A novel stereocilia defect in sensory hair cells of the deaf mouse mutant Tasmanian devil

Grxcr1 regulates hair bundle morphogenesis and is required for normal mechanoelectrical transduction in mouse cochlear hair cells

Tasmanian devil (tde) mice are deaf and exhibit circling behaviour. Sensory hair cells of mutants show disorganised hair bundles with abnormally thin stereocilia. The origin of this mutation is the insertion of a transgene which disrupts expression of the Grxcr1 (glutaredoxin cysteine rich 1) gene. We report here that Grxcr1 exons and transcript sequences are not affected by the transgene insertion in tde homozygous (tde/tde) mice. Furthermore, 5'RACE PCR experiments showed the presence of two different transcripts of the Grxcr1 gene, expressed in both tde/tde and in wild-type controls. However, quantitative analysis of Grxcr1 transcripts revealed a significantly decreased mRNA level in tde/tde mice. The key stereociliary proteins ESPN, MYO7A, EPS8 and PTPRQ were distributed in hair bundles of homozygous tde mutants in a similar pattern compared with control mice. We found that the abnormal morphology of the stereociliary bundle was associated with a reduction in the size and Ca2+-sensitivity of the mechanoelectrical transducer (MET) current. We propose that GRXCR1 is key for the normal growth of the stereociliary bundle prior to the onset of hearing, and in its absence hair cells are unable to mature into fully functional sensory receptors.

Murine GRXCR1 Has a Different Function Than GRXCR2 in the Morphogenesis of Stereocilia

Mutations in human glutaredoxin domain-containing cysteine-rich protein 1 (GRXCR1) and its paralog GRXCR2 have been linked to hearing loss in humans. Although both GRXCR1 and GRXCR2 are required for the morphogenesis of stereocilia in cochlear hair cells, a fundamental question that remains unclear is whether GRXCR1 and GRXCR2 have similar functions in hair cells. Previously, we found that GRXCR2 is critical for the stereocilia morphogenesis by regulating taperin localization at the base of stereocilia. Reducing taperin expression level rescues the morphological defects of stereocilia and hearing loss in Grxcr2-deficient mice. So far, functions of GRXCR1 in mammalian hair cells are still unclear. Grxcr1-deficient hair cells have very thin stereocilia with less F-actin content inside, which is different from Grxcr2-deficient hair cells. In contrast to GRXCR2, which is concentrated at the base of stereocilia, GRXCR1 is diffusely distributed throughout the stereocilia. Notably, GRXCR1 interacts with GRXCR2. In Grxcr1-deficient hair cells, the expression level of GRXCR2 and taperin is reduced. Remarkably, different from that in Grxcr2-deficient mice, reducing taperin expression level does not rescue the morphological defects of stereocilia or hearing loss in Grxcr1-deficient mice. Thus, our findings suggest that GRXCR1 has different functions than GRXCR2 during the morphogenesis of stereocilia.

Grxcr1 Promotes Hair Bundle Development by Destabilizing the Physical Interaction between Harmonin and Sans Usher Syndrome Proteins

Morphogenesis and mechanoelectrical transduction of the hair cell mechanoreceptor depend on the correct assembly of Usher syndrome (USH) proteins into highly organized macromolecular complexes. Defects in these proteins lead to deafness and vestibular areflexia in USH patients. Mutations in a non-USH protein, glutaredoxin domain-containing cysteine-rich 1 (GRXCR1), cause non-syndromic sensorineural deafness. To understand the deglutathionylating enzyme function of GRXCR1 in deafness, we generated two grxcr1 zebrafish mutant alleles. We found that hair bundles are thinner in homozygous grxcr1 mutants, similar to the USH1 mutants ush1c (Harmonin) and ush1ga (Sans). In vitro assays showed that glutathionylation promotes the interaction between Ush1c and Ush1ga and that Grxcr1 regulates mechanoreceptor development by preventing physical interaction between these proteins without affecting the assembly of another USH1 protein complex, the Ush1c-Cadherin23-Myosin7aa tripartite complex. By elucidating the molecular mechanism through which Grxcr1 functions, we also identify a mechanism that dynamically regulates the formation of Usher protein complexes.

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.

