinetics in VLGR1-deficient FAs. We showed that VLGR1 regulates cell migration by controlling the FA turnover during their assembly and expect novel insights into pathomechanisms related to pathogenic dysfunctions of VLGR1.

Control of stereocilia length during development of hair bundles

Assembly of the hair bundle, the sensory organelle of the inner ear, depends on differential growth of actin-based stereocilia. Separate rows of stereocilia, labeled 1 through 3 from tallest to shortest, lengthen or shorten during discrete time intervals during development. We used lattice structured illumination microscopy and surface rendering to measure dimensions of stereocilia from mouse apical inner hair cells during early postnatal development; these measurements revealed a sharp transition at postnatal day 8 between stage III (row 1 and 2 widening; row 2 shortening) and stage IV (final row 1 lengthening and widening). Tip proteins that determine row 1 lengthening did not accumulate simultaneously during stages III and IV; while the actin-bundling protein EPS8 peaked at the end of stage III, GNAI3 peaked several days later-in early stage IV-and GPSM2 peaked near the end of stage IV. To establish the contributions of key macromolecular assemblies to bundle structure, we examined mouse mutants that eliminated tip links (Cdh23v2J or Pcdh15av3J), transduction channels (TmieKO), or the row 1 tip complex (Myo15ash2). Cdh23v2J/v2J and Pcdh15av3J/av3J bundles had adjacent stereocilia in the same row that were not matched in length, revealing that a major role of these cadherins is to synchronize lengths of side-by-side stereocilia. Use of the tip-link mutants also allowed us to distinguish the role of transduction from effects of transduction proteins themselves. While levels of GNAI3 and GPSM2, which stimulate stereocilia elongation, were greatly attenuated at the tips of TmieKO/KO row 1 stereocilia, they accumulated normally in Cdh23v2J/v2J and Pcdh15av3J/av3J stereocilia. These results reinforced the suggestion that the transduction proteins themselves facilitate localization of proteins in the row 1 complex. By contrast, EPS8 concentrates at tips of all TmieKO/KO, Cdh23v2J/v2J, and Pcdh15av3J/av3J stereocilia, correlating with the less polarized distribution of stereocilia lengths in these bundles. These latter results indicated that in wild-type hair cells, the transduction complex prevents accumulation of EPS8 at the tips of shorter stereocilia, causing them to shrink (rows 2 and 3) or disappear (row 4 and microvilli). Reduced rhodamine-actin labeling at row 2 stereocilia tips of tip-link and transduction mutants suggests that transduction's role is to destabilize actin filaments there. These results suggest that regulation of stereocilia length occurs through EPS8 and that CDH23 and PCDH15 regulate stereocilia lengthening beyond their role in gating mechanotransduction channels.

RBM24 is required for mouse hair cell development through regulating pre-mRNA alternative splicing and mRNA stability

As the sensory receptor cells in vertebrate inner ear and lateral lines, hair cells are characterized by the hair bundle that consists of one tubulin-based kinocilium and dozens of actin-based stereocilia on the apical surface of each hair cell. Hair cell development is tightly regulated, and deficits in this process usually lead to hearing loss and/or balance dysfunctions. RNA-binding motif protein 24 (RBM24) is an RNA-binding protein that is specifically expressed in the hair cells in the inner ear. Previously, we showed that RBM24 affects hair cell development in zebrafish by regulating messenger RNA (mRNA) stability. In the present work, we further investigate the role of RBM24 in hearing and balance using conditional knockout mice. Our results show that Rbm24 knockout results in severe hearing and balance deficits. Hair cell development is significantly affected in Rbm24 knockout cochlea, as the hair bundles are poorly developed and eventually degenerated. Hair bundle disorganization is also observed in Rbm24 knockout vestibular hair cells, although to a lesser extent. Consistently, significant hair cell loss is observed in the cochlea but not vestibule. RNAseq analysis identified several genes whose mRNA stability or pre-mRNA alternative splicing is affected by Rbm24 knockout. Among them are Cdh23, Pcdh15, and Myo7a, which have been shown to play important roles in stereocilia development as well as mechano-electrical transduction. Taken together, our present work suggests that RBM24 is required for mouse hair cell development through regulating pre-mRNA alternative splicing as well as mRNA stability.

The morphological and functional diversity of apical microvilli

Sensory neurons use specialized apical processes to perceive external stimuli and monitor internal body conditions. The apical apparatus can include cilia, microvilli, or both, and is adapted for the functions of the particular cell type. Photoreceptors detect light through a large, modified cilium (outer segment), that is supported by a surrounding ring of microvilli-like calyceal processes (CPs). Although first reported 150 years ago, CPs remain poorly understood. As a basis for future study, we therefore conducted a review of existing literature about sensory cell microvilli, which can act either as the primary sensory detector or as support for a cilia-based detector. While all microvilli are finger-like cellular protrusions with an actin core, the processes vary across cell types in size, number, arrangement, dynamics, and function. We summarize the current state of knowledge about CPs and the characteristics of the microvilli found on inner ear hair cells (stereocilia) and cerebral spinal fluid-contacting neurons, with comparisons to the brush border of the intestinal and renal epithelia. The structure, stability, and dynamics of the actin core are regulated by a complement of actin-binding proteins, which includes both common components and unique features when compared across cell types. Further, microvilli are often supported by lateral links, a glycocalyx, and a defined extracellular matrix, each adapted to the function and environment of the cell. Our comparison of microvillar features will inform further research into how CPs support photoreceptor function, and also provide a general basis for investigations into the structure and functions of apical microvilli found on sensory neurons.

Identifying and Overcoming Challenges in Developing Effective Treatments for Usher 1B: A Workshop Report

Purpose

To identify challenges and opportunities for the development of treatments for Usher syndrome (USH) type 1B.

Methods

In September 2021, the Foundation Fighting Blindness hosted a virtual workshop of clinicians, academic and industry researchers, advocates, and affected individuals and their families to discuss the challenges and opportunities for USH1B treatment development.

