Mapping of genetic modifiers of Eya1 bor/bor in CAST/EiJ and BALB/cJ that suppress cochlear aplasia and associated deafness

Mice homozygous for the hypomorphic allele Eya1 ( bor ) exhibit cochlear aplasia, with associated deafness, and renal hypoplasia, similar to Branchio-Oto-Renal syndrome (BOR) in humans. Although much is known about the genetics of the disease, little is known about the factors that modify its phenotypic expression. We have recently detailed two modifier loci (Mead1 and Mead2) in a C3HeB/FeJ-Eya1 ( bor/+ ) x C57BL/6 J intercross that suppress the ear-related phenotypes in our hypomorphic mutants. In this study we report corroborating evidence for our initial finding with the identification of two modifier loci mapping to the same region in CAST/EiJ and BALB/cJ. Furthermore, we describe an additional locus (Mead3) on chromosome 19 in CAST/EiJ, within which the previously cloned suppressor Nxf1 resides. The suppression effect on cochlear coiling was studied on congenic line(s) for each protective allele. The penetrance and suppressor strength of these alleles vary by strain and locus. Eya1 ( bor/bor ) hypomorphs, when homozygous for each of the three protective alleles (CAST/EiJ, C57BL/6 J, or BALB/cJ) at the Mead1 or Mead2 locus, exhibit completely penetrant suppression of cochlear agenesis. At the Mead1 locus, the C57BL/6 J and BALB/cJ alleles have comparable strengths. At the Mead2 locus, the C57BL/6 J and CAST/EiJ alleles have comparable strengths. In contrast, mice with genotype Eya1 ( bor/bor )Mead3(CAST/CAST) exhibit incomplete penetrance (50%) and a wide range of cochlear coiling (1/4-1(1/2) turns). The identification of these additional modifier alleles could provide crucial clues for evaluating the candidate genes.     

Eya1 acts upstream of Tbx1, Neurogenin 1, NeuroD and the neurotrophins BDNF and NT-3 during inner ear development

Cell fate specification during inner ear development is dependent upon regional gene expression within the otic vesicle. One of the earliest cell fate determination steps in this system is the specification of neural precursors, and regulators of this process include the Atonal-related basic helix-loop-helix genes, Ngn1 and NeuroD and the T-box gene, Tbx1. In this study we demonstrate that Eya1 signaling is critical to the normal expression patterns of Tbx1, Ngn1, and NeuroD in the developing mouse otocyst. We discuss a potential mechanism for the absence of neural precursors in the Eya1-/- inner ears and the primary and secondary mechanisms for the loss of cochleovestibular ganglion cells in the Eya1bor/bor hypomorphic mutant.     

Altered gating properties of functional Cx26 mutants associated with recessive non-syndromic hearing loss

Connexins (Cx) form gap junctions that allow the exchange of small metabolites and ions. In the inner ear, Cx26 is the major gap junction protein and mutations in the Cx26-encoding gene, GJB2, are the most frequent cause of autosomal recessive non-syndromic hearing loss (DFNB1). We have functionally analyzed five Cx26 mutations associated with DFNB1, comprising the following single amino-acid substitutions: T8M, R143W, V153I, N206S and L214P. Coupling of cells expressing wild-type or mutant Cx26 was measured in the paired Xenopus oocyte assay. We found that the R143W, V153I and L214P mutations were unable to form functional channels. In contrast, the T8M and N206S mutants did electrically couple cells, though their voltage gating properties were different from wild-type Cx26 channels. The electrical coupling of oocytes expressing the T8M and N206S mutants suggest that these channels may retain high permeability to potassium ions. Therefore, deafness associated with Cx26 mutations may not only depend on reduced potassium re-circulation in the inner ear. Instead, abnormalities in the exchange of other metabolites through the cochlear gap junction network may also produce deafness.     

bor1-1, an Arabidopsis thaliana mutant that requires a high level of boron.

bor1-1 (high boron requiring), an Arabidopsis thaliana mutant that requires a high level of B, was isolated. When the B concentration in the medium was reduced to 3 microM, the expansion of rosette leaves was severely affected in bor1-1 but not in wild-type plants. In a medium containing 30 microM B the mutant grew normally but showed female sterility, whereas the wild type was able to set seeds. These defects of the bor1-1 mutant were not detected with supplementation of 100 microM B. In vivo concentrations of B in bor1-1 mutants were lower than those of the wild type, especially in the inflorescence stems. Tracer experiments using 10B suggested that the mutant has defects in uptake and/or translocation of B. The mutation was mapped on the lower arm of chromosome 2.     

