A deafness mutation isolates a second role for the tectorial membrane in hearing

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

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

ZP2 cleavage blocks polyspermy by modulating the architecture of the egg coat

Following the fertilization of an egg by a single sperm, the egg coat or zona pellucida (ZP) hardens and polyspermy is irreversibly blocked. These events are associated with the cleavage of the N-terminal region (NTR) of glycoprotein ZP2, a major subunit of ZP filaments. ZP2 processing is thought to inactivate sperm binding to the ZP, but its molecular consequences and connection with ZP hardening are unknown. Biochemical and structural studies show that cleavage of ZP2 triggers its oligomerization. Moreover, the structure of a native vertebrate egg coat filament, combined with AlphaFold predictions of human ZP polymers, reveals that two protofilaments consisting of type I (ZP3) and type II (ZP1/ZP2/ZP4) components interlock into a left-handed double helix from which the NTRs of type II subunits protrude. Together, these data suggest that oligomerization of cleaved ZP2 NTRs extensively cross-links ZP filaments, rigidifying the egg coat and making it physically impenetrable to sperm.

The cochlear matrisome: Importance in hearing and deafness

The extracellular matrix (ECM) consists in a complex meshwork of collagens, glycoproteins, and proteoglycans, which serves a scaffolding function and provides viscoelastic properties to the tissues. ECM acts as a biomechanical support, and actively participates in cell signaling to induce tissular changes in response to environmental forces and soluble cues. Given the remarkable complexity of the inner ear architecture, its exquisite structure-function relationship, and the importance of vibration-induced stimulation of its sensory cells, ECM is instrumental to hearing. Many factors of the matrisome are involved in cochlea development, function and maintenance, as evidenced by the variety of ECM proteins associated with hereditary deafness. This review describes the structural and functional ECM components in the auditory organ and how they are modulated over time and following injury.

Distortion Product Otoacoustic Emissions in Mice Above and Below the Eliciting Primaries

Normal hearing is associated with cochlear nonlinearity. When two tones (f1 and f2) are presented, the intracochlear response contains additional components that can be recorded from the ear canal as distortion product otoacoustic emissions (DPOAEs). Although the most prominent intermodulation distortion component is at 2f1-f2, other cubic distortion products are also generated. Because these measurements are noninvasive, they are used in humans and in animal models to detect hearing loss. This study evaluated how loss of sensitivity affects DPOAEs with frequencies above and below the stimulating primaries, i.e., for upper sideband (USB) components like 2f2-f1 and for lower sideband (LSB) components like 2f1-f2. DPOAEs were recorded in several mouse mutants with varying degrees of hearing loss associated with structural changes to the tectorial membrane (TM), or with loss of outer hair cell (OHC) somatic electromotility due to lack of prestin or to the expression of a non-functional prestin. In mice with changes in sensitivity, magnitude reductions were observed for 2f1-f2 relative to controls with mice lacking prestin showing the greatest changes. In contrast, 2f2-f1 was minimally affected by reductions in cochlear gain due to changes in the TM or by the loss of OHC somatic electromotility. In addition, TM mutants with spontaneous otoacoustic emissions (SOAEs) generated larger responses than controls at 2f2-f1 when its frequency was similar to that for the SOAEs. Although cochlear pathologies appear to affect USB and LSB DPOAEs in different ways, both 2f1-f2 and 2f2-f1 reflect nonlinearities associated with the transducer channels. However, in mice, the component at 2f2-f1 does not appear to receive enhancement due to prestin's motor action.

Stimulus-frequency otoacoustic emissions and middle-ear pressure gains in a finite-element mouse model

For evoked otoacoustic emissions (OAEs), the stimulus and emission signals traverse the middle ear (ME) in forward and reverse directions, respectively. In this study, a fully coupled three-dimensional finite-element model of the mouse ear canal (EC), ME, and cochlea was used to calculate ME pressure gains, impedances, and reflectances at the EC-entrance and stapes-footplate-cochlear-fluid interfaces. The cochlear model incorporates a series of interdigitated Y-shaped structures sandwiched between the basilar membrane and reticular lamina, each comprised of a Deiters' cell, its phalangeal-process extension, and an outer hair cell (OHC). By introducing random perturbations to the OHC gains, stimulation-frequency otoacoustic emissions (SFOAEs) were generated. Raising the perturbation magnitude from 10% to 80% increased the SFOAE magnitude by up to 24 dB in the 10-30 kHz frequency range. Increasing or decreasing the stiffness of the stapes annular ligament and eardrum by a factor of 8 changed the SFOAEs by up to 30 dB, but the round-trip ME gain as measured could not account for this. A modified round-trip ME gain, with reflections removed at the EC-entrance and stapes-cochlea boundaries, eliminated a ±10 dB discrepancy and allowed ME changes to be quantitatively associated with changes in measured OAEs.

The impact of targeted ablation of one row of outer hair cells and Deiters' cells on cochlear amplification

The mammalian cochlea contains three rows of outer hair cells (OHCs) that amplify the basilar membrane traveling wave with high gain and exquisite tuning. The pattern of OHC loss caused by typical methods of producing hearing loss in animal models (noise, ototoxic exposure, or aging) is variable and not consistent along the length of the cochlea. Thus, it is difficult to use these approaches to understand how forces from multiple OHCs summate to create normal cochlear amplification. Here, we selectively removed the third row of OHCs and Deiters' cells in adult mice and measured cochlear amplification. In the mature cochlear epithelia, expression of the Wnt target gene Lgr5 is restricted to the third row of Deiters' cells, the supporting cells directly underneath the OHCs. Diphtheria toxin administration to Lgr5DTR-EGFP/+ mice selectively ablated the third row of Deiters' cells and the third row of OHCs. Basilar membrane vibration in vivo demonstrated disproportionately lower reduction in cochlear amplification by about 13.5 dB. On a linear scale, this means that the 33% reduction in OHC number led to a 79% reduction in gain. Thus, these experimental data describe the impact of reducing the force of cochlear amplification by a specific amount. Furthermore, these data argue that because OHC forces progressively and sequentially amplify the traveling wave as it travels to its peak, the loss of even a relatively small number of OHCs, when evenly distributed longitudinally, will cause a substantial reduction in cochlear amplification.NEW & NOTEWORTHY Normal cochlear physiology involves force production from three rows of outer hair cells to amplify and tune the traveling wave. Here, we used a genetic approach to target and ablate the third row of outer hair cells in the mouse cochlea and found it reduced cochlear amplification by 79%. This means that the loss of even a relatively small number of OHCs, when evenly distributed, causes a substantial reduction in cochlear amplification.