Results

The workshop began with insights from individuals affected by USH1B. Presentation topics included myosin VIIA protein function in the ear and eye and its role in disease pathology; challenges with the USH1B mouse model most used in disease research to date; new investigations into alternative disease models that may provide closer analogues to USH1B in the human retina, including retinal organoids and large animal models; and learnings from and limitations of available disease natural history data. Participants discussed the need for an open dialogue between researchers and regulators to design USH1B clinical trials with appropriate outcome measures of vision improvement, along with multimodal imaging of the retina and other testing approaches that can help inform trial designs. The workshop concluded with presentations and a roundtable reviewing emerging treatments, including USH1B-targeted genetic augmentation therapy and gene-agnostic approaches.

Conclusions

Initiatives like this workshop are important to foster all stakeholders in support of achieving the shared goal of treating and curing USH1B.

Translational Relevance

Presentations and discussions focused on overcoming disease modeling and clinical trial design challenges to facilitate development, testing, and implementation of effective USH1B treatments.

Molecular structures and conformations of protocadherin-15 and its complexes on stereocilia elucidated by cryo-electron tomography

Mechanosensory transduction (MT), the conversion of mechanical stimuli into electrical signals, underpins hearing and balance and is carried out within hair cells in the inner ear. Hair cells harbor actin-filled stereocilia, arranged in rows of descending heights, where the tips of stereocilia are connected to their taller neighbors by a filament composed of protocadherin 15 (PCDH15) and cadherin 23 (CDH23), deemed the ‘tip link.’ Tension exerted on the tip link opens an ion channel at the tip of the shorter stereocilia, thus converting mechanical force into an electrical signal. While biochemical and structural studies have provided insights into the molecular composition and structure of isolated portions of the tip link, the architecture, location, and conformational states of intact tip links, on stereocilia, remains unknown. Here, we report in situ cryo-electron microscopy imaging of the tip link in mouse stereocilia. We observe individual PCDH15 molecules at the tip and shaft of stereocilia and determine their stoichiometry, conformational heterogeneity, and their complexes with other filamentous proteins, perhaps including CDH23. The PCDH15 complexes occur in clusters, frequently with more than one copy of PCDH15 at the tip of stereocilia, suggesting that tip links might consist of more than one copy of PCDH15 complexes and, by extension, might include multiple MT complexes.

Electron Microscopy Techniques for Investigating Structure and Composition of Hair-Cell Stereociliary Bundles

Hair cells—the sensory cells of the vertebrate inner ear—bear at their apical surfaces a bundle of actin-filled protrusions called stereocilia, which mediate the cells’ mechanosensitivity. Hereditary deafness is often associated with morphological disorganization of stereocilia bundles, with the absence or mislocalization within stereocilia of specific proteins. Thus, stereocilia bundles are closely examined to understand most animal models of hereditary hearing loss. Because stereocilia have a diameter less than a wavelength of light, light microscopy is not adequate to reveal subtle changes in morphology or protein localization. Instead, electron microscopy (EM) has proven essential for understanding stereocilia bundle development, maintenance, normal function, and dysfunction in disease. Here we review a set of EM imaging techniques commonly used to study stereocilia, including optimal sample preparation and best imaging practices. These include conventional and immunogold transmission electron microscopy (TEM) and scanning electron microscopy (SEM), as well as focused-ion-beam scanning electron microscopy (FIB-SEM), which enables 3-D serial reconstruction of resin-embedded biological structures at a resolution of a few nanometers. Parameters for optimal sample preparation, fixation, immunogold labeling, metal coating and imaging are discussed. Special attention is given to protein localization in stereocilia using immunogold labeling. Finally, we describe the advantages and limitations of these EM techniques and their suitability for different types of studies.

Synaptic Release Potentiation at Aging Auditory Ribbon Synapses

Age-related hidden hearing loss is often described as a cochlear synaptopathy that results from a progressive degeneration of the inner hair cell (IHC) ribbon synapses. The functional changes occurring at these synapses during aging are not fully understood. Here, we characterized this aging process in IHCs of C57BL/6J mice, a strain which is known to carry a cadherin-23 mutation and experiences early hearing loss with age. These mice, while displaying a large increase in auditory brainstem thresholds due to 50% loss of IHC synaptic ribbons at middle age (postnatal day 365), paradoxically showed enhanced acoustic startle reflex suggesting a hyperacusis-like response. The auditory defect was associated with a large shrinkage of the IHCs' cell body and a drastic enlargement of their remaining presynaptic ribbons which were facing enlarged postsynaptic AMPAR clusters. Presynaptic Ca²⁺ microdomains and the capacity of IHCs to sustain high rates of exocytosis were largely increased, while on the contrary the expression of the fast-repolarizing BK channels, known to negatively control transmitter release, was decreased. This age-related synaptic plasticity in IHCs suggested a functional potentiation of synaptic transmission at the surviving synapses, a process that could partially compensate the decrease in synapse number and underlie hyperacusis.

Central auditory deficits associated with genetic forms of peripheral deafness

Since the 1990s, the study of inherited hearing disorders, mostly those detected at birth, in the prelingual period or in young adults, has led to the identification of their causal genes. The genes responsible for more than 140 isolated (non-syndromic) and about 400 syndromic forms of deafness have already been discovered. Studies of mouse models of these monogenic forms of deafness have provided considerable insight into the molecular mechanisms of hearing, particularly those involved in the development and/or physiology of the auditory sensory organ, the cochlea. In parallel, studies of these models have also made it possible to decipher the pathophysiological mechanisms underlying hearing impairment. This has led a number of laboratories to investigate the potential of gene therapy for curing these forms of deafness. Proof-of-concept has now been obtained for the treatment of several forms of deafness in mouse models, paving the way for clinical trials of cochlear gene therapy in patients in the near future. Nevertheless, peripheral deafness may also be associated with central auditory dysfunctions and may extend well beyond the auditory system itself, as a consequence of alterations to the encoded sensory inputs or involvement of the causal deafness genes in the development and/or functioning of central auditory circuits. Investigating the diversity, causes and underlying mechanisms of these central dysfunctions, the ways in which they could impede the expected benefits of hearing restoration by peripheral gene therapy, and determining how these problems could be remedied is becoming a research field in its own right. Here, we provide an overview of the current knowledge about the central deficits associated with genetic forms of deafness.

