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

Grxcr2 is required for stereocilia morphogenesis in the cochlea

Hearing and balance depend upon the precise morphogenesis and mechanosensory function of stereocilia, the specialized structures on the apical surface of sensory hair cells in the inner ear. Previous studies of Grxcr1 mutant mice indicated a critical role for this gene in control of stereocilia dimensions during development. In this study, we analyzed expression of the paralog Grxcr2 in the mouse and evaluated auditory and vestibular function of strains carrying targeted mutations of the gene. Peak expression of Grxcr2 occurs during early postnatal development of the inner ear and GRXCR2 is localized to stereocilia in both the cochlea and in vestibular organs. Homozygous Grxcr2 deletion mutants exhibit significant hearing loss by 3 weeks of age that is associated with developmental defects in stereocilia bundle orientation and organization. Despite these bundle defects, the mechanotransduction apparatus assembles in relatively normal fashion as determined by whole cell electrophysiological evaluation and FM1-43 uptake. Although Grxcr2 mutants do not exhibit overt vestibular dysfunction, evaluation of vestibular evoked potentials revealed subtle defects of the mutants in response to linear accelerations. In addition, reduced Grxcr2 expression in a hypomorphic mutant strain is associated with progressive hearing loss and bundle defects. The stereocilia localization of GRXCR2, together with the bundle pathologies observed in the mutants, indicate that GRXCR2 plays an intrinsic role in bundle orientation, organization, and sensory function in the inner ear during development and at maturity.

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.

TRPV6, TRPM6 and TRPM7 Do Not Contribute to Hair-Cell Mechanotransduction

Hair cells of the inner ear transduce mechanical stimuli like sound or head movements into electrical signals, which are propagated to the central nervous system. The hair-cell mechanotransduction channel remains unidentified. We tested whether three transient receptor channel (TRP) family members, TRPV6, TRPM6 and TRPM7, were necessary for transduction. TRPV6 interacted with USH1C (harmonin), a scaffolding protein that participates in transduction. Using a cysteine-substitution knock-in mouse line and methanethiosulfonate (MTS) reagents selective for this allele, we found that inhibition of TRPV6 had no effect on transduction in mouse cochlear hair cells. TRPM6 and TRPM7 each interacted with the tip-link component PCDH15 in cultured eukaryotic cells, which suggested they might be part of the transduction complex. Cochlear hair cell transduction was not affected by manipulations of Mg2+, however, which normally perturbs TRPM6 and TRPM7. To definitively examine the role of these two channels in transduction, we showed that deletion of either or both of their genes selectively in hair cells had no effect on auditory function. We suggest that TRPV6, TRPM6 and TRPM7 are unlikely to be the pore-forming subunit of the hair-cell transduction channel.

Rescue of Outer Hair Cells with Antisense Oligonucleotides in Usher Mice Is Dependent on Age of Treatment

The absence of functional outer hair cells is a component of several forms of hereditary hearing impairment, including Usher syndrome, the most common cause of concurrent hearing and vision loss. Antisense oligonucleotide (ASO) treatment of mice with the human Usher mutation, Ush1c c.216G>A, corrects gene expression and significantly improves hearing, as measured by auditory-evoked brainstem responses (ABRs), as well as inner and outer hair cell (IHC and OHC) bundle morphology. However, it is not clear whether the improvement in hearing achieved by ASO treatment involves the functional rescue of outer hair cells. Here, we show that Ush1c c.216AA mice lack OHC function as evidenced by the absence of distortion product otoacoustic emissions (DPOAEs) in response to low-, mid-, and high-frequency tone pairs. This OHC deficit is rescued by treatment with an ASO that corrects expression of Ush1c c.216G>A. Interestingly, although rescue of inner hairs cells, as measured by ABR, is achieved by ASO treatment as late as 7 days after birth, rescue of outer hair cells, measured by DPOAE, requires treatment before post-natal day 5. These results suggest that ASO-mediated rescue of both IHC and OHC function is age dependent and that the treatment window is different for the different cell types. The timing of treatment for congenital hearing disorders is of critical importance for the development of drugs such ASO-29 for hearing rescue.

Cone degeneration is triggered by the absence of USH1 proteins but prevented by antioxidant treatments

Usher syndrome type 1 (USH1) is a major cause of inherited deafness and blindness in humans. The eye disorder is often referred to as retinitis pigmentosa, which is characterized by a secondary cone degeneration following the rod loss. The development of treatments to prevent retinal degeneration has been hampered by the lack of clear evidence for retinal degeneration in mutant mice deficient for the Ush1 genes, which instead faithfully mimic the hearing deficit. We show that, under normal housing conditions, Ush1g^−/− and Ush1c^−/− albino mice have dysfunctional cone photoreceptors whereas pigmented knockout animals have normal photoreceptors. The key involvement of oxidative stress in photoreceptor apoptosis and the ensued retinal gliosis were further confirmed by their prevention when the mutant mice are reared under darkness and/or supplemented with antioxidants. The primary degeneration of cone photoreceptors contrasts with the typical forms of retinitis pigmentosa. Altogether, we propose that oxidative stress probably accounts for the high clinical heterogeneity among USH1 siblings, which also unveils potential targets for blindness prevention.