Age-related degradation of tectorial membrane dynamics with loss of CEACAM16

Studies of genetic disorders of sensorineural hearing loss have been instrumental in delineating mechanisms that underlie the remarkable sensitivity and selectivity that are hallmarks of mammalian hearing. For example, genetic modifications of TECTA and TECTB, which are principal proteins that comprise the tectorial membrane (TM), have been shown to alter auditory thresholds and frequency tuning in ways that can be understood in terms of changes in the mechanical properties of the TM. Here, we investigate effects of genetic modification targeting CEACAM16, a third important TM protein. Loss of CEACAM16 has been recently shown to lead to progressive reductions in sensitivity. Whereas age-related hearing losses have previously been linked to changes in sensory receptor cells, the role of the TM in progressive hearing loss is largely unknown. Here, we show that TM stiffness and viscosity are significantly reduced in adult mice that lack functional CEACAM16 relative to age-matched wild-type controls. By contrast, these same mechanical properties of TMs from juvenile mice that lack functional CEACAM16 are more similar to those of wild-type mice. Thus, changes in hearing phenotype align with changes in TM material properties and can be understood in terms of the same TM wave properties that were previously used to characterize modifications of TECTA and TECTB. These results demonstrate that CEACAM16 is essential for maintaining TM mechanical and wave properties, which in turn are necessary for sustaining the remarkable sensitivity and selectivity of mammalian hearing with increasing age.

A genome-wide meta-analysis uncovers six sequence variants conferring risk of vertigo

Vertigo is the leading symptom of vestibular disorders and a major risk factor for falls. In a genome-wide association study of vertigo (Ncases = 48,072, Ncontrols = 894,541), we uncovered an association with six common sequence variants in individuals of European ancestry, including missense variants in ZNF91, OTOG, OTOGL, and TECTA, and a cis-eQTL for ARMC9. The association of variants in ZNF91, OTOGL, and OTOP1 was driven by an association with benign paroxysmal positional vertigo. Using previous reports of sequence variants associating with age-related hearing impairment and motion sickness, we found eight additional variants that associate with vertigo. Although disorders of the auditory and the vestibular system may co-occur, none of the six genome-wide significant vertigo variants were associated with hearing loss and only one was associated with age-related hearing impairment. Our results uncovered sequence variants associating with vertigo in a genome-wide association study and implicated genes with known roles in inner ear development, maintenance, and disease.

Zona Pellucida Genes and Proteins: Essential Players in Mammalian Oogenesis and Fertility

All mammalian oocytes and eggs are surrounded by a relatively thick extracellular matrix (ECM), the zona pellucida (ZP), that plays vital roles during oogenesis, fertilization, and preimplantation development. Unlike ECM surrounding somatic cells, the ZP is composed of only a few glycosylated proteins, ZP1–4, that are unique to oocytes and eggs. ZP1–4 have a large region of polypeptide, the ZP domain (ZPD), consisting of two subdomains, ZP-N and ZP-C, separated by a short linker region, that plays an essential role in polymerization of nascent ZP proteins into crosslinked fibrils. Both subdomains adopt immunoglobulin (Ig)-like folds for their 3-dimensional structure. Mouse and human ZP genes are encoded by single-copy genes located on different chromosomes and are highly expressed in the ovary by growing oocytes during late stages of oogenesis. Genes encoding ZP proteins are conserved among mammals, and their expression is regulated by cis-acting sequences located close to the transcription start-site and by the same/similar trans-acting factors. Nascent ZP proteins are synthesized, packaged into vesicles, secreted into the extracellular space, and assembled into long, crosslinked fibrils that have a structural repeat, a ZP2-ZP3 dimer, and constitute the ZP matrix. Fibrils are oriented differently with respect to the oolemma in the inner and outer layers of the ZP. Sequence elements in the ZPD and the carboxy-terminal propeptide of ZP1–4 regulate secretion and assembly of nascent ZP proteins. The presence of both ZP2 and ZP3 is required to assemble ZP fibrils and ZP1 and ZP4 are used to crosslink the fibrils. Inactivation of mouse ZP genes by gene targeting has a detrimental effect on ZP formation around growing oocytes and female fertility. Gene sequence variations in human ZP genes due to point, missense, or frameshift mutations also have a detrimental effect on ZP formation and female fertility. The latter mutations provide additional support for the role of ZPD subdomains and other regions of ZP polypeptide in polymerization of human ZP proteins into fibrils and matrix.

Spontaneous otoacoustic emissions are biomarkers for mice with tectorial membrane defects

Cochlear function depends on the operation of a coupled feedback loop, incorporating outer hair cells (OHCs), and structured to assure that inner hair cells (IHCs) convey frequency specific acoustic information to the brain, even at very low sound levels. Although our knowledge of OHC function and its contribution to cochlear amplification has expanded, the importance of the tectorial membrane (TM) to the processing of mechanical inputs has not been fully elucidated. In addition, there are a surprising number of genetic mutations that affect TM structure and that produce hearing loss in humans. By synthesizing old and new results obtained on several mouse mutants, we learned that animals with abnormal TMs are prone to generate spontaneous otoacoustic emissions (SOAE), which are uncommon in most wildtype laboratory animals. Because SOAEs are not produced in TM mutants or in humans when threshold shifts exceed approximately 25 dB, some degree of cochlear amplification is required. However, amplification by itself is not sufficient because normal mice are rarely spontaneous emitters. Since SOAEs reflect active cochlear operation, TM mutants are valuable for studying the oscillatory nature of the amplification process and the structures associated with its stabilization. Inasmuch as the mouse models were selected to mirror human auditory disorders, using SOAEs as a noninvasive clinical tool may assist the classification of individuals with genetic defects that influence the active mechanisms responsible for sensitivity and frequency selectivity, the hallmarks of mammalian hearing.

MET currents and otoacoustic emissions from mice with a detached tectorial membrane indicate the extracellular matrix regulates Ca2+ near stereocilia

KEY POINTS

The aim was to determine whether detachment of the tectorial membrane (TM) from the organ of Corti in Tecta/Tectb-/- mice affects the biophysical properties of cochlear outer hair cells (OHCs). Tecta/Tectb-/- mice have highly elevated hearing thresholds, but OHCs mature normally. Mechanoelectrical transducer (MET) channel resting open probability (Po ) in mature OHC is ∼50% in endolymphatic [Ca2+ ], resulting in a large standing depolarizing MET current that would allow OHCs to act optimally as electromotile cochlear amplifiers. MET channel resting Po in vivo is also high in Tecta/Tectb-/- mice, indicating that the TM is unlikely to statically bias the hair bundles of OHCs. Distortion product otoacoustic emissions (DPOAEs), a readout of active, MET-dependent, non-linear cochlear amplification in OHCs, fail to exhibit long-lasting adaptation to repetitive stimulation in Tecta/Tectb-/- mice. We conclude that during prolonged, sound-induced stimulation of the cochlea the TM may determine the extracellular Ca2+ concentration near the OHC's MET channels.