Inner hair cell stereocilia are embedded in the tectorial membrane

Mammalian hearing depends on sound-evoked displacements of the stereocilia of inner hair cells (IHCs), which cause the endogenous mechanoelectrical transducer channels to conduct inward currents of cations including Ca²⁺. Due to their presumed lack of contacts with the overlaying tectorial membrane (TM), the putative stimulation mechanism for these stereocilia is by means of the viscous drag of the surrounding endolymph. However, despite numerous efforts to characterize the TM by electron microscopy and other techniques, the exact IHC stereocilia-TM relationship remains elusive. Here we show that Ca²⁺-rich filamentous structures, that we call Ca²⁺ ducts, connect the TM to the IHC stereocilia to enable mechanical stimulation by the TM while also ensuring the stereocilia access to TM Ca²⁺. Our results call for a reassessment of the stimulation mechanism for the IHC stereocilia and the TM role in hearing.

Interaction of protocadherin-15 with the scaffold protein whirlin supports its anchoring of hair-bundle lateral links in cochlear hair cells

The hair bundle of cochlear hair cells is the site of auditory mechanoelectrical transduction. It is formed by three rows of stiff microvilli-like protrusions of graduated heights, the short, middle-sized, and tall stereocilia. In developing and mature sensory hair cells, stereocilia are connected to each other by various types of fibrous links. Two unconventional cadherins, protocadherin-15 (PCDH15) and cadherin-23 (CDH23), form the tip-links, whose tension gates the hair cell mechanoelectrical transduction channels. These proteins also form transient lateral links connecting neighboring stereocilia during hair bundle morphogenesis. The proteins involved in anchoring these diverse links to the stereocilia dense actin cytoskeleton remain largely unknown. We show that the long isoform of whirlin (L-whirlin), a PDZ domain-containing submembrane scaffold protein, is present at the tips of the tall stereocilia in mature hair cells, together with PCDH15 isoforms CD1 and CD2; L-whirlin localization to the ankle-link region in developing hair bundles moreover depends on the presence of PCDH15-CD1 also localizing there. We further demonstrate that L-whirlin binds to PCDH15 and CDH23 with moderate-to-high affinities in vitro. From these results, we suggest that L-whirlin is part of the molecular complexes bridging PCDH15-, and possibly CDH23-containing lateral links to the cytoskeleton in immature and mature stereocilia.

Serial scanning electron microscopy of anti-PKHD1L1 immuno-gold labeled mouse hair cell stereocilia bundles

Serial electron microscopy techniques have proven to be a powerful tool in biology. Unfortunately, the data sets they generate lack robust and accurate automated segmentation algorithms. In this data descriptor publication, we introduce a serial focused ion beam scanning electron microscopy (FIB-SEM) dataset consisting of six outer hair cell (OHC) stereocilia bundles, and the supranuclear part of the hair cell bodies. Also presented are the manual segmentations of stereocilia bundles and the gold bead labeling of PKHD1L1, a coat protein of hair cell stereocilia important for hearing in mice. This depository includes all original data and several intermediate steps of the manual analysis, as well as the MATLAB algorithm used to generate a three-dimensional distribution map of gold labels. They serve as a reference dataset, and they enable reproduction of our analysis, evaluation and improvement of current methods of protein localization, and training of algorithms for accurate automated segmentation.

Rho-family G-proteins are required for the recovery of traumatized hair bundle mechanoreceptors in the sea anemone, Nematostella vectensis

Immersing anemones in calcium-free seawater disorganizes hair bundle mechanoreceptors on tentacles of sea anemones while causing a loss of vibration sensitivity. Remarkably, anemone hair bundles recover after being returned to calcium-containing seawater. Reorganization of actin in stereocilia likely follows during the recovery of normal morphology of hair bundles after such immersion. Previous studies have reported that Rho G-proteins are located in the stereocilia of hair bundles in sea anemones where they participate in polymerizing actin in stereocilia upon activation of specific chemoreceptors. We here find that immersing anemones in calcium-free seawater significantly reduces the abundance of hair bundles. A partial recovery of abundance of hair bundles occurs within 3 h post-immersion, but a full recovery of abundance does not occur even 6 h after specimens are returned to calcium-containing seawater. Anemones recovering from immersion in calcium-free seawater feature hair bundles that are significantly wider at their tips than in controls. The hair bundles subsequently narrow at their tips, becoming comparable to those of untreated controls within 6 h. Stereocilia of hair bundles are significantly longer in experimental animals than in controls at 2 h of recovery before shortening to lengths comparable to untreated controls at 6 h. In the presence of Rho inhibitors, the recovery in abundance of hair bundles through 6 h is delayed or inhibited. Likewise, in the presence of Rho inhibitors, stereocilia fail to significantly elongate within 2 h of recovery. These data suggest that Rho G-proteins participate in the normal recovery of abundance and recovery of normal morphology of experimentally damaged hair bundle mechanoreceptors.

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.

Cellular Differences in the Cochlea of CBA and B6 Mice May Underlie Their Difference in Susceptibility to Hearing Loss

Hearing is an extremely delicate sense that is particularly vulnerable to insults from environment, including drugs and noise. Unsurprisingly, mice of different genetic backgrounds show different susceptibility to hearing loss. In particular, CBA/CaJ (CBA) mice maintain relatively stable hearing over age while C57BL/6J (B6) mice show a steady decline of hearing, making them a popular model for early onset hearing loss. To reveal possible underlying mechanisms, we examined cellular differences in the cochlea of these two mouse strains. Although the ABR threshold and Wave I latency are comparable between them, B6 mice have a smaller Wave I amplitude. This difference is probably due to fewer spiral ganglion neurons found in B6 mice, as the number of ribbon synapses per inner hair cell (IHC) is comparable between the two mouse strains. Next, we compared the outer hair cell (OHC) function and we found OHCs from B6 mice are larger in size but the prestin density is similar among them, consistent with the finding that they share similar hearing thresholds. Lastly, we examined the IHC function and we found IHCs from B6 mice have a larger Ca²⁺ current, release more synaptic vesicles and recycle synaptic vesicles more quickly. Taken together, our results suggest that excessive exocytosis from IHCs in B6 mice may raise the probability of glutamate toxicity in ribbon synapses, which could accumulate over time and eventually lead to early onset hearing loss.