Rescue of peripheral vestibular function in Usher syndrome mice using a splice-switching antisense oligonucleotide

Usher syndrome type 1C (USH1C/harmonin) is associated with profound retinal, auditory and vestibular dysfunction. We have previously reported on an antisense oligonucleotide (ASO-29) that dramatically improves auditory function and balance behavior in mice homozygous for the harmonin mutation Ush1c c.216G > A following a single systemic administration. The findings were suggestive of improved vestibular function; however, no direct vestibular assessment was made. Here, we measured vestibular sensory evoked potentials (VsEPs) to directly assess vestibular function in Usher mice. We report that VsEPs are absent or abnormal in Usher mice, indicating profound loss of vestibular function. Strikingly, Usher mice receiving ASO-29 treatment have normal or elevated vestibular response thresholds when treated during a critical period between postnatal day 1 and 5, respectively. In contrast, treatment of mice with ASO-29 treatment at P15 was minimally effective at rescuing vestibular function. Interestingly, ASO-29 treatment at P1, P5 or P15 resulted in sufficient vestibular recovery to support normal balance behaviors, suggesting a therapeutic benefit to balance with ASO-29 treatment at P15 despite the profound vestibular functional deficits that persist with treatment at this later time. These findings provide the first direct evidence of an effective treatment of peripheral vestibular function in a mouse model of USH1C and reveal the potential for using antisense technology to treat vestibular dysfunction.

A Model for Link Pruning to Establish Correctly Polarized and Oriented Tip Links in Hair Bundles

Tip links are thought to gate the mechanically sensitive transduction channels of hair cells, but how they form during development and regeneration remains mysterious. In particular, it is unclear how tip links are strung between stereocilia so that they are oriented parallel to a single axis; why their polarity is uniform despite their constituent molecules' intrinsic asymmetry; and why only a single tip link is present at each tip-link position. We present here a series of simple rules that reasonably explain why these phenomena occur. In particular, our model relies on each of the two ends of the tip link having distinct Ca²⁺-dependent stability and being connected to different motor complexes. A simulation employing these rules allowed us to explore the parameter space for the model, demonstrating the importance of the feedback between transduction channels and angled links, links that are 60° off-axis with respect to mature tip links. We tested this key aspect of the model by examining angled links in chick cochlea hair cells. As implied by the assumptions used to generate the model, we found that angled links were stabilized if there was no tip link at the tip of the upper stereocilium, and appeared when transduction channels were blocked. The model thus plausibly explains how tip-link formation and pruning can occur.

Antisense oligonucleotide therapy rescues disruptions in organization of exploratory movements associated with Usher syndrome type 1C in mice

Usher syndrome, Type 1C (USH1C) is an autosomal recessive inherited disorder in which a mutation in the gene encoding harmonin is associated with multi-sensory deficits (i.e., auditory, vestibular, and visual). USH1C (Usher) mice, engineered with a human USH1C mutation, exhibit these multi-sensory deficits by circling behavior and lack of response to sound. Administration of an antisense oligonucleotide (ASO) therapeutic that corrects expression of the mutated USH1C gene, has been shown to increase harmonin levels, reduce circling behavior, and improve vestibular and auditory function. The current study evaluates the organization of exploratory movements to assess spatial organization in Usher mice and determine the efficacy of ASO therapy in attenuating any such deficits. Usher and heterozygous mice received the therapeutic ASO, ASO-29, or a control, non-specific ASO treatment at postnatal day five. Organization of exploratory movements was assessed under dark and light conditions at two and six-months of age. Disruptions in exploratory movement organization observed in control-treated Usher mice were consistent with impaired use of self-movement and environmental cues. In general, ASO-29 treatment rescued organization of exploratory movements at two and six-month testing points. These observations are consistent with ASO-29 rescuing processing of multiple sources of information and demonstrate the potential of ASO therapies to ameliorate topographical disorientation associated with other genetic disorders.

Emerging Gene Therapies for Genetic Hearing Loss

Gene therapy, or the treatment of human disease using genetic material, for inner ear dysfunction is coming of age. Recent progress in developing gene therapy treatments for genetic hearing loss has demonstrated tantalizing proof-of-principle in animal models. While successful translation of this progress into treatments for humans awaits, there is growing interest from patients, scientists, clinicians, and industry. Nonetheless, it is clear that a number of hurdles remain, and expectations for total restoration of auditory function should remain tempered until these challenges have been overcome. Here, we review progress, prospects, and challenges for gene therapy in the inner ear. We focus on technical aspects, including routes of gene delivery to the inner ear, choice of vectors, promoters, inner ear targets, therapeutic strategies, preliminary success stories, and points to consider for translating of these successes to the clinic.

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.

Local gene therapy durably restores vestibular function in a mouse model of Usher syndrome type 1G

Our understanding of the mechanisms underlying inherited forms of inner ear deficits has considerably improved during the past 20 y, but we are still far from curative treatments. We investigated gene replacement as a strategy for restoring inner ear functions in a mouse model of Usher syndrome type 1G, characterized by congenital profound deafness and balance disorders. These mice lack the scaffold protein sans, which is involved both in the morphogenesis of the stereociliary bundle, the sensory antenna of inner ear hair cells, and in the mechanoelectrical transduction process. We show that a single delivery of the sans cDNA by the adenoassociated virus 8 to the inner ear of newborn mutant mice reestablishes the expression and targeting of the protein to the tips of stereocilia. The therapeutic gene restores the architecture and mechanosensitivity of stereociliary bundles, improves hearing thresholds, and durably rescues these mice from the balance defects. Our results open up new perspectives for efficient gene therapy of cochlear and vestibular disorders by showing that even severe dysmorphogenesis of stereociliary bundles can be corrected.