ABSTRACT

The tectorial membrane (TM) is an acellular structure of the cochlea that is attached to the stereociliary bundles of the outer hair cells (OHCs), electromotile cells that amplify motion of the cochlear partition and sharpen its frequency selectivity. Although the TM is essential for hearing, its role is still not fully understood. In Tecta/Tectb-/- double knockout mice, in which the TM is not coupled to the OHC stereocilia, hearing sensitivity is considerably reduced compared with that of wild-type animals. In vivo, the OHC receptor potentials, assessed using cochlear microphonics, are symmetrical in both wild-type and Tecta/Tectb-/- mice, indicating that the TM does not bias the hair bundle resting position. The functional maturation of hair cells is also unaffected in Tecta/Tectb-/- mice, and the resting open probability of the mechanoelectrical transducer (MET) channel reaches values of ∼50% when the hair bundles of mature OHCs are bathed in an endolymphatic-like Ca2+ concentration (40 μM) in vitro. The resultant large MET current depolarizes OHCs to near -40 mV, a value that would allow optimal activation of the motor protein prestin and normal cochlear amplification. Although the set point of the OHC receptor potential transfer function in vivo may therefore be determined primarily by endolymphatic Ca2+ concentration, repetitive acoustic stimulation fails to produce adaptation of MET-dependent otoacoustic emissions in vivo in the Tecta/Tectb-/- mice. Therefore, the TM is likely to contribute to the regulation of Ca2+ levels around the stereocilia, and thus adaptation of the OHC MET channel during prolonged sound stimulation.

Cryo-EM structure of native human uromodulin, a zona pellucida module polymer

Assembly of extracellular filaments and matrices mediating fundamental biological processes such as morphogenesis, hearing, fertilization, and antibacterial defense is driven by a ubiquitous polymerization module known as zona pellucida (ZP) "domain". Despite the conservation of this element from hydra to humans, no detailed information is available on the filamentous conformation of any ZP module protein. Here, we report a cryo-electron microscopy study of uromodulin (UMOD)/Tamm-Horsfall protein, the most abundant protein in human urine and an archetypal ZP module-containing molecule, in its mature homopolymeric state. UMOD forms a one-start helix with an unprecedented 180-degree twist between subunits enfolded by interdomain linkers that have completely reorganized as a result of propeptide dissociation. Lateral interaction between filaments in the urine generates sheets exposing a checkerboard of binding sites to capture uropathogenic bacteria, and UMOD-based models of heteromeric vertebrate egg coat filaments identify a common sperm-binding region at the interface between subunits.

Comparing spontaneous and stimulus frequency otoacoustic emissions in mice with tectorial membrane defects

The global standing-wave model for generation of spontaneous otoacoustic emissions (SOAEs) suggests that they are amplitude-stabilized standing waves and that the spacing between SOAEs corresponds to the interval over which the phase changes by one cycle as determined from the phase-gradient delays of stimulus frequency otoacoustic emissions (SFOAEs). Because data characterizing the relationship between spontaneous and evoked emissions in nonhuman mammals are limited, we examined SOAEs and SFOAEs in tectorial membrane (TM) mutants and their controls. Computations indicate that the spacing between adjacent SOAEs is predicted by the SFOAE phase-gradient delays for TM mutants lacking Ceacam16, where SOAE frequencies are greater than ~20 kHz and the mutants retain near-normal hearing when young. Mice with a missense mutation in Tecta (Tecta^Y1870C/+), as well as mice lacking Otoancorin (Otoa^-/-), were also examined. Although these mutants exhibit hearing loss, they generate SOAEs with average frequencies of 11 kHz in Tecta^Y1870C/+ and 6 kHz in Otoa^-/-. In these animals, the spacing between adjacent SOAEs is larger than predicted by the SFOAE phase delays. It is also demonstrated that mice do not exhibit the strong frequency-dependence in signal coding that characterizes species with good low-frequency hearing. In fact, a transition occurs near the apical end of the mouse cochlea rather than at the mid-point along the cochlear partition. Hence, disagreements with the standing-wave model are not easily explained by a transition in tuning ratios between apical and basal regions of the cochlea, especially for SOAEs generated in Tecta^Y1870C/+mice.

Zona Pellucida Proteins, Fibrils, and Matrix

The zona pellucida (ZP) is an extracellular matrix that surrounds all mammalian oocytes, eggs, and early embryos and plays vital roles during oogenesis, fertilization, and preimplantation development. The ZP is composed of three or four glycosylated proteins, ZP1–4, that are synthesized, processed, secreted, and assembled into long, cross-linked fibrils by growing oocytes. ZP proteins have an immunoglobulin-like three-dimensional structure and a ZP domain that consists of two subdomains, ZP-N and ZP-C, with ZP-N of ZP2 and ZP3 required for fibril assembly. A ZP2–ZP3 dimer is located periodically along ZP fibrils that are cross-linked by ZP1, a protein with a proline-rich N terminus. Fibrils in the inner and outer regions of the ZP are oriented perpendicular and parallel to the oolemma, respectively, giving the ZP a multilayered appearance. Upon fertilization of eggs, modification of ZP2 and ZP3 results in changes in the ZP's physical and biological properties that have important consequences. Certain structural features of ZP proteins suggest that they may be amyloid-like proteins.

Abnormal Tectorial Membranes in Sensorineural Hearing Loss: A Human Temporal Bone Study

HYPOTHESIS

This study evaluates the morphological changes of the tectorial membrane (TM) in conjunction with degeneration of hair cells, interdental cells, and presence of endolymphatic hydrops (EH) in sensorineural hearing loss (HL) in the human using histopathology techniques.

BACKGROUND

The TM plays an important role in mechanical transduction of acoustic energy, and pathology of the TM may result in HL.

METHODS

All temporal bone (TB) specimens from the Massachusetts Eye and Ear Otopathology Laboratory from patients with various causes of sensorineural HL and morphological abnormalities of the TM were evaluated. Cases with a history of cochlear trauma (other than acoustic trauma) and/or severe postmortem artifacts were excluded. The TBs were processed histologically, and the status of hair cells, supporting cells, interdental cells, presence of EH, and postmortem time were tabulated.