Absence of plastin 1 causes abnormal maintenance of hair cell stereocilia and a moderate form of hearing loss in mice

Hearing relies on the mechanosensory inner and outer hair cells (OHCs) of the organ of Corti, which convert mechanical deflections of their actin-rich stereociliary bundles into electrochemical signals. Several actin-associated proteins are essential for stereocilia formation and maintenance, and their absence leads to deafness. One of the most abundant actin-bundling proteins of stereocilia is plastin 1, but its function has never been directly assessed. Here, we found that plastin 1 knock-out (Pls1 KO) mice have a moderate and progressive form of hearing loss across all frequencies. Auditory hair cells developed normally in Pls1 KO, but in young adult animals, the stereocilia of inner hair cells were reduced in width and length. The stereocilia of OHCs were comparatively less affected; however, they also showed signs of degeneration in ageing mice. The hair bundle stiffness and the acquisition of the electrophysiological properties of hair cells were unaffected by the absence of plastin 1, except for a significant change in the adaptation properties, but not the size of the mechanoelectrical transducer currents. These results show that in contrast to other actin-bundling proteins such as espin, harmonin or Eps8, plastin 1 is dispensable for the initial formation of stereocilia. However, the progressive hearing loss and morphological defects of hair cells in adult Pls1 KO mice point at a specific role for plastin 1 in the preservation of adult stereocilia and optimal hearing. Hence, mutations in the human PLS1 gene may be associated with relatively mild and progressive forms of hearing loss.

A reduction in Ptprq associated with specific features of the deafness phenotype of the miR-96 mutant mouse diminuendo

miR-96 is a microRNA, a non-coding RNA gene which regulates a wide array of downstream genes. The miR-96 mouse mutant diminuendo exhibits deafness and arrested hair cell functional and morphological differentiation. We have previously shown that several genes are markedly downregulated in the diminuendo organ of Corti; one of these is Ptprq, a gene known to be important for maturation and maintenance of hair cells. In order to study the contribution that downregulation of Ptprq makes to the diminuendo phenotype, we carried out microarrays, scanning electron microscopy and single hair cell electrophysiology to compare diminuendo mutants (heterozygous and homozygous) with mice homozygous for a functional null allele of Ptprq. In terms of both morphology and electrophysiology, the auditory phenotype of mice lacking Ptprq resembles that of diminuendo heterozygotes, while diminuendo homozygotes are more severely affected. A comparison of transcriptomes indicates there is a broad similarity between diminuendo homozygotes and Ptprq-null mice. The reduction in Ptprq observed in diminuendo mice appears to be a major contributor to the morphological, transcriptional and electrophysiological phenotype, but does not account for the complete diminuendo phenotype.

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

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

Homozygosity mapping reveals mutations of GRXCR1 as a cause of autosomal-recessive nonsyndromic hearing impairment

We identified overlapping homozygous regions within the DFNB25 locus in two Dutch and ten Pakistani families with sensorineural autosomal-recessive nonsyndromic hearing impairment (arNSHI). Only one of the families, W98-053, was not consanguineous, and its sibship pointed toward a reduced critical region of 0.9 Mb. This region contained the GRXCR1 gene, and the orthologous mouse gene was described to be mutated in the pirouette (pi) mutant with resulting hearing loss and circling behavior. Sequence analysis of the GRXCR1 gene in hearing-impaired family members revealed splice-site mutations in two Dutch families and a missense and nonsense mutation, respectively, in two Pakistani families. The splice-site mutations are predicted to cause frameshifts and premature stop codons. In family W98-053, this could be confirmed by cDNA analysis. GRXCR1 is predicted to contain a GRX-like domain. GRX domains are involved in reversible S-glutathionylation of proteins and thereby in the modulation of activity and/or localization of these proteins. The missense mutation is located in this domain, whereas the nonsense and splice-site mutations may result in complete or partial absence of the GRX-like domain or of the complete protein. Hearing loss in patients with GRXCR1 mutations is congenital and is moderate to profound. Progression of the hearing loss was observed in family W98-053. Vestibular dysfunction was observed in some but not all affected individuals. Quantitative analysis of GRXCR1 transcripts in fetal and adult human tissues revealed a preferential expression of the gene in fetal cochlea, which may explain the nonsyndromic nature of the hearing impairment.

A hearing and vestibular phenotyping pipeline to identify mouse mutants with hearing impairment

We describe a protocol for the production of mice carrying N-ethyl-N-nitrosourea (ENU) mutations and their screening for auditory and vestibular phenotypes. In comparison with the procedures describing individual phenotyping tests, this protocol integrates a set of tests for the comprehensive determination of the causes of hearing loss. It comprises a primary screen of relatively simple auditory and vestibular tests. A variety of secondary phenotyping protocols are also described for further investigating the deaf and vestibular mutants identified in the primary screen. The screen can be applied to potentially thousands of mutant mice, produced either by ENU or other mutagenesis approaches. Primary screening protocols take no longer than a few minutes, apart from ABR testing which takes upto 3.5 h per mouse. These protocols have been applied for the identification of mouse models of human deafness and are a key component for investigating the genes and genetic pathways involved in hereditary deafness.

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

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

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

A locus for autosomal dominant progressive non-syndromic hearing loss, DFNA27, is on chromosome 4q12-13.1

We ascertained a large North American family, LMG2, segregating progressive, non-syndromic, sensorineural hearing loss. A genome-wide scan identified significant evidence for linkage (maximum logarithm of the odds (LOD) score = 4.67 at theta = 0 for D4S398) to markers in a 5.7-cM interval on chromosome 4q12-13.1. The DFNA27 interval spans 8.85 Mb and includes at least 61 predicted and 8 known genes. We sequenced eight genes and excluded them as candidates for the DFNA27 gene.