A homozygous MYO7A mutation associated to Usher syndrome and unilateral auditory neuropathy spectrum disorder

Usher syndrome (USH) is an autosomal recessive disorder characterized by sensorineural hearing loss, progressive visual loss and night blindness due to retinitis pigmentosa (RP), with or without vestibular dysfunction. The purpose of this study was to detect the causative gene in a consanguineous Chinese family with USH. A c.3696_3706del (p.R1232Sfs*72) variant in the myosin VIIa gene (MYO7A) was identified in the homozygous state by exome sequencing. The co‑segregation of the MYO7A c.3696_3706del variant with the phenotype of deafness and progressive visual loss in the USH family was confirmed by Sanger sequencing. The variant was absent in 200 healthy controls. Therefore, the c.3696_3706del variant may disrupt the interaction between myosin VIIa and other USH1 proteins, and impair melanosome transport in retinal pigment epithelial cells. Notably, bilateral auditory brainstem responses were absent in two patients of the USH family, while distortion product otoacoustic emissions were elicited in the right ears of the two patients, consistent with clinical diagnosis of unilateral auditory neuropathy spectrum disorder. These data suggested that the homozygous c.3696_3706del variant in the MYO7A gene may be the disease‑causing mutation for the disorder in this family. These findings broaden the phenotype spectrum of the MYO7A gene, and may facilitate understanding of the molecular pathogenesis of the disease, and genetic counseling for the family.

Auditory cortex interneuron development requires cadherins operating hair-cell mechanoelectrical transduction

Many genetic forms of congenital deafness affect the sound reception antenna of cochlear sensory cells, the hair bundle. The resulting sensory deprivation jeopardizes auditory cortex (AC) maturation. Early prosthetic intervention should revive this process. Nevertheless, this view assumes that no intrinsic AC deficits coexist with the cochlear ones, a possibility as yet unexplored. We show here that many GABAergic interneurons, from their generation in the medial ganglionic eminence up to their settlement in the AC, express two cadherin-related (cdhr) proteins, cdhr23 and cdhr15, that form the hair bundle tip links gating the mechanoelectrical transduction channels. Mutant mice lacking either protein showed a major decrease in the number of parvalbumin interneurons specifically in the AC, and displayed audiogenic reflex seizures. Cdhr15- and Cdhr23-expressing interneuron precursors in Cdhr23^-/- and Cdhr15^-/- mouse embryos, respectively, failed to enter the embryonic cortex and were scattered throughout the subpallium, consistent with the cell polarity abnormalities we observed in vitro. In the absence of adhesion G protein-coupled receptor V1 (adgrv1), another hair bundle link protein, the entry of Cdhr23- and Cdhr15-expressing interneuron precursors into the embryonic cortex was also impaired. Our results demonstrate that a population of newborn interneurons is endowed with specific cdhr proteins necessary for these cells to reach the developing AC. We suggest that an "early adhesion code" targets populations of interneuron precursors to restricted neocortical regions belonging to the same functional area. These findings open up new perspectives for auditory rehabilitation and cortical therapies in patients.

Conformational switch of harmonin, a submembrane scaffold protein of the hair cell mechanoelectrical transduction machinery

Mutations in the gene encoding harmonin, a multi‐PDZ domain‐containing submembrane protein, cause Usher syndrome type 1 (congenital deafness and balance disorder, and early‐onset sight loss). The structure of the protein and biological activities of its three different classes of splice isoforms (a, b, and c) remain poorly understood. Combining biochemical and biophysical analyses, we show that harmonin‐a1 can switch between open and closed conformations through intramolecular binding of its C‐terminal PDZ‐binding motif to its N‐terminal supramodule NTD‐PDZ1 and through a flexible PDZ2‐PDZ3 linker. This conformational switch presumably extends to most harmonin isoforms, and it is expected to have an impact on the interaction with some binding partners, as shown here for cadherin‐related 23, another component of the hair cell mechanoelectrical transduction machinery.

Myosin 7 and its adaptors link cadherins to actin

Cadherin linkages between adjacent stereocilia and microvilli are essential for mechanotransduction and maintaining their organization. They are anchored to actin through interaction of their cytoplasmic domains with related tripartite complexes consisting of a class VII myosin and adaptor proteins: Myo7a/SANS/Harmonin in stereocilia and Myo7b/ANKS4B/Harmonin in microvilli. Here, we determine high-resolution structures of Myo7a and Myo7b C-terminal MyTH4-FERM domain (MF2) and unveil how they recognize harmonin using a novel binding mode. Systematic definition of interactions between domains of the tripartite complex elucidates how the complex assembles and prevents possible self-association of harmonin-a. Several Myo7a deafness mutants that map to the surface of MF2 disrupt harmonin binding, revealing the molecular basis for how they impact the formation of the tripartite complex and disrupt mechanotransduction. Our results also suggest how switching between different harmonin isoforms can regulate the formation of networks with Myo7a motors and coordinate force sensing in stereocilia.

Cadherin is essential for mechanotransduction and myosin-adaptor-harmonin complexes anchor it to actin. Here the authors present the structures of myosin 7 MF2 domains bound to the harmonin PDZ3c domain and give insights into myosin-adaptor-harmonin complex assembly.

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.