RESULTS

Two thousand two hundred ninety TBs from 1340 individuals were evaluated, and 164 of 748 TBs from the otological disorders in which the TM were abnormal, met the inclusion criteria. The most common disorders were idiopathic sudden deafness (57.1%), genetic etiology (53.7%), and ototoxicity (40.0%), as compared with cases with presbycusis (2.9%). EH was found in 33.3% of all cases with an identified abnormality of the TM.Abnormalities of the TM were 1) deformed, 2) shrunken, 3) detached from the limbus, 4) encapsulated, or 5) missing. Encapsulated, shrunken and missing patterns (36, 35, 31%, respectively) were the most common.

CONCLUSION

A relative high prevalence of EH among disorders with TM abnormalities suggests a possible common pathophysiology in both. In addition, anatomic abnormalities of the TM may play a role in the pathophysiology of HL in these disorders.

The Tectorial Membrane: Mechanical Properties and Functions

The tectorial membrane (TM) is widely believed to play a critical role in determining the remarkable sensitivity and frequency selectivity that are hallmarks of mammalian hearing. Recently developed mouse models of human hearing disorders have provided new insights into the molecular, nanomechanical mechanisms that underlie resonance and traveling wave properties of the TM. Herein we review recent experimental and theoretical results detailing TM morphology, local poroelastic and electromechanical interactions, and global spread of excitation via TM traveling waves, with direct implications for cochlear mechanisms.

Accelerated Age-Related Degradation of the Tectorial Membrane in the Ceacam16βgal/βgal Null Mutant Mouse, a Model for Late-Onset Human Hereditary Deafness DFNB113

CEACAM16 is a non-collagenous protein of the tectorial membrane, an extracellular structure of the cochlea essential for normal hearing. Dominant and recessive mutations in CEACAM16 have been reported to cause postlingual and progressive forms of deafness in humans. In a previous study of young Ceacam16βgal/βgal null mutant mice on a C57Bl/6J background, the incidence of spontaneous otoacoustic emissions (SOAEs) was greatly increased relative to Ceacam16+/+ and Ceacam16+/βgal mice, but auditory brain-stem responses (ABRs) and distortion product otoacoustic emissions (DPOAEs) were near normal, indicating auditory thresholds were not significantly affected. To determine if the loss of CEACAM16 leads to hearing loss at later ages in this mouse line, cochlear structure and auditory function were examined in Ceacam16+/+, Ceacam16+/βgal and Ceacam16βgal/βgal mice at 6 and 12 months of age and compared to that previously described at 1 month. Analysis of older Ceacam16βgal/βgal mice reveals a progressive loss of matrix from the core of the tectorial membrane that is more extensive in the apical, low-frequency regions of the cochlea. In Ceacam16βgal/βgal mice at 6-7 months, the DPOAE magnitude at 2f1-f2 and the incidence of SOAEs both decrease relative to young animals. By ∼12 months, SOAEs and DPOAEs are not detected in Ceacam16βgal/βgal mice and ABR thresholds are increased by up to ∼40 dB across frequency, despite a complement of hair cells similar to that present in Ceacam16+/+ mice. Although SOAE incidence decreases with age in Ceacam16βgal/βgal mice, it increases in aging heterozygous Ceacam16+/βgal mice and is accompanied by a reduction in the accumulation of CEACAM16 in the tectorial membrane relative to controls. An apically-biased loss of matrix from the core of the tectorial membrane, similar to that observed in young Ceacam16βgal/βgal mice, is also seen in Ceacam16+/+ and Ceacam16+/βgal mice, and other strains of wild-type mice, but at much later ages. The loss of Ceacam16 therefore accelerates age-related degeneration of the tectorial membrane leading, as in humans with mutations in CEACAM16, to a late-onset progressive form of hearing loss.

Reducing tectorial membrane viscoelasticity enhances spontaneous otoacoustic emissions and compromises the detection of low level sound

The mammalian cochlea is able to detect faint sounds due to the presence of an active nonlinear feedback mechanism that boosts cochlear vibrations of low amplitude. Because of this feedback, self-sustained oscillations called spontaneous otoacoustic emissions (SOAEs) can often be measured in the ear canal. Recent experiments in genetically modified mice have demonstrated that mutations of the genes expressed in the tectorial membrane (TM), an extracellular matrix located in the cochlea, can significantly enhance the generation of SOAEs. Multiple untested mechanisms have been proposed to explain these unexpected results. In this work, a physiologically motivated computational model of a mammalian species commonly studied in auditory research, the gerbil, is used to demonstrate that altering the viscoelastic properties of the TM tends to affect the linear stability of the cochlea, SOAE generation and the cochlear response to low amplitude stimuli. These results suggest that changes in TM properties might be the underlying cause for SOAE enhancement in some mutant mice. Furthermore, these theoretical findings imply that the TM contributes to keeping the mammalian cochlea near an oscillatory instability, which promotes high sensitivity and the detection of low level stimuli.

Control of hearing sensitivity by tectorial membrane calcium

A new mechanism that contributes to control of hearing sensitivity is described here. We show that an accessory structure in the hearing organ, the tectorial membrane, affects the function of inner ear sensory cells by storing calcium ions. When the calcium store is depleted, by brief exposure to rock concert-level sounds or by the introduction of calcium chelators, the sound-evoked responses of the sensory cells decrease. Upon restoration of tectorial membrane calcium, sensory cell function returns. This previously unknown mechanism contributes to explaining the temporary numbness in the ear that follows from listening to sounds that are too loud, a phenomenon that most people experience at some point in their lives.

When sound stimulates the stereocilia on the sensory cells in the hearing organ, Ca²⁺ ions flow through mechanically gated ion channels. This Ca²⁺ influx is thought to be important for ensuring that the mechanically gated channels operate within their most sensitive response region, setting the fraction of channels open at rest, and possibly for the continued maintenance of stereocilia. Since the extracellular Ca²⁺ concentration will affect the amount of Ca²⁺ entering during stimulation, it is important to determine the level of the ion close to the sensory cells. Using fluorescence imaging and fluorescence correlation spectroscopy, we measured the Ca²⁺ concentration near guinea pig stereocilia in situ. Surprisingly, we found that an acellular accessory structure close to the stereocilia, the tectorial membrane, had much higher Ca²⁺ than the surrounding fluid. Loud sounds depleted Ca²⁺ from the tectorial membrane, and Ca²⁺ manipulations had large effects on hair cell function. Hence, the tectorial membrane contributes to control of hearing sensitivity by influencing the ionic environment around the stereocilia.