Hair cell development: commitment through differentiation

The perceptions of sound, balance and acceleration are mediated through the vibration of stereociliary bundles located on the lumenal surfaces of mechanosensory hair cells located within the inner ear. In mammals, virtually all hair cells are generated during a relatively brief period in embryogenesis with any subsequent hair cell loss leading to a progressive and permanent loss of sensitivity. In light of the importance of these cells, considerable effort has been focused on understanding the molecular genetic pathways that regulate their development. The results of these studies have begun to elucidate the signaling molecules that regulate several key events in hair cell development. In particular, significant progress has been made in the understanding of hair cell commitment, survival and differentiation. In addition, several aspects of the development of the stereociliary bundle, including its elongation and orientation, have recently been examined. This review will summarize results from each of these developmental events and describe the molecular signaling pathways involved.

A Myo7a mutation cosegregates with stereocilia defects and low-frequency hearing impairment

A phenotype-driven approach was adopted in the mouse to identify molecules involved in ear development and function. Mutant mice were obtained using N-ethyl- N-nitrosourea (ENU) mutagenesis and were screened for dominant mutations that affect hearing and/or balance. Heterozygote headbanger ( Hdb/+) mutants display classic behavior indicative of vestibular dysfunction including hyperactivity and head bobbing, and they show a Preyer reflex in response to sound but have raised cochlear thresholds especially at low frequencies. Scanning electron microscopy of the surface of the organ of Corti revealed abnormal stereocilia bundle development from an early age that was more severe in the apex than the base. Utricular stereocilia were long, thin, and wispy. Homozygotes showed a similar but more severe phenotype. The headbanger mutation has been mapped to a 1.5-cM region on mouse Chromosome 7 in the region of the unconventional myosin gene Myo7a, and mutation screening revealed an A>T transversion that is predicted to cause an isoleucine-to-phenylalanine amino acid substitution (I178F) in a conserved region in the motor-encoding domain of the gene. Protein analysis revealed reduced levels of myosin VIIa expression in inner ears of headbanger mice. Headbanger represents a novel inner ear phenotype and provides a potential model for low-frequency-type human hearing loss.

Cochlear development: hair cells don their wigs and get wired

PURPOSE OF REVIEW

Hair cells and spiral ganglion neurons form functional pairings in the cochlea that transduce the mechanical energy of sound into signals that are carried to the brainstem. Mutations of genes affecting the development and maintenance of these two cell populations cause deafness in humans and other animals. This review highlights recent findings regarding the development of hair cell stereocilia and spiral ganglion neurons in the cochlea.

RECENT FINDINGS

Genes underlying Usher syndrome 1A have shed light on possible molecular participants in the development and structure of the hair cell stereocilia. Analysis of deaf mouse mutants have uncovered genes involved in stereocilia elongation and the orientation of the stereociliary bundles. Studies on the regulation of spiral ganglion neuronal survival and guidance suggest that the timing of expression of specific growth factors along the cochlear spiral is involved in maintaining the divergence of vestibular and cochlear nerve fibers.

SUMMARY

Examining human and mouse genes affecting hearing has not only provided insight into causes of human deafness, but has also opened a window into how stereociliary bundles are constructed and spiral ganglion neurons are preserved and guided during development. Synthesis of information from diverse lines of research pinpoints genes for screening or repair in the genetic medicine of the future and dramatizes the intimate relationship between strict adherence to complex developmental programs and hearing. In addition, future improvements in the efficacy of cochlear implants may depend on the preservation and manipulation of adult spiral ganglion neurons. Developmental mechanisms promise to yield insight into possible interventions to redirect or reconnect spiral ganglion neurons in damaged cochlea.

Radixin deficiency causes deafness associated with progressive degeneration of cochlear stereocilia

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

Mouse models for deafness: lessons for the human inner ear and hearing loss

In the field of hearing research, recent advances using the mouse as a model for human hearing loss have brought exciting insights into the molecular pathways that lead to normal hearing, and into the mechanisms that are disrupted once a mutation occurs in one of the critical genes. Inaccessible for most procedures other than high-resolution computed tomography (CT) scanning or invasive surgery, most studies on the ear in humans can only be performed postmortem. A major goal in hearing research is to gain a full understanding of how a sound is heard at the molecular level, so that diagnostic and eventually therapeutic interventions can be developed that can treat the diseased inner ear before permanent damage has occurred, such as hair cell loss. The mouse, with its advantages of short gestation time, ease of selective matings, and similarity of the genome and inner ear to humans, is truly a remarkable resource for attaining this goal and investigating the intrigues of the human ear.