Over-expression of myosin7A in cochlear hair cells of circling mice

Circling mouse (C57BL/6J-cir/cir) deleted the transmembrane inner ear (Tmie) gene is an animal model for human non-syndromic recessive deafness, DFNB6. In circling mouse, hair cells in the cochlea have degenerated and hair bundles have become irregularity as time goes on. Tmie protein carries out a function of the mechanoelectrical transduction channel in cochlear hair cells. Myosin7a (MYO7A) protein has key roles in development of the cochlear hair bundles as well as in the function of cochlear hair cells. To find whether Tmie protein interacts with MYO7A proteins in the cochlea postnatal developmental stage, we investigated expression of the MYO7A proteins in the cochlear hair cells of circling mice by western blot analysis and whole mount immunofluorescence at postnatal day 5 (P5). The expression of MYO7A showed statistically significant increase in the cochlea of C57BL/6J-+/cir and C57BL/6J-cir/cir mice than that of C57BL/6J-+/+ mice. The MYO7A intensity of the cochlear hair cells also increased in C57BL/6J-+/cir and C57BL/6J-cir/cir mice compared with those of C57BL/6J-+/+ mice. Taken together, the results indicate that Tmie protein may have an important role with MYO7A protein in the development and maintenance of the stereociliary bundles during postnatal developmental stage of the cochlea.

Ca2+-Induced Rigidity Change of the Myosin VIIa IQ Motif-Single α Helix Lever Arm Extension

Several unconventional myosins contain a highly charged single α helix (SAH) immediately following the calmodulin (CaM) binding IQ motifs, functioning to extend lever arms of these myosins. How such SAH is connected to the IQ motifs and whether the conformation of the IQ motifs-SAH segments are regulated by Ca²⁺ fluctuations are not known. Here, we demonstrate by solving its crystal structure that the predicted SAH of myosin VIIa (Myo7a) forms a stable SAH. The structure of Myo7a IQ5-SAH segment in complex with apo-CaM reveals that the SAH sequence can extend the length of the Myo7a lever arm. Although Ca²⁺-CaM remains bound to IQ5-SAH, the Ca²⁺-induced CaM binding mode change softens the conformation of the IQ5-SAH junction, revealing a Ca²⁺-induced lever arm flexibility change for Myo7a. We further demonstrate that the last IQ motif of several other myosins also binds to both apo- and Ca²⁺-CaM, suggesting a common Ca²⁺-induced conformational regulation mechanism.

Gene therapy restores auditory and vestibular function in a mouse model of Usher syndrome type 1c

Because there are currently no biological treatments for deafness, we sought to advance gene therapy approaches to treat genetic deafness. We reasoned that gene delivery systems that target auditory and vestibular sensory cells with high efficiency would be required to restore complex auditory and balance function. We focused on Usher Syndrome, a devastating genetic disorder that causes blindness, balance disorders and profound deafness, and used a knock-in mouse model, Ush1c c.216G>A, which carries a cryptic splice site mutation found in French-Acadian patients with Usher Syndrome type IC (USH1C). Following delivery of wild-type Ush1c into the inner ears of neonatal Ush1c c.216G>A mice, we find recovery of gene and protein expression, restoration of sensory cell function, rescue of complex auditory function and recovery of hearing and balance behavior to near wild-type levels. The data represent unprecedented recovery of inner ear function and suggest that biological therapies to treat deafness may be suitable for translation to humans with genetic inner ear disorders.

Myosin-7b Promotes Distal Tip Localization of the Intermicrovillar Adhesion Complex

Transporting epithelial cells interact with the luminal environment using a tightly packed array of microvilli known as the brush border. During intestinal epithelial differentiation, microvillar packing and organization are driven by cadherin-dependent adhesion complexes that localize to the distal tips of microvilli, where they drive physical interactions between neighboring protrusions. Although enrichment of the "intermicrovillar adhesion complex" (IMAC) at distal tips is required for proper function, the mechanism driving tip accumulation of these factors remains unclear. Here, we report that the actin-based motor myosin-7b (Myo7b) promotes the accumulation of IMAC components at microvillar tips. Myo7b is highly enriched at the tips of microvilli in both kidney and intestinal brush borders, and loss of Myo7b in differentiating intestinal epithelial cells disrupts intermicrovillar adhesion and, thus, brush border assembly. Analysis of cells lacking Myo7b revealed that IMAC components and the resulting intermicrovillar adhesion links are mislocalized along the microvillar axis rather than enriched at the distal tips. We also found that Myo7b motor domains are capable of supporting tip-directed transport. However, motor activity is supplemented by other passive targeting mechanisms that together drive highly efficient IMAC accumulation at the tips. These findings illuminate the molecular basis of IMAC enrichment at microvillar tips and hold important implications for understanding apical morphogenesis in transporting and sensory epithelial tissues.

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.

Cytoskeleton Molecular Motors: Structures and Their Functions in Neuron

Cells make use of molecular motors to transport small molecules, macromolecules and cellular organelles to target region to execute biological functions, which is utmost important for polarized cells, such as neurons. In particular, cytoskeleton motors play fundamental roles in neuron polarization, extension, shape and neurotransmission. Cytoskeleton motors comprise of myosin, kinesin and cytoplasmic dynein. F-actin filaments act as myosin track, while kinesin and cytoplasmic dynein move on microtubules. Cytoskeleton motors work together to build a highly polarized and regulated system in neuronal cells via different molecular mechanisms and functional regulations. This review discusses the structures and working mechanisms of the cytoskeleton motors in neurons.