Cochlear outer hair cell horizontal top connectors mediate mature stereocilia bundle mechanics

Outer hair cell (OHC) stereocilia bundle deflection opens mechanoelectrical transduction channels at the tips of the stereocilia from the middle and short rows, while bundle cohesion is maintained owing to the presence of horizontal top connectors. Here, we used a quantitative noncontact atomic force microscopy method to investigate stereocilia bundle stiffness and damping, when stimulated at acoustic frequencies and nanometer distances from the bundle. Stereocilia bundle mechanics were determined in stereocilin-deficient mice lacking top connectors and with detached tectorial membrane (Strc -/-/Tecta -/- double knockout) and heterozygous littermate controls (Strc +/-/Tecta -/-). A substantial decrease in bundle stiffness and damping by ~60 and ~74% on postnatal days P13 to P15 was observed when top connectors were absent. Additionally, we followed bundle mechanics during OHC top connectors development between P9 and P15 and quantified the observed increase in OHC bundle stiffness and damping in Strc +/-/Tecta -/- mice while no significant change was detected in Strc -/-/Tecta -/- animals.

Centriole Positioning: Not Just a Little Dot in the Cell

Organelle positioning as many other morphological parameters in a cell is not random. Centriole positioning as centrosomes or ciliary basal bodies is not an exception to this rule in cell biology. Indeed, centriole positioning is a tightly regulated process that occurs during development, and it is critical for many organs to function properly, not just during development but also in the adulthood. In this book chapter, we overview our knowledge on centriole positioning in different and highly specialized animal cells like photoreceptor or ependymal cells. We will also discuss recent advances in the discovery of molecular pathways involved in this process, mostly related to the cytoskeleton and the cell polarity pathways. And finally, we present quantitative methods that have been used to assess centriole positioning in different cell types although mostly in epithelial cells.

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

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

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

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

The Prospects of Cadherin-23 as a Mediator of Homophilic Cell-Cell Adhesion

Cadherins (calcium-dependent adhesion proteins) constitute a family of cell surface proteins that mediate cell-cell adhesion and actively participate in tissue morphogenesis and in mediating tissue integrity. The ecto-domains of cadherins from opposing cell surfaces interact with each other to form the load-bearing trans-dimers and mechanically hold cells together. The "classical" cadherins and desmosomes that form separate groups in cadherin superfamily are mostly explored for their roles in cell-cell adhesion. However, majority of cadherins in cells belong to "nonclassical" group which is poorly explored in the context of their cell-binding properties. This review focuses on the role of "nonclassical" cadherin, cadherin-23, in cell-cell adhesion. Overall, this review highlights the need for further investigations on the role of "nonclassical" cadherin-23 in cell-cell adhesion.

The roles of USH1 proteins and PDZ domain-containing USH proteins in USH2 complex integrity in cochlear hair cells

Usher syndrome (USH) is the most common cause of inherited deaf-blindness, manifested as USH1, USH2 and USH3 clinical types. The protein products of USH2 causative and modifier genes, USH2A, ADGRV1, WHRN and PDZD7, interact to assemble a multiprotein complex at the ankle link region of the mechanosensitive stereociliary bundle in hair cells. Defects in this complex cause stereociliary bundle disorganization and hearing loss. The four USH2 proteins also interact in vitro with USH1 proteins including myosin VIIa, USH1G (SANS), CIB2 and harmonin. However, it is unclear whether the interactions between USH1 and USH2 proteins occur in vivo and whether USH1 proteins play a role in USH2 complex assembly in hair cells. In this study, we identified a novel interaction between myosin VIIa and PDZD7 by FLAG pull-down assay. We further investigated the role of the above-mentioned four USH1 proteins in the cochlear USH2 complex assembly using USH1 mutant mice. We showed that only myosin VIIa is indispensable for USH2 complex assembly at ankle links, indicating the potential transport and/or anchoring role of myosin VIIa for USH2 proteins in hair cells. However, myosin VIIa is not required for USH2 complex assembly in photoreceptors. We further showed that, while PDZ protein harmonin is not involved, its paralogous USH2 proteins, PDZD7 and whirlin, function synergistically in USH2 complex assembly in cochlear hair cells. In summary, our studies provide novel insight into the functional relationship between USH1 and USH2 proteins in the cochlea and the retina as well as the disease mechanisms underlying USH1 and USH2.

Usher syndrome type 1-associated cadherins shape the photoreceptor outer segment

Usher syndrome type 1 (USH1) causes combined hearing and sight defects, but USH1 protein function in the retina is unclear. Schietroma et al. use Xenopus to model the deficiency in two USH1 proteins—protocadherin-15 and cadherin-23—and identify crucial roles for these molecules in shaping the photoreceptor outer segment.

Usher syndrome type 1 (USH1) causes combined hearing and sight defects, but how mutations in USH1 genes lead to retinal dystrophy in patients remains elusive. The USH1 protein complex is associated with calyceal processes, which are microvilli of unknown function surrounding the base of the photoreceptor outer segment. We show that in Xenopus tropicalis, these processes are connected to the outer-segment membrane by links composed of protocadherin-15 (USH1F protein). Protocadherin-15 deficiency, obtained by a knockdown approach, leads to impaired photoreceptor function and abnormally shaped photoreceptor outer segments. Rod basal outer disks displayed excessive outgrowth, and cone outer segments were curved, with lamellae of heterogeneous sizes, defects also observed upon knockdown of Cdh23, encoding cadherin-23 (USH1D protein). The calyceal processes were virtually absent in cones and displayed markedly reduced F-actin content in rods, suggesting that protocadherin-15–containing links are essential for their development and/or maintenance. We propose that calyceal processes, together with their associated links, control the sizing of rod disks and cone lamellae throughout their daily renewal.

Plasma Membrane Targeting of Protocadherin 15 Is Regulated by the Golgi-Associated Chaperone Protein PIST

Protocadherin 15 (PCDH15) is a core component of hair cell tip-links and crucial for proper function of inner ear hair cells. Mutations of PCDH15 gene cause syndromic and nonsyndromic hearing loss. At present, the regulatory mechanisms responsible for the intracellular transportation of PCDH15 largely remain unknown. Here we show that PIST, a Golgi-associated, PDZ domain-containing protein, interacts with PCDH15. The interaction is mediated by the PDZ domain of PIST and the C-terminal PDZ domain-binding interface (PBI) of PCDH15. Through this interaction, PIST retains PCDH15 in the trans-Golgi network (TGN) and reduces the membrane expression of PCDH15. We have previously showed that PIST regulates the membrane expression of another tip-link component, cadherin 23 (CDH23). Taken together, our finding suggests that PIST regulates the intracellular trafficking and membrane targeting of the tip-link proteins CDH23 and PCDH15.