Redundant functions of Rac GTPases in inner ear morphogenesis

Development of the mammalian inner ear requires coordination of cell proliferation, cell fate determination and morphogenetic movements. While significant progress has been made in identifying developmental signals required for inner ear formation, less is known about how distinct signals are coordinated by their downstream mediators. Members of the Rac family of small GTPases are known regulators of cytoskeletal remodeling and numerous other cellular processes. However, the function of Rac GTPases in otic development is largely unexplored. Here, we show that Rac1 and Rac3 redundantly regulate many aspects of inner ear morphogenesis. While no morphological defects were observed in Rac3(-/-) mice, Rac1(CKO); Rac3(-/-) double mutants displayed enhanced vestibular and cochlear malformations compared to Rac1(CKO) single mutants. Moreover, in Rac1(CKO); Rac3(-/-) mutants, we observed compromised E-cadherin-mediated cell adhesion, reduced cell proliferation and increased cell death in the early developing otocyst, leading to a decreased size and malformation of the membranous labyrinth. Finally, cochlear extension was severely disrupted in Rac1(CKO); Rac3(-/-) mutants, accompanied by a loss of epithelial cohesion and formation of ectopic sensory patches underneath the cochlear duct. The compartmentalized expression of otic patterning genes within the Rac1(CKO); Rac3(-/-) mutant otocyst was largely normal, however, indicating that Rac proteins regulate inner ear morphogenesis without affecting cell fate specification. Taken together, our results reveal an essential role for Rac GTPases in coordinating cell adhesion, cell proliferation, cell death and cell movements during otic development.     

The zebrafish dog-eared mutation disrupts eya1, a gene required for cell survival and differentiation in the inner ear and lateral line

To understand the molecular basis of sensory organ development and disease, we have cloned and characterized the zebrafish mutation dog-eared (dog) that is defective in formation of the inner ear and lateral line sensory systems. The dog locus encodes the eyes absent-1 (eya1) gene and single point mutations were found in three independent dog alleles, each prematurely truncating the expressed protein within the Eya domain. Moreover, morpholino-mediated knockdown of eya1 gene function phenocopies the dog-eared mutation. In zebrafish, the eya1 gene is widely expressed in placode-derived sensory organs during embryogenesis but Eya1 function appears to be primarily required for survival of sensory hair cells in the developing ear and lateral line neuromasts. Increased levels of apoptosis occur in the migrating primordia of the posterior lateral line in dog embryos and as well as in regions of the developing otocyst that are mainly fated to give rise to sensory cells of the cristae. Importantly, mutation of the EYA1 or EYA4 gene causes hereditary syndromic deafness in humans. Determination of eya gene function during zebrafish organogenesis will facilitate understanding the molecular etiology of human vestibular and hearing disorders.     

Multiple quantitative trait loci modify cochlear hair cell degeneration in the Beethoven (Tmc1Bth) mouse model of progressive hearing loss DFNA36

Dominant mutations of transmembrane channel-like gene 1 (TMC1) cause progressive sensorineural hearing loss in humans and Beethoven (Tmc1Bth/+) mice. Here we show that Tmc1Bth/+ mice on a C3HeB/FeJ strain background have selective degeneration of inner hair cells while outer hair cells remain structurally and functionally intact. Inner hair cells primarily function as afferent sensory cells, whereas outer hair cells are electromotile amplifiers of auditory stimuli that can be functionally assessed by distortion product otoacoustic emission (DPOAE) analysis. When C3H-Tmc1Bth/Bth is crossed with either C57BL/6J or DBA/2J wild-type mice, F1 hybrid Tmc1Bth/+ progeny have increased hearing loss associated with increased degeneration of outer hair cells and diminution of DPOAE amplitudes but no difference in degeneration of inner hair cells. We mapped at least one quantitative trait locus (QTL), Tmc1m1, for DPOAE amplitude on chromosome 2 in [(C/B)F1xC]N2-Tmc1Bth/+ backcross progeny, and three other QTL on chromosomes 11 (Tmc1m2), 12 (Tmc1m3), and 5 (Tmc1m4) in [(C/D)F1xC]N2-Tmc1Bth/+ progeny. The polygenic basis of outer hair cell degeneration in Beethoven mice provides a model system for the dissection of common, complex hearing loss phenotypes, such as presbycusis, that involve outer hair cell degeneration in humans.     