Cadherin 23-C Regulates Microtubule Networks by Modifying CAMSAP3's Function

Cadherin-related 23 (CDH23) is an adhesive protein important for hearing and vision, while CAMSAP3/Marshalin is a microtubule (MT) minus-end binding protein that regulates MT networks. Although both CDH23 and CAMSAP3/Marshalin are expressed in the organ of Corti, and carry several protein-protein interaction domains, no functional connection between these two proteins has been proposed. In this report, we demonstrate that the C isoform of CDH23 (CDH23-C) directly binds to CAMSAP3/Marshalin and modifies its function by inhibiting CAMSAP3/Marshalin-induced bundle formation, a process that requires a tubulin-binding domain called CKK. We further identified a conserved N-terminal region of CDH23-C that binds to the CKK domain. This CKK binding motif (CBM) is adjacent to the domain that interacts with harmonin, a binding partner of CDH23 implicated in deafness. Because the human Usher Syndrome 1D-associated mutation, CDH23 R3175H, maps to the CBM, we created a matched mutation in mouse CDH23-C at R55H. Both in vivo and in vitro assays decreased the ability of CDH23-C to interact with CAMSAP3/Marshalin, indicating that the interaction between CDH23 and CAMSAP3/Marshalin plays a vital role in hearing and vision. Together, our data suggest that CDH23-C is a CAMSAP3/Marshalin-binding protein that can modify MT networks indirectly through its interaction with CAMSAP3/Marshalin.

Distinct roles of Eps8 in the maturation of cochlear and vestibular hair cells

Several genetic mutations affecting the development and function of mammalian hair cells have been shown to cause deafness but not vestibular defects, most likely because vestibular deficits are sometimes centrally compensated. The study of hair cell physiology is thus a powerful direct approach to ascertain the functional status of the vestibular end organs. Deletion of Epidermal growth factor receptor pathway substrate 8 (Eps8), a gene involved in actin remodeling, has been shown to cause deafness in mice. While both inner and outer hair cells from Eps8 knockout (KO) mice showed abnormally short stereocilia, inner hair cells (IHCs) also failed to acquire mature-type ion channels. Despite the fact that Eps8 is also expressed in vestibular hair cells, Eps8 KO mice show no vestibular deficits. In the present study we have investigated the properties of vestibular Type I and Type II hair cells in Eps8-KO mice and compared them to those of cochlear IHCs. In the absence of Eps8, vestibular hair cells show normally long kinocilia, significantly shorter stereocilia and a normal pattern of basolateral voltage-dependent ion channels. We have also found that while vestibular hair cells from Eps8 KO mice show normal voltage responses to injected sinusoidal currents, which were used to mimic the mechanoelectrical transducer current, IHCs lose their ability to synchronize their responses to the stimulus. We conclude that the absence of Eps8 produces a weaker phenotype in vestibular hair cells compared to cochlear IHCs, since it affects the hair bundle morphology but not the basolateral membrane currents. This difference is likely to explain the absence of obvious vestibular dysfunction in Eps8 KO mice.

Kinetic Adaptations of Myosins for Their Diverse Cellular Functions

Members of the myosin superfamily are involved in all aspects of eukaryotic life. Their function ranges from the transport of organelles and cargos to the generation of membrane tension, and the contraction of muscle. The diversity of physiological functions is remarkable, given that all enzymatically active myosins follow a conserved mechanoenzymatic cycle in which the hydrolysis of ATP to ADP and inorganic phosphate is coupled to either actin-based transport or tethering of actin to defined cellular compartments. Kinetic capacities and limitations of a myosin are determined by the extent to which actin can accelerate the hydrolysis of ATP and the release of the hydrolysis products and are indispensably linked to its physiological tasks. This review focuses on kinetic competencies that - together with structural adaptations - result in myosins with unique mechanoenzymatic properties targeted to their diverse cellular functions.

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.

Class III myosins shape the auditory hair bundles by limiting microvilli and stereocilia growth

Analysis of mice deficient for myosin IIIa and myosin IIIb shows that class III myosins limit the elongation of stereocilia and of subsequently regressing microvilli, thus contributing to the early hair bundle shaping.

The precise architecture of hair bundles, the arrays of mechanosensitive microvilli-like stereocilia crowning the auditory hair cells, is essential to hearing. Myosin IIIa, defective in the late-onset deafness form DFNB30, has been proposed to transport espin-1 to the tips of stereocilia, thereby promoting their elongation. We show that Myo3a^−/−Myo3b^−/− mice lacking myosin IIIa and myosin IIIb are profoundly deaf, whereas Myo3a-cKO Myo3b^−/− mice lacking myosin IIIb and losing myosin IIIa postnatally have normal hearing. Myo3a^−/−Myo3b^−/− cochlear hair bundles display robust mechanoelectrical transduction currents with normal kinetics but show severe embryonic abnormalities whose features rapidly change. These include abnormally tall and numerous microvilli or stereocilia, ungraded stereocilia bundles, and bundle rounding and closure. Surprisingly, espin-1 is properly targeted to Myo3a^−/−Myo3b^−/− stereocilia tips. Our results uncover the critical role that class III myosins play redundantly in hair-bundle morphogenesis; they unexpectedly limit the elongation of stereocilia and of subsequently regressing microvilli, thus contributing to the early hair bundle shaping.