The acquisition of mechano-electrical transducer current adaptation in auditory hair cells requires myosin VI

KEY POINTS

The transduction of sound into electrical signals occurs at the hair bundles atop sensory hair cells in the cochlea, by means of mechanosensitive ion channels, the mechano-electrical transducer (MET) channels. The MET currents decline during steady stimuli; this is termed adaptation and ensures they always work within the most sensitive part of their operating range, responding best to rapidly changing (sound) stimuli. In this study we used a mouse model (Snell's waltzer) for hereditary deafness in humans that has a mutation in the gene encoding an unconventional myosin, myosin VI, which is present in the hair bundles. We found that in the absence of myosin VI the MET current fails to acquire its characteristic adaptation as the hair bundles develop. We propose that myosin VI supports the acquisition of adaptation by removing key molecules from the hair bundle that serve a temporary, developmental role.

ABSTRACT

Mutations in Myo6, the gene encoding the (F-actin) minus end-directed unconventional myosin, myosin VI, cause hereditary deafness in mice (Snell's waltzer) and humans. In the sensory hair cells of the cochlea, myosin VI is expressed in the cell bodies and along the stereocilia that project from the cells' apical surface. It is required for maintaining the structural integrity of the mechanosensitive hair bundles formed by the stereocilia. In this study we investigate whether myosin VI contributes to mechano-electrical transduction. We report that Ca(2+) -dependent adaptation of the mechano-electrical transducer (MET) current, which serves to keep the transduction apparatus operating within its most sensitive range, is absent in outer and inner hair cells from homozygous Snell's waltzer mutant mice, which fail to express myosin VI. The operating range of the MET channels is also abnormal in the mutants, resulting in the absence of a resting MET current. We found that cadherin 23, a component of the hair bundle's transient lateral links, fails to be downregulated along the length of the stereocilia in maturing Myo6 mutant mice. MET currents of heterozygous littermates appear normal. We propose that myosin VI, by removing key molecules from developing hair bundles, is required for the development of the MET apparatus and its Ca(2+) -dependent adaptation.

Differential effects of Cdh23(753A) on auditory and vestibular functional aging in C57BL/6J mice

The C57BL/6J (B6) mouse strain carries a cadherin 23 mutation (Cdh23(753A), also known as Ahl), which affects inner ear structures and results in age-related hearing loss. The B6.CAST strain harbors the wild type Cdh23 gene, and hence, the influence of Ahl is absent. The purpose of the present study was to characterize the effect of age and gender on gravity receptor function in B6 and B6.CAST strains and to compare functional aging between auditory and vestibular modalities. Auditory sensitivity declined at significantly faster rates than gravity receptor sensitivity for both strains. Indeed, vestibular functional aging was minimal for both strains. The comparatively smaller loss of macular versus cochlear sensitivity in both the B6 and B6.CAST strains suggests that the contribution of Ahl to the aging of the vestibular system is minimal, and thus very different than its influence on aging of the auditory system. Alternatively, there exist unidentified genes or gene modifiers that serve to slow the degeneration of gravity receptor structures and maintain gravity receptor sensitivity into advanced age.

Heterozygous mutation of Ush1g/Sans in mice causes early-onset progressive hearing loss, which is recovered by reconstituting the strain-specific mutation in Cdh23

Most clinical reports have suggested that patients with congenital profound hearing loss have recessive mutations in deafness genes, whereas dominant alleles are associated with progressive hearing loss (PHL). Jackson shaker (Ush1g^js) is a mouse model of recessive deafness that exhibits congenital profound deafness caused by the homozygous mutation of Ush1g/Sans on chromosome 11. We found that C57BL/6J-Ush1g^js/+ heterozygous mice exhibited early-onset PHL (ePHL) accompanied by progressive degeneration of stereocilia in the cochlear outer hair cells. Interestingly, ePHL did not develop in mutant mice with the C3H/HeN background, thus suggesting that other genetic factors are required for ePHL development. Therefore, we performed classical genetic analyses and found that the occurrence of ePHL in Ush1g^js/+ mice was associated with an interval in chromosome 10 that contains the cadherin 23 gene (Cdh23), which is also responsible for human deafness. To confirm this mutation effect, we generated C57BL/6J-Ush1g^js/+, Cdh23^c.753A/G double-heterozygous mice by using the CRISPR/Cas9-mediated Cdh23^c.753A>G knock-in method. The Cdh23^c.753A/G mice harbored a one-base substitution (A for G), and the homozygous A allele caused moderate hearing loss with aging. Analyses revealed the complete recovery of ePHL and stereocilia degeneration in C57BL/6J-Ush1g^js/+ mice. These results clearly show that the development of ePHL requires at least two mutant alleles of the Ush1g and Cdh23 genes. Our results also suggest that because the SANS and CDH23 proteins form a complex in the stereocilia, the interaction between these proteins may play key roles in the maintenance of stereocilia and the prevention of ePHL.

Shaping the intestinal brush border

Epithelial cells from diverse tissues, including the enterocytes that line the intestinal tract, remodel their apical surface during differentiation to form a brush border: an array of actin-supported membrane protrusions known as microvilli that increases the functional capacity of the tissue. Although our understanding of how epithelial cells assemble, stabilize, and organize apical microvilli is still developing, investigations of the biochemical and physical underpinnings of these processes suggest that cells coordinate cytoskeletal remodeling, membrane-cytoskeleton cross-linking, and extracellular adhesion to shape the apical brush border domain.