Conditional deletion of calcium-modulating cyclophilin ligand causes deafness in mice

Calcium-modulating cyclophilin ligand (Caml) is a ubiquitously expressed cytoplasmic protein that is involved in multiple signaling and developmental pathways. An observation in our laboratory of a protein-protein interaction between Caml and the cytoplasmic region of Cadherin23 led us to speculate that Caml might be important in the inner ear and play a role in the development and/or function of hair cells. To address this question, we generated a mouse line in which Caml expression was eliminated in Atoh1-expressing cells of the inner ear upon administration of tamoxifen. Tamoxifen was administered immediately after birth to neonates to assess the effect of loss of Caml in the inner ear during postnatal development. Hearing in treated animals was tested by auditory brain stem response (ABR) analysis and cochlear pathology was evaluated by light microscopy. Lack of Caml expression in the inner ear leads to severe loss of cochlear hair cells and complete deafness. Elucidating the role of Caml in the inner ear will aid our understanding of the molecular pathways important for auditory development and function.     

Preoperative computed tomography of temporal bones and its use in selecting patients with sensorineural deafness for cochlear implantation

Cochlear implantation (CI) involves the insertion of electrode systems in the inner ear in order to restore hearing in patients with sensorineural deafness. Positive CI results are closely related with careful selection of candidates. Preoperative computed tomography (CT) and its results are decisive in this selection. Temporal bone CT was made in 40 patients aged 1 to 57 years who had sensorineural deafness. The following changes were detected: malformations of the inner ear, the inner auditory canal, meningititis-induced cochlear obliteration, post-traumatic temporal bone changes, local auditory ossicle malformation, inflammatory middle ear changes, the high position of the jugular vein bulb, and its diverticula. Temporal bone CT plays an important role in selecting candidates for CI and circumvents potential difficulties and complications at CI.     

Targeted ablation of connexin26 in the inner ear epithelial gap junction network causes hearing impairment and cell death

Mutations in the gene encoding the gap junction protein connexin26 (Cx26) are responsible for the autosomal recessive isolated deafness, DFNB1, which accounts for half of the cases of prelingual profound hereditary deafness in Caucasian populations. To date, in vivo approaches to decipher the role of Cx26 in the inner ear have been hampered by the embryonic lethality of the Cx26 knockout mice. To overcome this difficulty, we performed targeted ablation of Cx26 specifically in one of the two cellular networks that it underlies in the inner ear, namely, the epithelial network. We show that homozygous mutant mice, Cx26(OtogCre), have hearing impairment, but no vestibular dysfunction. The inner ear developed normally. However, on postnatal day 14 (P14), i.e., soon after the onset of hearing, cell death appeared and eventually extended to the cochlear epithelial network and sensory hair cells. Cell death initially affected only the supporting cells of the genuine sensory cell (inner hair cell, IHC), thus suggesting that it could be triggered by the IHC response to sound stimulation. Altogether, our results demonstrate that the Cx26-containing epithelial gap junction network is essential for cochlear function and cell survival. We conclude that prevention of cell death in the sensory epithelium is essential for any attempt to restore the auditory function in DFNB1 patients.     

Establishment of a knock-in mouse model with the SLC26A4 c.919-2A>G mutation and characterization of its pathology

Recessive mutations in the SLC26A4 gene are a common cause of hereditary hearing impairment worldwide. Previous studies have demonstrated that different SLC26A4 mutations may have different pathogenetic mechanisms. In the present study, we established a knock-in mouse model (i.e., Slc26a4(tm1Dontuh/tm1Dontuh) mice) homozygous for the c.919-2A>G mutation, which is a common mutation in East Asians. Mice were then subjected to audiologic assessment, a battery of vestibular evaluations, and inner ear morphological studies. All Slc26a4(tm1Dontuh/tm1Dontuh) mice revealed profound hearing loss, whereas 46% mice demonstrated pronounced head tilting and circling behaviors. There was a significant difference in the vestibular performance between wild-type and Slc26a4(tm1Dontuh/tm1Dontuh) mice, especially those exhibiting circling behavior. Inner ear morphological examination of Slc26a4(tm1Dontuh/tm1Dontuh) mice revealed an enlarged endolymphatic duct, vestibular aqueduct and sac, atrophy of stria vascularis, deformity of otoconia in the vestibular organs, consistent degeneration of cochlear hair cells, and variable degeneration of vestibular hair cells. Audiologic and inner ear morphological features of Slc26a4(tm1Dontuh/tm1Dontuh) mice were reminiscent of those observed in humans. These features were also similar to those previously reported in both knock-out Slc26a4(-/-) mice and Slc26a4(loop/loop) mice with the Slc26a4 p.S408F mutation, albeit the severity of vestibular hair cell degeneration appeared different among the three mouse strains.     