Adhesion GPCR-Related Protein Networks

Adhesion G protein-coupled receptors (aGPCRs/ADGRs) are unique receptors that combine cell adhesion and signaling functions. Protein networks related to ADGRs exert diverse functions, e.g., in tissue polarity, cell migration, nerve cell function, or immune response, and are regulated via different mechanisms. The large extracellular domain of ADGRs is capable of mediating cell-cell or cell-matrix protein interactions. Their intracellular surface and domains are coupled to downstream signaling pathways and often bind to scaffold proteins, organizing membrane-associated protein complexes. The cohesive interplay between ADGR-related network components is essential to prevent severe disease-causing damage in numerous cell types. Consequently, in recent years, attention has focused on the decipherment of the precise molecular composition of ADGR protein complexes and interactomes in various cellular modules. In this chapter, we discuss the affiliation of ADGR networks to cellular modules and how they can be regulated, pinpointing common features in the networks related to the diverse ADGRs. Detailed decipherment of the composition of protein networks should provide novel targets for the development of novel therapies with the aim to cure human diseases related to ADGRs.

It takes two

Two forms of an unconventional myosin motor protein have separate functions in the growth and maintenance of hair bundles in auditory hair cells.

Zebrafish Models for the Mechanosensory Hair Cell Dysfunction in Usher Syndrome 3 Reveal That Clarin-1 Is an Essential Hair Bundle Protein

Usher syndrome type III (USH3) is characterized by progressive loss of hearing and vision, and varying degrees of vestibular dysfunction. It is caused by mutations that affect the human clarin-1 protein (hCLRN1), a member of the tetraspanin protein family. The missense mutation CLRN1(N48K), which affects a conserved N-glycosylation site in hCLRN1, is a common causative USH3 mutation among Ashkenazi Jews. The affected individuals hear at birth but lose that function over time. Here, we developed an animal model system using zebrafish transgenesis and gene targeting to provide an explanation for this phenotype. Immunolabeling demonstrated that Clrn1 localized to the hair cell bundles (hair bundles). The clrn1 mutants generated by zinc finger nucleases displayed aberrant hair bundle morphology with diminished function. Two transgenic zebrafish that express either hCLRN1 or hCLRN1(N48K) in hair cells were produced to examine the subcellular localization patterns of wild-type and mutant human proteins. hCLRN1 localized to the hair bundles similarly to zebrafish Clrn1; in contrast, hCLRN1(N48K) largely mislocalized to the cell body with a small amount reaching the hair bundle. We propose that this small amount of hCLRN1(N48K) in the hair bundle provides clarin-1-mediated function during the early stages of life; however, the presence of hCLRN1(N48K) in the hair bundle diminishes over time because of intracellular degradation of the mutant protein, leading to progressive loss of hair bundle integrity and hair cell function. These findings and genetic tools provide an understanding and path forward to identify therapies to mitigate hearing loss linked to the CLRN1 mutation.

SIGNIFICANCE STATEMENT

Mutations in the clarin-1 gene affect eye and ear function in humans. Individuals with the CLRN1(N48K) mutation are born able to hear but lose that function over time. Here, we develop an animal model system using zebrafish transgenesis and gene targeting to provide an explanation for this phenotype. This approach illuminates the role of clarin-1 and the molecular mechanism linked to the CLRN1(N48K) mutation in sensory hair cells of the inner ear. Additionally, the investigation provided an in vivo model to guide future drug discovery to rescue the hCLRN1(N48K) in hair cells.

Individual USH2 proteins make distinct contributions to the ankle link complex during development of the mouse cochlear stereociliary bundle

Usher syndrome (USH) is the leading cause of inherited deaf-blindness, with type 2 (USH2) being the most common clinical form. Studies suggest that proteins encoded by USH2 causative genes assemble into the ankle link complex (ALC) at the hair cell stereociliary bundle; however, little is known about the in vivo assembly and function of this complex. Using various USH2 mutant mice, we showed by immunofluorescence that USH2 proteins play different roles in cochlear ALC assembly, with G protein-coupled receptor 98 being the most important protein. Complex assembly likely occurs at the stereociliary bundle but not along the protein transport route in the cell body. Stereociliary morphological defects in USH2 mutant mice suggest roles for the ALC in regulating inner hair cell stereociliary growth and differentiation as well as outer hair cell stereociliary rigidity and organization during development. These roles are unique from the bundle cohesion role of Usher syndrome type 1 protein complexes. Loss of individual USH2 gene expressions leads to variable morphological and functional consequences, correlating with the severity of ALC disruption. This finding suggests a potential genotype-phenotype correlation in USH2 patients. In summary, this study provides novel insights into the molecular mechanism underlying cochlear stereociliary bundle development and hearing loss pathogenesis of various USH2 subtypes. Our thorough phenotypical characterization of USH2 mouse models is essential for future use of these animal models in therapeutic development.