Usher syndrome: Hearing loss, retinal degeneration and associated abnormalities

Usher syndrome (USH), clinically and genetically heterogeneous, is the leading genetic cause of combined hearing and vision loss. USH is classified into three types, based on the hearing and vestibular symptoms observed in patients. Sixteen loci have been reported to be involved in the occurrence of USH and atypical USH. Among them, twelve have been identified as causative genes and one as a modifier gene. Studies on the proteins encoded by these USH genes suggest that USH proteins interact among one another and function in multiprotein complexes in vivo. Although their exact functions remain enigmatic in the retina, USH proteins are required for the development, maintenance and function of hair bundles, which are the primary mechanosensitive structure of inner ear hair cells. Despite the unavailability of a cure, progress has been made to develop effective treatments for this disease. In this review, we focus on the most recent discoveries in the field with an emphasis on USH genes, protein complexes and functions in various tissues as well as progress toward therapeutic development for USH.

Using injectoporation to deliver genes to mechanosensory hair cells

Mechanosensation, the transduction of mechanical force into electrochemical signals, allows organisms to detect touch and sound, to register movement and gravity, and to sense changes in cell volume and shape. The hair cells of the mammalian inner ear are the mechanosensors for the detection of sound and head movement. The analysis of gene function in hair cells has been hampered by the lack of an efficient gene transfer method. Here we describe a method termed injectoporation that combines tissue microinjection with electroporation to express cDNAs and shRNAs in mouse cochlear hair cells. Injectoporation allows for gene transfer into dozens of hair cells, and it is compatible with the analysis of hair cell function using imaging approaches and electrophysiology. Tissue dissection and injectoporation can be carried out within a few hours, and the tissue can be cultured for days for subsequent functional analyses.

Genetics of auditory mechano-electrical transduction

The hair bundles of cochlear hair cells play a central role in the auditory mechano-electrical transduction (MET) process. The identification of MET components and of associated molecular complexes by biochemical approaches is impeded by the very small number of hair cells within the cochlea. In contrast, human and mouse genetics have proven to be particularly powerful. The study of inherited forms of deafness led to the discovery of several essential proteins of the MET machinery, which are currently used as entry points to decipher the associated molecular networks. Notably, MET relies not only on the MET machinery but also on several elements ensuring the proper sound-induced oscillation of the hair bundle or the ionic environment necessary to drive the MET current. Here, we review the most significant advances in the molecular bases of the MET process that emerged from the genetics of hearing.

Compound heterozygosity of the functionally null Cdh23(v-ngt) and hypomorphic Cdh23(ahl) alleles leads to early-onset progressive hearing loss in mice

The waltzer (v) mouse mutant harbors a mutation in Cadherin 23 (Cdh23) and is a model for Usher syndrome type 1D, which is characterized by congenital deafness, vestibular dysfunction, and prepubertal onset of progressive retinitis pigmentosa. In mice, functionally null Cdh23 mutations affect stereociliary morphogenesis and the polarity of both cochlear and vestibular hair cells. In contrast, the murine Cdh23^ahl allele, which harbors a hypomorphic mutation, causes an increase in susceptibility to age-related hearing loss in many inbred strains. We produced congenic mice by crossing mice carrying the v niigata (Cdh23^v-ngt) null allele with mice carrying the hypomorphic Cdh23^ahl allele on the C57BL/6J background, and we then analyzed the animals’ balance and hearing phenotypes. Although the Cdh23^v-ngt/ahl compound heterozygous mice exhibited normal vestibular function, their hearing ability was abnormal: the mice exhibited higher thresholds of auditory brainstem response (ABR) and rapid age-dependent elevation of ABR thresholds compared with Cdh23^ahl/ahl homozygous mice. We found that the stereocilia developed normally but were progressively disrupted in Cdh23^v-ngt/ahl mice. In hair cells, CDH23 localizes to the tip links of stereocilia, which are thought to gate the mechanoelectrical transduction channels in hair cells. We hypothesize that the reduction of Cdh23 gene dosage in Cdh23^v-ngt/ahl mice leads to the degeneration of stereocilia, which consequently reduces tip link tension. These findings indicate that CDH23 plays an important role in the maintenance of tip links during the aging process.

The retinal phenotype of Usher syndrome: pathophysiological insights from animal models

The Usher syndrome (USH) is the most prevalent cause of inherited deaf-blindness. Three clinical subtypes, USH1-3, have been defined, and ten USH genes identified. The hearing impairment due to USH gene defects has been shown to result from improper organisation of the hair bundle, the sound receptive structure of sensory hair cells. In contrast, the cellular basis of the visual defect is less well understood as this phenotype is absent in almost all the USH mouse models that faithfully mimic the human hearing impairment. Structural and molecular interspecies discrepancies regarding photoreceptor calyceal processes and the association with the distribution of USH1 proteins have recently been unravelled, and have led to the conclusion that a defect in the USH1 protein complex-mediated connection between the photoreceptor outer segment and the surrounding calyceal processes (in both rods and cones), and the inner segment (in rods only), probably causes the USH1 retinal dystrophy in humans.

Auditory hair cell centrioles undergo confined Brownian motion throughout the developmental migration of the kinocilium

Planar polarization of the forming hair bundle, the mechanosensory antenna of auditory hair cells, depends on the poorly characterized center-to-edge displacement of a primary cilium, the kinocilium, at their apical surface. Taking advantage of the gradient of hair cell differentiation along the cochlea, we reconstituted a map of the kinocilia displacements in the mouse embryonic cochlea. We then developed a cochlear organotypic culture and video-microscopy approach to monitor the movements of the kinocilium basal body (mother centriole) and its daughter centriole, which we analyzed using particle tracking and modeling. We found that both hair cell centrioles undergo confined Brownian movements around their equilibrium positions, under the apparent constraint of a radial restoring force of ∼0.1 pN. This magnitude depended little on centriole position, suggesting nonlinear interactions with constraining, presumably cytoskeletal elements. The only dynamic change observed during the period of kinocilium migration was a doubling of the centrioles' confinement area taking place early in the process. It emerges from these static and dynamic observations that kinocilia migrate gradually in parallel with the organization of hair cells into rows during cochlear neuroepithelium extension. Analysis of the confined motion of hair cell centrioles under normal and pathological conditions should help determine which structures contribute to the restoring force exerting on them.