Spontaneous Otoacoustic Emissions in TectaY1870C/+ Mice Reflect Changes in Cochlear Amplification and How It Is Controlled by the Tectorial Membrane

Spontaneous otoacoustic emissions (SOAEs) recorded from the ear canal in the absence of sound reflect cochlear amplification, an outer hair cell (OHC) process required for the extraordinary sensitivity and frequency selectivity of mammalian hearing. Although wild-type mice rarely emit, those with mutations that influence the tectorial membrane (TM) show an incidence of SOAEs similar to that in humans. In this report, we characterized mice with a missense mutation in Tecta, a gene required for the formation of the striated-sheet matrix within the core of the TM. Mice heterozygous for the Y1870C mutation (Tecta^Y1870C/+) are prolific emitters, despite a moderate hearing loss. Additionally, Kimura’s membrane, into which the OHC stereocilia insert, separates from the main body of the TM, except at apical cochlear locations. Multimodal SOAEs are also observed in Tecta^Y1870C/+ mice where energy is present at frequencies that are integer multiples of a lower-frequency SOAE (the primary). Second-harmonic SOAEs, at twice the frequency of a lower-frequency primary, are the most frequently observed. These secondary SOAEs are found in spatial regions where stimulus-evoked OAEs are small or in the noise floor. Introduction of high-level suppressors just above the primary SOAE frequency reduce or eliminate both primary and second-harmonic SOAEs. In contrast, second-harmonic SOAEs are not affected by suppressors, either above or below the second-harmonic SOAE frequency, even when they are much larger in amplitude. Hence, second-harmonic SOAEs do not appear to be spatially separated from their primaries, a finding that has implications for cochlear mechanics and the consequences of changes to TM structure.

Structure of Zona Pellucida Module Proteins

The egg coat, an extracellular matrix made up of glycoprotein filaments, plays a key role in animal fertilization by acting as a gatekeeper for sperm. Egg coat components polymerize using a common zona pellucida (ZP) "domain" module that consists of two related immunoglobulin-like domains, called ZP-N and ZP-C. The ZP module has also been recognized in a large number of other secreted proteins with different biological functions, whose mutations are linked to severe human diseases. During the last decade, tremendous progress has been made toward understanding the atomic architecture of the ZP module and the structural basis of its polymerization. Moreover, sperm-binding regions at the N-terminus of mollusk and mammalian egg coat subunits were found to consist of domain repeats that also adopt a ZP-N fold. This discovery revealed an unexpected link between invertebrate and vertebrate fertilization and led to the first structure of an egg coat-sperm protein recognition complex. In this review we summarize these exciting findings, discuss their functional implications, and outline future challenges that must be addressed in order to develop a comprehensive view of this family of biomedically important extracellular molecules.

Structure, Function, and Development of the Tectorial Membrane: An Extracellular Matrix Essential for Hearing

The tectorial membrane is an extracellular matrix that lies over the apical surface of the auditory epithelia in the inner ears of reptiles, birds, and mammals. Recent studies have shown it is composed of a small set of proteins, some of which are only produced at high levels in the ear and many of which are the products of genes that, when mutated, cause nonsyndromic forms of human hereditary deafness. Quite how the proteins of the tectorial membrane are assembled within the lumen of the inner ear to form a structure that is precisely regulated in its size and physical properties along the length of a tonotopically organized hearing organ is a question that remains to be fully answered. In this brief review we will summarize what is known thus far about the structure, protein composition, and function of the tectorial membrane in birds and mammals, describe how the tectorial membrane develops, and discuss major events that have occurred during the evolution of this extracellular matrix.

The Mouse Egg's Zona Pellucida

All mammalian eggs are surrounded by a highly specialized extracellular matrix (ECM), called the zona pellucida (ZP), that functions before, during, and after fertilization. Unlike somatic cell ECM the mouse ZP is composed of three different proteins, ZP1-3, that are synthesized and secreted by growing oocytes and assembled into long interconnected fibrils. ECM or vitelline envelope (VE) that surrounds fish, reptilian, amphibian, and avian eggs also consists of a limited number of proteins all closely related to ZP1-3. Messenger RNAs encoding ZP1-3 are expressed only by growing oocytes at very high levels from single-copy genes present on different chromosomes. Processing at the amino- and carboxy-termini of nascent ZP1-3 permits secretion of mature proteins into the extracellular space and assembly into fibrils and matrix. Structural features of nascent ZP proteins prevent assembly within secretory vesicles of growing oocytes. Homozygous knockout female mice that fail to synthesize either ZP2 or ZP3 are unable to construct a ZP, ovulate few if any eggs, and are infertile. ZP1-3 have a common structural feature, the ZP domain (ZPD), that has been conserved through 600 million years of evolution and is essential for ZP protein assembly into fibrils. The ZPD consists of two subdomains, each with four conserved cysteine residues present as two intramolecular disulfides, and resembles an immunoglobulin (Ig) domain found in a wide variety of proteins that have diverse functions, from receptors to mechanical transducers. ZP2 and ZP3 function as receptors for acrosome-reacted and acrosome-intact sperm, respectively, during fertilization of ovulated eggs, but are inactivated as sperm receptors as a result of fertilization.

A tectorin-based matrix and planar cell polarity genes are required for normal collagen-fibril orientation in the developing tectorial membrane

The tectorial membrane is an extracellular structure of the cochlea. It develops on the surface of the auditory epithelium and contains collagen fibrils embedded in a tectorin-based matrix. The collagen fibrils are oriented radially with an apically directed slant - a feature considered crucial for hearing. To determine how this pattern is generated, collagen-fibril formation was examined in mice lacking a tectorin-based matrix, epithelial cilia or the planar cell polarity genes Vangl2 and Ptk7 In wild-type mice, collagen-fibril bundles appear within a tectorin-based matrix at E15.5 and, as fibril number rapidly increases, become co-aligned and correctly oriented. Epithelial width measurements and data from Kif3acKO mice suggest, respectively, that radial stretch and cilia play little, if any, role in determining normal collagen-fibril orientation; however, evidence from tectorin-knockout mice indicates that confinement is important. PRICKLE2 distribution reveals the planar cell polarity axis in the underlying epithelium is organised along the length of the cochlea and, in mice in which this polarity is disrupted, the apically directed collagen offset is no longer observed. These results highlight the importance of the tectorin-based matrix and epithelial signals for precise collagen organisation in the tectorial membrane.