Developmental regulation of planar cell polarity and hair-bundle morphogenesis in auditory hair cells: lessons from human and mouse genetics

Hearing loss is the most common and costly sensory defect in humans and genetic causes underlie a significant proportion of affected individuals. In mammals, sound is detected by hair cells (HCs) housed in the cochlea of the inner ear, whose function depends on a highly specialized mechanotransduction organelle, the hair bundle. Understanding the factors that regulate the development and functional maturation of the hair bundle is crucial for understanding the pathophysiology of human deafness. Genetic analysis of deafness genes in animal models, together with complementary forward genetic screens and conditional knock-out mutations in essential genes, have provided great insights into the molecular machinery underpinning hair-bundle development and function. In this review, we highlight recent advances in our understanding of hair-bundle morphogenesis, with an emphasis on the molecular pathways governing hair-bundle polarity and orientation. We next discuss the proteins and structural elements important for hair-cell mechanotransduction as well as hair-bundle cohesion and maintenance. In addition, developmental signals thought to regulate tonotopic features of HCs are introduced. Finally, novel approaches that complement classic genetics for studying the molecular etiology of human deafness are presented. WIREs Dev Biol 2016, 5:85-101. doi: 10.1002/wdev.202 For further resources related to this article, please visit the WIREs website.

Structure and Regulation of the Movement of Human Myosin VIIA

Human myosin VIIA (HM7A) is responsible for human Usher syndrome type 1B, which causes hearing and visual loss in humans. Here we studied the regulation of HM7A. The actin-activated ATPase activity of full-length HM7A (HM7AFull) was lower than that of tail-truncated HM7A (HM7AΔTail). Deletion of the C-terminal 40 amino acids and mutation of the basic residues in this region (R2176A or K2179A) abolished the inhibition. Electron microscopy revealed that HM7AFull is a monomer in which the tail domain bends back toward the head-neck domain to form a compact structure. This compact structure is extended at high ionic strength or in the presence of Ca(2+). Although myosin VIIA has five isoleucine-glutamine (IQ) motifs, the neck length seems to be shorter than the expected length of five bound calmodulins. Supporting this observation, the IQ domain bound only three calmodulins in Ca(2+), and the first IQ motif failed to bind calmodulin in EGTA. These results suggest that the unique IQ domain of HM7A is important for the tail-neck interaction and, therefore, regulation. Cellular studies revealed that dimer formation of HM7A is critical for its translocation to filopodial tips and that the tail domain (HM7ATail) markedly reduced the filopodial tip localization of the HM7AΔTail dimer, suggesting that the tail-inhibition mechanism is operating in vivo. The translocation of the HM7AFull dimer was significantly less than that of the HM7AΔTail dimer, and R2176A/R2179A mutation rescued the filopodial tip translocation. These results suggest that HM7A can transport its cargo molecules, such as USH1 proteins, upon release of the tail-dependent inhibition.

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.

Clarin-1 acts as a modulator of mechanotransduction activity and presynaptic ribbon assembly

Clarin-1 is a four-transmembrane protein expressed by hair cells and photoreceptors. Mutations in its corresponding gene are associated with Usher syndrome type 3, characterized by late-onset and progressive hearing and vision loss in humans. Mice carrying mutations in the clarin-1 gene have hair bundle dysmorphology and a delay in synapse maturation. In this paper, we examined the expression and function of clarin-1 in zebrafish hair cells. We observed protein expression as early as 1 d postfertilization. Knockdown of clarin-1 resulted in inhibition of FM1-43 incorporation, shortening of the kinocilia, and mislocalization of ribeye b clusters. These phenotypes were fully prevented by co-injection with clarin-1 transcript, requiring its C-terminal tail. We also observed an in vivo interaction between clarin-1 and Pcdh15a. Altogether, our results suggest that clarin-1 is functionally important for mechanotransduction channel activity and for proper localization of synaptic components, establishing a critical role for clarin-1 at the apical and basal poles of hair cells.

Reaction-diffusion model of hair-bundle morphogenesis

The hair bundle, an apical specialization of the hair cell composed of several rows of regularly organized stereocilia and a kinocilium, is essential for mechanotransduction in the ear. Its precise organization allows the hair bundle to convert mechanical stimuli to electrical signals; mutations that alter the bundle's morphology often cause deafness. However, little is known about the proteins involved in the process of morphogenesis and how the structure of the bundle arises through interactions between these molecules. We present a mathematical model based on simple reaction-diffusion mechanisms that can reproduce the shape and organization of the hair bundle. This model suggests that the boundary of the cell and the kinocilium act as signaling centers that establish the bundle's shape. The interaction of two proteins forms a hexagonal Turing pattern--a periodic modulation of the concentrations of the morphogens, sustained by local activation and long-range inhibition of the reactants--that sets a blueprint for the location of the stereocilia. Finally we use this model to predict how different alterations to the system might impact the shape and organization of the hair bundle.