Non-syndromic hereditary sensorineural hearing loss: review of the genes involved

BACKGROUND

Hereditary sensorineural hearing loss is the most frequently occurring birth defect. It has profound effects for the individual and is a substantial burden on society. Insight into disease mechanisms can help to broaden therapeutic options and considerably lower lifetime social costs. In the past few decades, the identification of genes that can cause this type of hearing loss has developed rapidly.

OBJECTIVE

This paper provides a concise overview of the currently known genes involved in non-syndromic hereditary hearing loss and their function in the inner ear.

Usher protein functions in hair cells and photoreceptors

The 10 different genes associated with the deaf/blind disorder, Usher syndrome, encode a number of structurally and functionally distinct proteins, most expressed as multiple isoforms/protein variants. Functional characterization of these proteins suggests a role in stereocilia development in cochlear hair cells, likely owing to adhesive interactions in hair bundles. In mature hair cells, homodimers of the Usher cadherins, cadherin 23 and protocadherin 15, interact to form a structural fiber, the tip link, and the linkages that anchor the taller stereocilia's actin cytoskeleton core to the shorter adjacent stereocilia and the elusive mechanotransduction channels, explaining the deafness phenotype when these molecular interactions are perturbed. The conundrum is that photoreceptors lack a synonymous mechanotransduction apparatus, and so a common theory for Usher protein function in the two neurosensory cell types affected in Usher syndrome is lacking. Recent evidence linking photoreceptor cell dysfunction in the shaker 1 mouse model for Usher syndrome to light-induced protein translocation defects, combined with localization of an Usher protein interactome at the periciliary region of the photoreceptors suggests Usher proteins might regulate protein trafficking between the inner and outer segments of photoreceptors. A distinct Usher protein complex is trafficked to the ribbon synapses of hair cells, and synaptic defects have been reported in Usher mutants in both hair cells and photoreceptors. This review aims to clarify what is known about Usher protein function at the synaptic and apical poles of hair cells and photoreceptors and the prospects for identifying a unifying pathobiological mechanism to explain deaf/blindness in Usher syndrome.

Molecular remodeling of tip links underlies mechanosensory regeneration in auditory hair cells

Sound detection by inner ear hair cells requires tip links that interconnect mechanosensory stereocilia and convey force to yet unidentified transduction channels. Current models postulate a static composition of the tip link, with protocadherin 15 (PCDH15) at the lower and cadherin 23 (CDH23) at the upper end of the link. In terminally differentiated mammalian auditory hair cells, tip links are subjected to sound-induced forces throughout an organism's life. Although hair cells can regenerate disrupted tip links and restore hearing, the molecular details of this process are unknown. We developed a novel implementation of backscatter electron scanning microscopy to visualize simultaneously immuno-gold particles and stereocilia links, both of only a few nanometers in diameter. We show that functional, mechanotransduction-mediating tip links have at least two molecular compositions, containing either PCDH15/CDH23 or PCDH15/PCDH15. During regeneration, shorter tip links containing nearly equal amounts of PCDH15 at both ends appear first. Whole-cell patch-clamp recordings demonstrate that these transient PCDH15/PCDH15 links mediate mechanotransduction currents of normal amplitude but abnormal Ca(2+)-dependent decay (adaptation). The mature PCDH15/CDH23 tip link composition is re-established later, concomitant with complete recovery of adaptation. Thus, our findings provide a molecular mechanism for regeneration and maintenance of mechanosensory function in postmitotic auditory hair cells and could help identify elusive components of the mechanotransduction machinery.

Noddy, a mouse harboring a missense mutation in protocadherin-15, reveals the impact of disrupting a critical interaction site between tip-link cadherins in inner ear hair cells

In hair cells of the inner ear, sound or head movement increases tension in fine filaments termed tip links, which in turn convey force to mechanosensitive ion channels to open them. Tip links are formed by a tetramer of two cadherin proteins: protocadherin 15 (PCDH15) and cadherin 23 (CDH23), which have 11 and 27 extracellular cadherin (EC) repeats, respectively. Mutations in either protein cause inner ear disorders in mice and humans. We showed recently that these two cadherins bind tip-to-tip in a "handshake" mode that involves the EC1 and EC2 repeats of both proteins. However, a paucity of appropriate animal models has slowed our understanding both of the interaction and of how mutations of residues within the predicted interface compromise tip link integrity. Here, we present noddy, a new mouse model for hereditary deafness. Identified in a forward genetic screen, noddy homozygotes lack inner ear function. Mapping and sequencing showed that noddy mutant mice harbor an isoleucine-to-asparagine (I108N) mutation in the EC1 repeat of PCDH15. Residue I108 interacts with CDH23 EC2 in the handshake and its mutation impairs the interaction in vitro. The noddy mutation allowed us to determine the consequences of blocking the handshake in vivo: tip link formation and bundle morphology are disrupted, and mechanotransduction channels fail to remain open at rest. These results offer new insights into the interaction between PCDH15 and CDH23 and help explain the etiology of human deafness linked to mutations in the tip-link interface.

TMHS is an integral component of the mechanotransduction machinery of cochlear hair cells

Hair cells are mechanosensors for the perception of sound, acceleration, and fluid motion. Mechanotransduction channels in hair cells are gated by tip links, which connect the stereocilia of a hair cell in the direction of their mechanical sensitivity. The molecular constituents of the mechanotransduction channels of hair cells are not known. Here, we show that mechanotransduction is impaired in mice lacking the tetraspan TMHS. TMHS binds to the tip-link component PCDH15 and regulates tip-link assembly, a process that is disrupted by deafness-causing Tmhs mutations. TMHS also regulates transducer channel conductance and is required for fast channel adaptation. TMHS therefore resembles other ion channel regulatory subunits such as the transmembrane alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid (AMPA) receptor regulatory proteins (TARPs) of AMPA receptors that facilitate channel transport and regulate the properties of pore-forming channel subunits. We conclude that TMHS is an integral component of the hair cell's mechanotransduction machinery that functionally couples PCDH15 to the transduction channel.