Prevalence of TECTA mutation in patients with mid-frequency sensorineural hearing loss

BACKGROUND

To date, 102 genes have been reported as responsible for non-syndromic hearing loss, some of which are associated with specific audiogram features. Four genes have been reported as causative for mid-frequency sensorineural hearing loss (MFSNHL), among which TECTA is the most frequently reported; however, the prevalence of TECTA mutations is unknown. To elucidate the prevalence of TECTA mutation in MFSNHL and clarify genotype-phenotype correlations, we analyzed the genetic and clinical features of patients with MFSNHL.

METHODS

Subjects with bilateral non-syndromic hearing loss were prescreened for GJB2 and m.1555A > G and m.3243A > G mitochondrial DNA mutations, and patients with inner ear malformations were excluded. We selected MFSNHL patients whose audiograms met the U-shaped criterion proposed by the GENDEAF study group, along with those with shallow U-shaped audiograms, for TECTA analysis. All TECTA exons were analyzed by Sanger sequencing. Novel missense variants were classified as possibly pathogenic, non-pathogenic, and variants of uncertain significance, based on genetic data. To evaluate novel possibly pathogenic variants, we predicted changes in protein structure by molecular modeling.

RESULTS

Pathogenic and possibly pathogenic variants of TECTA were found in 4 (6.0%) of 67 patients with MFSNHL. In patients with U-shaped audiograms, none (0%) of 21 had pathogenic or possibly pathogenic variants. In patients with shallow U-shaped audiograms, four (8.7%) of 46 had pathogenic or possibly pathogenic variants. Two novel possibly pathogenic variants were identified and two previously reported mutations were considered as variant of unknown significance. The clinical features of patients with pathogenic and possibly pathogenic variants were consistent with those in previous studies. Pathogenic or possibly pathogenic variants were identified in 3 of 23 families (13.0%) which have the family histories compatible with autosomal dominant and 1 of 44 families (2.3%) which have the family histories compatible with sporadic or autosomal recessive.

CONCLUSIONS

TECTA mutations were identified in 6.0% of MFSNHL. These mutations were more frequent in patients with shallow U-shaped audiograms than those with U-shaped audiograms, and in families which have the family histories compatible with autosomal dominant than those with the family histories compatible with sporadic or autosomal recessive.

An update of common autosomal recessive non-syndromic hearing loss genes in Iranian population

Autosomal-recessive genes are responsible for about 80% of the hereditary non-syndromic hearing loss (NSHL) cases. In Iran, due to consanguineous marriages, NSHL is the second most frequent disability after intellectual disability, occurring one in 16 individuals. Enormous heterogeneity in the genetic pathology of hearing loss causes a major challenge in identification of responsible genes. In Iran, GJB2 is responsible for the most cases of pre-lingual and non-syndromic hearing loss (with frequency of 16.7%) which followed by other genes with lower frequency. Although several studies have indicated that a large proportion of both syndromic and non-syndromic hearing loss in Iranian populations are caused by defects in just a few genes, new detection strategies such as NGS (Next-generation sequencing) have increased the spectrum of responsible mutations. However, by applying this technique in Iran patients screening, the role of lots of novel related genes have been reported. In this review, we aim to describe function of these genes and their contribution to non-syndromic genetic hearing loss in Iranian population and we classify the genes by their functions.

A connexin30 mutation rescues hearing and reveals roles for gap junctions in cochlear amplification and micromechanics

Accelerated age-related hearing loss disrupts high-frequency hearing in inbred CD-1 mice. The p.Ala88Val (A88V) mutation in the gene coding for the gap-junction protein connexin30 (Cx30) protects the cochlear basal turn of adult CD-1Cx30^A88V/A88V mice from degeneration and rescues hearing. Here we report that the passive compliance of the cochlear partition and active frequency tuning of the basilar membrane are enhanced in the cochleae of CD-1Cx30^A88V/A88V compared to CBA/J mice with sensitive high-frequency hearing, suggesting that gap junctions contribute to passive cochlear mechanics and energy distribution in the active cochlea. Surprisingly, the endocochlear potential that drives mechanoelectrical transduction currents in outer hair cells and hence cochlear amplification is greatly reduced in CD-1Cx30^A88V/A88V mice. Yet, the saturating amplitudes of cochlear microphonic potentials in CD-1Cx30^A88V/A88V and CBA/J mice are comparable. Although not conclusive, these results are compatible with the proposal that transmembrane potentials, determined mainly by extracellular potentials, drive somatic electromotility of outer hair cells.

A point mutation in the gap-junction protein connexin 30 stops early onset age-related hearing loss. Here, the authors show that gap junctions contribute to cochlear micromechanics and that cochlear amplification is likely controlled by extracellular potentials in vicinity of the cochlear sensory cells.

Mice Haploinsufficient for Ets1 and Fli1 Display Middle Ear Abnormalities and Model Aspects of Jacobsen Syndrome

E26 transformation-specific 1 (ETS1) and friend leukemia integration 1 (FLI1) are members of the ETS family of transcription factors, of which there are 28 in humans. Both genes are hemizygous in Jacobsen syndrome, an 11q contiguous gene deletion disorder involving thrombocytopenia, facial dysmorphism, growth and mental retardation, malformation of the heart and other organs, and hearing impairment associated with recurrent ear infections. To determine whether any of these defects are because of hemizygosity for ETS1 and FLI1, we characterized the phenotype of mice heterozygous for mutant alleles of Ets1 and Fli1. Fli1(+/-) mice displayed mild thrombocytopenia, as did Ets1(+/-)Fli1(+/-) animals. Fli1(+/-) and Ets1(+/-)Fli1(+/-) mice also displayed craniofacial abnormalities, including a small middle ear cavity, short nasal bone, and malformed interface between the nasal bone process and cartilaginous nasal septum. They exhibited hearing impairment, otitis media, fusions of ossicles to the middle ear wall, and deformed stapes. Hearing impairment was more penetrant and stapes malformations were more severe in Ets1(+/-)Fli1(+/-) mice than in Fli1(+/-) mice, indicating partial functional redundancy of these transcription factors during auditory development. Our findings indicate that the short nose, otitis media, and hearing impairment in Jacobsen syndrome are likely because of hemizygosity for ETS1 and FLI1.