The role of Six1 in mammalian auditory system development

The homeobox Six genes, homologues to Drosophila sine oculis (so) gene, are expressed in multiple organs during mammalian development. However, their roles during auditory system development have not been studied. We report that Six1 is required for mouse auditory system development. During inner ear development, Six1 expression was first detected in the ventral region of the otic pit and later is restricted to the middle and ventral otic vesicle within which, respectively, the vestibular and auditory epithelia form. By contrast, Six1 expression is excluded from the dorsal otic vesicle within which the semicircular canals form. Six1 is also expressed in the vestibuloacoustic ganglion. At E15.5, Six1 is expressed in all sensory epithelia of the inner ear. Using recently generated Six1 mutant mice, we found that all Six1(+/-) mice showed some degree of hearing loss because of a failure of sound transmission in the middle ear. By contrast, Six1(-/-) mice displayed malformations of the auditory system involving the outer, middle and inner ears. The inner ear development in Six1(-/-) embryos arrested at the otic vesicle stage and all components of the inner ear failed to form due to increased cell death and reduced cell proliferation in the otic epithelium. Because we previously reported that Six1 expression in the otic vesicle is Eya1 dependent, we first clarified that Eya1 expression was unaffected in Six1(-/-) otic vesicle, further demonstrating that the Drosophila Eya-Six regulatory cassette is evolutionarily conserved during mammalian inner ear development. We also analyzed several other otic markers and found that the expression of Pax2 and Pax8 was unaffected in Six1(-/-) otic vesicle. By contrast, Six1 is required for the activation of Fgf3 expression and the maintenance of Fgf10 and Bmp4 expression in the otic vesicle. Furthermore, loss of Six1 function alters the expression pattern of Nkx5.1 and Gata3, indicating that Six1 is required for regional specification of the otic vesicle. Finally, our data suggest that the interaction between Eya1 and Six1 is crucial for the morphogenesis of the cochlea and the posterior ampulla during inner ear development. These analyses establish a role for Six1 in early growth and patterning of the otic vesicle.     

Conductance and block of hair-cell mechanotransducer channels in transmembrane channel–like protein mutants

Transmembrane channel–like (TMC) proteins TMC1 and TMC2 are crucial to the function of the mechanotransducer (MT) channel of inner ear hair cells, but their precise function has been controversial. To provide more insight, we characterized single MT channels in cochlear hair cells from wild-type mice and mice with mutations in Tmc1, Tmc2, or both. Channels were recorded in whole-cell mode after tip link destruction with BAPTA or after attenuating the MT current with GsMTx-4, a peptide toxin we found to block the channels with high affinity. In both cases, the MT channels in outer hair cells (OHCs) of wild-type mice displayed a tonotopic gradient in conductance, with channels from the cochlear base having a conductance (110 pS) nearly twice that of those at the apex (62 pS). This gradient was absent, with channels at both cochlear locations having similar small conductances, with two different Tmc1 mutations. The conductance of MT channels in inner hair cells was invariant with cochlear location but, as in OHCs, was reduced in either Tmc1 mutant. The gradient of OHC conductance also disappeared in Tmc1/Tmc2 double mutants, in which a mechanically sensitive current could be activated by anomalous negative displacements of the hair bundle. This “reversed stimulus–polarity” current was seen with two different Tmc1/Tmc2 double mutants, and with Tmc1/Tmc2/Tmc3 triple mutants, and had a pharmacological sensitivity comparable to that of native MT currents for most antagonists, except dihydrostreptomycin, for which the affinity was less, and for curare, which exhibited incomplete block. The existence in the Tmc1/Tmc2 double mutants of MT channels with most properties resembling those of wild-type channels indicates that proteins other than TMCs must be part of the channel pore. We suggest that an external vestibule of the MT channel may partly account for the channel’s large unitary conductance, high Ca²⁺ permeability, and pharmacological profile, and that this vestibule is disrupted in Tmc mutants.     