Integrity and regeneration of mechanotransduction machinery regulate aminoglycoside entry and sensory cell death

Sound perception requires functional hair cell mechanotransduction (MET) machinery, including the MET channels and tip-link proteins. Prior work showed that uptake of ototoxic aminoglycosides (AG) into hair cells requires functional MET channels. In this study, we examined whether tip-link proteins, including Cadherin 23 (Cdh23), regulate AG entry into hair cells. Using time-lapse microscopy on cochlear explants, we found rapid uptake of gentamicin-conjugated Texas Red (GTTR) into hair cells from three-day-old Cdh23^+/+ and Cdh23^v2J/+ mice, but failed to detect GTTR uptake in Cdh23^v2J/v2J hair cells. Pre-treatment of wildtype cochleae with the calcium chelator 1,2-bis(o-aminophenoxy) ethane-N,N,N',N'-tetraacetic acid (BAPTA) to disrupt tip-links also effectively reduced GTTR uptake into hair cells. Both Cdh23^v2J/v2J and BAPTA-treated hair cells were protected from degeneration caused by gentamicin. Six hours after BAPTA treatment, GTTR uptake remained reduced in comparison to controls; by 24 hours, drug uptake was comparable between untreated and BAPTA-treated hair cells, which again became susceptible to cell death induced by gentamicin. Together, these results provide genetic and pharmacologic evidence that tip-links are required for AG uptake and toxicity in hair cells. Because tip-links can spontaneously regenerate, their temporary breakage offers a limited time window when hair cells are protected from AG toxicity.

Cadherin defects in inherited human diseases

The tight control of cell-cell connectivity mediated by cadherins is a key issue in human health and disease. The human genome contains over 115 genes encoding cadherins and cadherin-like proteins. Defects in about 21 of these proteins (8 classical, 5 desmosomal, 8 atypical cadherins) have been linked to inherited disorders in humans, including skin and hair disorders, cardiomyopathies, sensory defects associated with deafness and blindness, and psychiatric disorders. With the advent of exome and genome sequencing techniques, we can anticipate the discovery of yet more evidence for the involvement of additional cadherins. Elucidation of the related physiopathological mechanisms underlying these conditions should help to clarify the roles played by these cadherins in tissues and the ways in which defects in different cadherins cause such a wide spectrum of associated phenotypes. These disorders also constitute disparate model systems for investigations of the relative contributions of mechanical adhesive strength and intracellular signaling pathways to the pathogenic process for a given cadherin.

The composition and role of cross links in mechanoelectrical transduction in vertebrate sensory hair cells

The key components of acousticolateralis systems (lateral line, hearing and balance) are sensory hair cells. At their apex, these cells have a bundle of specialized cellular protrusions, which are modified actin-containing microvilli, connected together by extracellular filaments called cross links. Stereociliary deflections open nonselective cation channels allowing ions from the extracellular environment into the cell, a process called mechanoelectrical transduction. This produces a receptor potential that causes the release of the excitatory neurotransmitter glutamate onto the terminals of the sensory nerve fibres, which connect to the cell base, causing nerve signals to be sent to the brain. Identification of the cellular mechanisms underlying mechanoelectrical transduction and of some of the proteins involved has been assisted by research into the genetics of deafness, molecular biology and mechanical measurements of function. It is thought that one type of cross link, the tip link, is composed of cadherin 23 and protocadherin 15, and gates the transduction channel when the bundle is deflected. Another type of link, called lateral (or horizontal) links, maintains optimal bundle cohesion and stiffness for transduction. This Commentary summarizes the information currently available about the structure, function and composition of the links and how they might be relevant to human hearing impairment.

Localization of Usher 1 proteins to the photoreceptor calyceal processes, which are absent from mice

The mechanisms underlying retinal dystrophy in Usher syndrome type I (USH1) remain unknown because mutant mice lacking any of the USH1 proteins-myosin VIIa, harmonin, cadherin-23, protocadherin-15, sans-do not display retinal degeneration. We found here that, in macaque photoreceptor cells, all USH1 proteins colocalized at membrane interfaces (i) between the inner and outer segments in rods and (ii) between the microvillus-like calyceal processes and the outer segment basolateral region in rods and cones. This pattern, conserved in humans and frogs, was mediated by the formation of an USH1 protein network, which was associated with the calyceal processes from the early embryonic stages of outer segment growth onwards. By contrast, mouse photoreceptors lacked calyceal processes and had no USH1 proteins at the inner-outer segment interface. We suggest that USH1 proteins form an adhesion belt around the basolateral region of the photoreceptor outer segment in humans, and that defects in this structure cause the retinal degeneration in USH1 patients.

Large protein assemblies formed by multivalent interactions between cadherin23 and harmonin suggest a stable anchorage structure at the tip link of stereocilia

Stereocilia tip links of inner ear hair cells are subjected to constant stretching during hair-bundle deflection, and accordingly are well designed to prevent from being broken by mechanical tensions. The roots of tip links, which couple tip links with the cytoskeleton, supposedly play important roles in withstanding large forces under stimulated conditions. The upper root of the tip link is mainly formed by the cytoplasmic tail of cadherin23 and its actin-anchoring protein harmonin. However, the detailed organization mode of the two proteins that gives rise to a strong upper root remains unclear. Here we show that the exon68-encoded peptide of cadherin23 can either interact with the N-terminal domain (NTD) of harmonin or form a homodimer. We demonstrate that the three harmonin binding sites of cadherin23, namely the NTD-binding motif, the exon68 peptide, and the C-terminal PDZ binding motif, do not synergize with each other in binding to harmonin, instead they facilitate formation of polymeric cadherin23/harmonin complexes. The exon68 peptide can promote the cadherin23/harmonin polymer formation via either binding to harmonin NTD or self-dimerization. We propose that the polymeric cadherin23/harmonin complex formed beneath the upper tip link membranes may serve as part of the stable rootlet structure for anchoring the tip links of stereocilia.

Advantages of a mouse model for human hearing impairment

Hearing is a major factor in human quality of life. Mouse models are important tools for discovering the genes that are responsible for genetic hearing loss, and these models often allow the processes that regulate the onset of deafness in humans to be analyzed. Thus far, in the study of hearing and deafness, at least 400 mutants with hearing impairments have been identified in laboratory mouse populations. Analysis of through a combination of genetic, morphological, and physiological studies is revealing valuable insights into the ontogenesis, morphogenesis, and function of the mammalian ear. This review discusses the advantages of the mouse models of human hearing impairment and highlights the identification of the molecules required for stereocilia development in the inner ear hair cells by analysis of various mouse mutants.