A structured interdomain linker directs self-polymerization of human uromodulin

Uromodulin (UMOD)/Tamm-Horsfall protein, the most abundant human urinary protein, plays a key role in chronic kidney diseases and is a promising therapeutic target for hypertension. Via its bipartite zona pellucida module (ZP-N/ZP-C), UMOD forms extracellular filaments that regulate kidney electrolyte balance and innate immunity, as well as protect against renal stones. Moreover, salt-dependent aggregation of UMOD filaments in the urine generates a soluble molecular net that captures uropathogenic bacteria and facilitates their clearance. Despite the functional importance of its homopolymers, no structural information is available on UMOD and how it self-assembles into filaments. Here, we report the crystal structures of polymerization regions of human UMOD and mouse ZP2, an essential sperm receptor protein that is structurally related to UMOD but forms heteropolymers. The structure of UMOD reveals that an extensive hydrophobic interface mediates ZP-N domain homodimerization. This arrangement is required for filament formation and is directed by an ordered ZP-N/ZP-C linker that is not observed in ZP2 but is conserved in the sequence of deafness/Crohn's disease-associated homopolymeric glycoproteins α-tectorin (TECTA) and glycoprotein 2 (GP2). Our data provide an example of how interdomain linker plasticity can modulate the function of structurally similar multidomain proteins. Moreover, the architecture of UMOD rationalizes numerous pathogenic mutations in both UMOD and TECTA genes.

Tectorins crosslink type II collagen fibrils and connect the tectorial membrane to the spiral limbus

All inner ear organs possess extracellular matrix appendices over the sensory epithelia that are crucial for their proper function. The tectorial membrane (TM) is a gelatinous acellular membrane located above the hearing sensory epithelium and is composed mostly of type II collagen, and α and β tectorins. TM molecules self-assemble in the endolymph fluid environment, interacting medially with the spiral limbus and distally with the outer hair cell stereocilia. Here, we used immunogold labeling in freeze-substituted mouse cochleae to assess the fine localization of both tectorins in distinct TM regions. We observed that the TM adheres to the spiral limbus through a dense thin matrix enriched in α- and β-tectorin, both likely bound to the membranes of interdental cells. Freeze-etching images revealed that type II collagen fibrils were crosslinked by short thin filaments (4±1.5nm, width), resembling another collagen type protein, or chains of globular elements (15±3.2nm, diameter). Gold-particles for both tectorins also localized adjacent to the type II collagen fibrils, suggesting that these globules might be composed essentially of α- and β-tectorins. Finally, the presence of gold-particles at the TM lower side suggests that the outer hair cell stereocilia membrane has a molecular partner to tectorins, probably stereocilin, allowing the physical connection between the TM and the organ of Corti.

Local mechanisms for loud sound-enhanced aminoglycoside entry into outer hair cells

Loud sound exposure exacerbates aminoglycoside ototoxicity, increasing the risk of permanent hearing loss and degrading the quality of life in affected individuals. We previously reported that loud sound exposure induces temporary threshold shifts (TTS) and enhances uptake of aminoglycosides, like gentamicin, by cochlear outer hair cells (OHCs). Here, we explore mechanisms by which loud sound exposure and TTS could increase aminoglycoside uptake by OHCs that may underlie this form of ototoxic synergy. Mice were exposed to loud sound levels to induce TTS, and received fluorescently-tagged gentamicin (GTTR) for 30 min prior to fixation. The degree of TTS was assessed by comparing auditory brainstem responses (ABRs) before and after loud sound exposure. The number of tip links, which gate the GTTR-permeant mechanoelectrical transducer (MET) channels, was determined in OHC bundles, with or without exposure to loud sound, using scanning electron microscopy. We found wide-band noise (WBN) levels that induce TTS also enhance OHC uptake of GTTR compared to OHCs in control cochleae. In cochlear regions with TTS, the increase in OHC uptake of GTTR was significantly greater than in adjacent pillar cells. In control mice, we identified stereociliary tip links at ~50% of potential positions in OHC bundles. However, the number of OHC tip links was significantly reduced in mice that received WBN at levels capable of inducing TTS. These data suggest that GTTR uptake by OHCs during TTS occurs by increased permeation of surviving, mechanically-gated MET channels, and/or non-MET aminoglycoside-permeant channels activated following loud sound exposure. Loss of tip links would hyperpolarize hair cells and potentially increase drug uptake via aminoglycoside-permeant channels expressed by hair cells. The effect of TTS on aminoglycoside-permeant channel kinetics will shed new light on the mechanisms of loud sound-enhanced aminoglycoside uptake, and consequently on ototoxic synergy.

Noninvasive in vivo imaging reveals differences between tectorial membrane and basilar membrane traveling waves in the mouse cochlea

Sound is encoded within the auditory portion of the inner ear, the cochlea, after propagating down its length as a traveling wave. For over half a century, vibratory measurements to study cochlear traveling waves have been made using invasive approaches such as laser Doppler vibrometry. Although these studies have provided critical information regarding the nonlinear processes within the living cochlea that increase the amplitude of vibration and sharpen frequency tuning, the data have typically been limited to point measurements of basilar membrane vibration. In addition, opening the cochlea may alter its function and affect the findings. Here we describe volumetric optical coherence tomography vibrometry, a technique that overcomes these limitations by providing depth-resolved displacement measurements at 200 kHz inside a 3D volume of tissue with picometer sensitivity. We studied the mouse cochlea by imaging noninvasively through the surrounding bone to measure sound-induced vibrations of the sensory structures in vivo, and report, to our knowledge, the first measures of tectorial membrane vibration within the unopened cochlea. We found that the tectorial membrane sustains traveling wave propagation. Compared with basilar membrane traveling waves, tectorial membrane traveling waves have larger dynamic ranges, sharper frequency tuning, and apically shifted positions of peak vibration. These findings explain discrepancies between previously published basilar membrane vibration and auditory nerve single unit data. Because the tectorial membrane directly overlies the inner hair cell stereociliary bundles, these data provide the most accurate characterization of the stimulus shaping the afferent auditory response available to date.

Modified protein expression in the tectorial membrane of the cochlea reveals roles for the striated sheet matrix

The tectorial membrane (TM) of the mammalian cochlea is a complex extracellular matrix which, in response to acoustic stimulation, displaces the hair bundles of outer hair cells (OHCs), thereby initiating sensory transduction and amplification. Here, using TM segments from the basal, high-frequency region of the cochleae of genetically modified mice (including models of human hereditary deafness) with missing or modified TM proteins, we demonstrate that frequency-dependent stiffening is associated with the striated sheet matrix (SSM). Frequency-dependent stiffening largely disappeared in all three TM mutations studied where the SSM was absent either entirely or at least from the stiffest part of the TM overlying the OHCs. In all three TM mutations, dissipation of energy is decreased at low (<8 kHz) and increased at high (>8 kHz) stimulus frequencies. The SSM is composed of polypeptides carrying fixed charges, and electrostatic interaction between them may account for frequency-dependent stiffness changes in the material properties of the TM. Through comparison with previous in vivo measurements, it is proposed that implementation of frequency-dependent stiffening of the TM in the OHC attachment region facilitates interaction among tones, backward transmission of energy, and amplification in the cochlea.