Cargo recognition and cargo-mediated regulation of unconventional myosins

Organized motions are hallmarks of living organisms. Such motions range from collective cell movements during development and muscle contractions at the macroscopic scale all the way down to cellular cargo (e.g., various biomolecules and organelles) transportation and mechanoforce sensing at more microscopic scales. Energy required for these biological motions is almost invariably provided by cellular chemical fuels in the form of nucleotide triphosphate. Biological systems have designed a group of nanoscale engines, known as molecular motors, to convert cellular chemical fuels into mechanical energy. Molecular motors come in various forms including cytoskeleton motors (myosin, kinesin, and dynein), nucleic-acid-based motors, cellular membrane-based rotary motors, and so on. The main focus of this Account is one subfamily of actin filament-based motors called unconventional myosins (other than muscle myosin II, the remaining myosins are collectively referred to as unconventional myosins). In general, myosins can use ATP to fuel two types of mechanomotions: dynamic tethering actin filaments with various cellular compartments or structures and actin filament-based intracellular transport. In contrast to rich knowledge accumulated over many decades on ATP hydrolyzing motor heads and their interactions with actin filaments, how various myosins recognize their specific cargoes and whether and how cargoes can in return regulate functions of motors are less understood. Nonetheless, a series of biochemical and structural investigations in the past few years, including works from our own laboratory, begin to shed lights on these latter questions. Some myosins (e.g., myosin-VI) can function both as cellular transporters and as mechanical tethers. To function as a processive transporter, myosins need to form dimers or multimers. To be a mechanical tether, a monomeric myosin is sufficient. It has been shown for myosin-VI that its cellular cargo proteins can play critical roles in determining the motor properties. Dab2, an adaptor protein linking endocytic vesicles with actin-filament-bound myosin-VI, can induce the motor to form a transport competent dimer. Such a cargo-mediated dimerization mechanism has also been observed in other myosins including myosin-V and myosin-VIIa. The tail domains of myosins are very diverse both in their lengths and protein domain compositions and thus enable motors to engage a broad range of different cellular cargoes. Remarkably, the cargo binding tail of one myosin alone often can bind to multiple distinct target proteins. A series of atomic structures of myosin-V/cargo complexes solved recently reveals that the globular cargo binding tail of the motor contains a number of nonoverlapping target recognition sites for binding to its cargoes including melanophilin, vesicle adaptors RILPL2, and vesicle-bound GTPase Rab11. The structures of the MyTH4-FERM tandems from myosin-VIIa and myosin-X in complex with their respective targets reveal that MyTH4 and FERM domains extensively interact with each other forming structural and functional supramodules in both motors and demonstrate that the structurally similar MyTH4-FERM tandems of the two motors display totally different target binding modes. These structural studies have also shed light on why numerous mutations found in these myosins can cause devastating human diseases such as deafness and blindness, intellectual disabilities, immune disorders, and diabetes.

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.

Myosin VIIA regulates microvillus morphogenesis and interacts with cadherin Cad99C in Drosophila oogenesis

Microvilli and related actin-based protrusions permit multiple interactions between cells and their environment. How the shape, length and arrangement of microvilli are determined remains largely unclear. To address this issue and explore the cooperation of the two main components of a microvillus, the central F-actin bundle and the enveloping plasma membrane, we investigated the expression and function of Myosin VIIA (Myo7A), which is encoded by crinkled (ck), and its interaction with cadherin Cad99C in the microvilli of the Drosophila follicular epithelium. Myo7A is present in the microvilli and terminal web of follicle cells, and associates with several other F-actin-rich structures in the ovary. Loss of Myo7A caused brush border defects and a reduction in the amount of the microvillus regulator Cad99C. We show that Myo7A and Cad99C form a molecular complex and that the cytoplasmic tail of Cad99C recruits Myo7A to microvilli. Our data indicate that Myo7A regulates the structure and spacing of microvilli, and interacts with Cad99C in vivo. A comparison of the mutant phenotypes suggests that Myo7A and Cad99C have co-dependent and independent functions in microvilli.

The physiology of mechanoelectrical transduction channels in hearing

Much is known about the mechanotransducer (MT) channels mediating transduction in hair cells of the vertrbrate inner ear. With the use of isolated preparations, it is experimentally feasible to deliver precise mechanical stimuli to individual cells and record the ensuing transducer currents. This approach has shown that small (1-100 nm) deflections of the hair-cell stereociliary bundle are transmitted via interciliary tip links to open MT channels at the tops of the stereocilia. These channels are cation-permeable with a high selectivity for Ca(2+); two channels are thought to be localized at the lower end of the tip link, each with a large single-channel conductance that increases from the low- to high-frequency end of the cochlea. Ca(2+) influx through open channels regulates their resting open probability, which may contribute to setting the hair cell resting potential in vivo. Ca(2+) also controls transducer fast adaptation and force generation by the hair bundle, the two coupled processes increasing in speed from cochlear apex to base. The molecular intricacy of the stereocilary bundle and the transduction apparatus is reflected by the large number of single-gene mutations that are linked to sensorineural deafness, especially those in Usher syndrome. Studies of such mutants have led to the discovery of many of the molecules of the transduction complex, including the tip link and its attachments to the stereociliary core. However, the MT channel protein is still not firmly identified, nor is it known whether the channel is activated by force delivered through accessory proteins or by deformation of the lipid bilayer.

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.

Complexes of Usher proteins preassemble at the endoplasmic reticulum and are required for trafficking and ER homeostasis

Usher syndrome (USH), the leading cause of hereditary combined hearing and vision loss, is characterized by sensorineural deafness and progressive retinal degeneration. Mutations in several different genes produce USH, but the proximal cause of sensory cell death remains mysterious. We adapted a proximity ligation assay to analyze associations among three of the USH proteins, Cdh23, Harmonin and Myo7aa, and the microtubule-based transporter Ift88 in zebrafish inner ear mechanosensory hair cells. We found that the proteins are in close enough proximity to form complexes and that these complexes preassemble at the endoplasmic reticulum (ER). Defects in any one of the three USH proteins disrupt formation and trafficking of the complex and result in diminished levels of the other proteins, generalized trafficking defects and ER stress that triggers apoptosis. ER stress, thus, contributes to sensory hair cell loss and provides a new target to explore for protective therapies for USH.

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.

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.