Tmprss3, a transmembrane serine protease deficient in human DFNB8/10 deafness, is critical for cochlear hair cell survival at the onset of hearing

Mutations in the type II transmembrane serine protease 3 (TMPRSS3) gene cause non-syndromic autosomal recessive deafness (DFNB8/10), characterized by congenital or childhood onset bilateral profound hearing loss. In order to explore the physiopathology of TMPRSS3 related deafness, we have generated an ethyl-nitrosourea-induced mutant mouse carrying a protein-truncating nonsense mutation in Tmprss3 (Y260X) and characterized the functional and histological consequences of Tmprss3 deficiency. Auditory brainstem response revealed that wild type and heterozygous mice have normal hearing thresholds up to 5 months of age, whereas Tmprss3(Y260X) homozygous mutant mice exhibit severe deafness. Histological examination showed degeneration of the organ of Corti in adult mutant mice. Cochlear hair cell degeneration starts at the onset of hearing, postnatal day 12, in the basal turn and progresses very rapidly toward the apex, reaching completion within 2 days. Given that auditory and vestibular deficits often co-exist, we evaluated the balancing abilities of Tmprss3(Y260X) mice by using rotating rod and vestibular behavioral tests. Tmprss3(Y260X) mice effectively displayed mild vestibular syndrome that correlated histologically with a slow degeneration of saccular hair cells. In situ hybridization in the developing inner ear showed that Tmprss3 mRNA is localized in sensory hair cells in the cochlea and the vestibule. Our results show that Tmprss3 acts as a permissive factor for cochlear hair cells survival and activation at the onset of hearing and is required for saccular hair cell survival. This mouse model will certainly help to decipher the molecular mechanisms underlying DFNB8/10 deafness and cochlear function.     

Molecular mechanisms underlying inner ear patterning defects in kreisler mutants

Prior studies have shown that kreisler mutants display early inner ear defects that are related to abnormal hindbrain development and signaling. These defects in kreisler mice have been linked to mutation of the kr/mafB gene. To investigate potential relevance of kr/mafB and abnormal hindbrain development in inner ear patterning, we analyzed the ear morphogenesis in kreisler mice using a paint-fill technique. We also examined the expression patterns of a battery of genes important for normal inner ear patterning and development. Our results indicate that the loss of dorsal otic structures such as the endolymphatic duct and sac is attributable to the downregulation of Gbx2, Dlx5 and Wnt2b in the dorsal region of the otocyst. In contrast, the expanded expression domain of Otx2 in the ventral otic region likely contributes to the cochlear phenotype seen in kreisler mutants. Sensory organ development is also markedly disrupted in kreisler mutants. This pattern of defects and gene expression changes is remarkably similar to that observed in Gbx2 mutants. Taken together, the data show an important role for hindbrain cues, and indirectly, kr/mafB, in guiding inner ear morphogenesis. The data also identify Gbx2, Dlx5, Wnt2b and Otx2 as key otic genes ultimately affected by perturbation of the kr/mafB-hindbrain pathway.     

The zinc finger transcription factor Gfi1, implicated in lymphomagenesis,is required for inner ear hair cell differentiation and survival

Gfi1 was first identified as causing interleukin 2-independent growth in T cells and lymphomagenesis in mice. Much work has shown that Gfi1 and Gfi1b, a second mouse homolog, play pivotal roles in blood cell lineage differentiation. However, neither Gfi1 nor Gfi1b has been implicated in nervous system development, even though their invertebrate homologues, senseless in Drosophila and pag-3 in C. elegans are expressed and required in the nervous system. We show that Gfi1 mRNA is expressed in many areas that give rise to neuronal cells during embryonic development in mouse, and that Gfi1 protein has a more restricted expression pattern. By E12.5 Gfi1 mRNA is expressed in both the CNS and PNS as well as in many sensory epithelia including the developing inner ear epithelia. At later developmental stages, Gfi1 expression in the ear is refined to the hair cells and neurons throughout the inner ear. Gfi1 protein is expressed in a more restricted pattern in specialized sensory cells of the PNS, including the eye, presumptive Merkel cells, the lung and hair cells of the inner ear. Gfi1 mutant mice display behavioral defects that are consistent with inner ear anomalies, as they are ataxic, circle, display head tilting behavior and do not respond to noise. They have a unique inner ear phenotype in that the vestibular and cochlear hair cells are differentially affected. Although Gfi1-deficient mice initially specify inner ear hair cells, these hair cells are disorganized in both the vestibule and cochlea. The outer hair cells of the cochlea are improperly innervated and express neuronal markers that are not normally expressed in these cells. Furthermore, Gfi1 mutant mice lose all cochlear hair cells just prior to and soon after birth through apoptosis. Finally, by five months of age there is also a dramatic reduction in the number of cochlear neurons. Hence, Gfi1 is expressed in the developing nervous system, is required for inner ear hair cell differentiation, and its loss causes programmed cell death.