Porosity controls spread of excitation in tectorial membrane traveling waves

Cochlear frequency selectivity plays a key role in our ability to understand speech, and is widely believed to be associated with cochlear amplification. However, genetic studies targeting the tectorial membrane (TM) have demonstrated both sharper and broader tuning with no obvious changes in hair bundle or somatic motility mechanisms. For example, cochlear tuning of Tectb(-/-) mice is significantly sharper than that of Tecta(Y1870C/+) mice, even though TM stiffnesses are similarly reduced relative to wild-type TMs. Here we show that differences in TM viscosity can account for these differences in tuning. In the basal cochlear turn, nanoscale pores of Tecta(Y1870C/+) TMs are significantly larger than those of Tectb(-/-) TMs. The larger pore size reduces shear viscosity (by ∼70%), thereby reducing traveling wave speed and increasing spread of excitation. These results demonstrate the previously unrecognized importance of TM porosity in cochlear and neural tuning.

Novel TECTA mutations identified in stable sensorineural hearing loss and their clinical implications

TECTA is a causative gene of autosomal dominant (DFNA8/A12) and autosomal recessive (DFNB 21) nonsyndromic sensorineural hearing loss (NSHL). Mutations in TECTA account for 4% of all autosomal dominant NSHL cases in some populations and are thus thought to be one of the major causes of autosomal dominant NSHL. A genotype-phenotype correlation for autosomal dominant mutations in the TECTA gene has been proposed. Two families (SB146 and SB149), which segregated moderate NSHL in an autosomal dominant fashion, were included in this study. We performed targeted resequencing of 134 known deafness genes (TRS-134) and bioinformatics analyses to find causative mutations for NSHL in these 2 families. Through TRS-134, we detected 2 novel mutations, i.e. c.3995G>T (p.C1332F) and c.5618C>T (p.T1873I), in the TECTA gene. These mutations cosegregated with NSHL in the studied families and were not detected in normal controls. The mutations c.3995G>T and c.5618C>T reside in the von Willebrand factor type D3-D4 (vWFD3-D4) interdomain of the zonadhesin (ZA) domain and the zona pellucida (ZP) domain, respectively. p.C1332F is the first mutation detected in the vWFD3-D4 interdomain of the ZA domain. The mutations p.C1332F and p.T1873I were associated with stable high-frequency and mid-frequency hearing loss, respectively. Notably, the cysteine residue mutated to phenylalanine in SB146 was not related to progression of sensorineural hearing loss, which argues against the previous hypothesis. Here we confirm a known genotype-phenotype correlation for the ZP domain and propose a hypothetical genotype-phenotype correlation which relates mutations in vWFD3-D4 to stable high-frequency NSHL in Koreans. This clinical feature makes subjects with the missense mutation in the vWFD3-D4 interdomain of TECTA potentially good candidates for middle ear implantation.

Loss of the tectorial membrane protein CEACAM16 enhances spontaneous, stimulus-frequency, and transiently evoked otoacoustic emissions

α-Tectorin (TECTA), β-tectorin (TECTB), and carcinoembryonic antigen-related cell adhesion molecule 16 (CEACAM) are secreted glycoproteins that are present in the tectorial membrane (TM), an extracellular structure overlying the hearing organ of the inner ear, the organ of Corti. Previous studies have shown that TECTA and TECTB are both required for formation of the striated-sheet matrix within which collagen fibrils of the TM are imbedded and that CEACAM16 interacts with TECTA. To learn more about the structural and functional significance of CEACAM16, we created a Ceacam16-null mutant mouse. In the absence of CEACAM16, TECTB levels are reduced, a clearly defined striated-sheet matrix does not develop, and Hensen's stripe, a prominent feature in the basal two-thirds of the TM in WT mice, is absent. CEACAM16 is also shown to interact with TECTB, indicating that it may stabilize interactions between TECTA and TECTB. Although brain-stem evoked responses and distortion product otoacoustic emissions are, for most frequencies, normal in young mice lacking CEACAM16, stimulus-frequency and transiently evoked emissions are larger. We also observed spontaneous otoacoustic emissions (SOAEs) in 70% of the homozygous mice. This incidence is remarkable considering that <3% of WT controls have SOAEs. The predominance of SOAEs >15 kHz correlates with the loss of Hensen's stripe. Results from mice lacking CEACAM16 are consistent with the idea that the organ of Corti evolved to maximize the gain of the cochlear amplifier while preventing large oscillations. Changes in TM structure appear to influence the balance between energy generation and dissipation such that the system becomes unstable.

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.

Whole-exome sequencing identifies a novel genotype-phenotype correlation in the entactin domain of the known deafness gene TECTA

Postlingual progressive hearing loss, affecting primarily the high frequencies, is the clinical finding in most cases of autosomal dominant nonsyndromic hearing loss (ADNSHL). The molecular genetic etiology of ADNSHL is extremely heterogeneous. We applied whole-exome sequencing to reveal the genetic etiology of high-frequency hearing loss in a mid-sized Korean family without any prior linkage data. Whole-exome sequencing of four family members (two affected and two unaffected), together with our filtering strategy based on comprehensive bioinformatics analyses, identified 21 potential pathogenic candidates. Sanger validation of an additional five family members excluded 20 variants, leaving only one novel variant, TECTA c.710C>T (p.T237I), as the strongest candidate. This variant resides in the entactin (ENT) domain and co-segregated perfectly with non-progressive high-frequency hearing loss in the family. It was absent among 700 ethnically matched control chromosomes, and the T237 residue is conserved among species, which supports its pathogenicity. Interestingly, this finding contrasted with a previously proposed genotype-phenotype correlation in which variants of the ENT domain of TECTA were associated with mid-frequency hearing loss. Based upon what we observed, we propose a novel “genotype to phenotype” correlation in the ENT domain of TECTA. Our results shed light on another important application of whole-exome sequencing: the establishment of a novel genotype-phenotype in the molecular genetic diagnosis of autosomal dominant hearing loss.

Despite a lack of otoacoustic emission, word recognition is not seriously influenced in a TECTA DFNA8/12 family

OBJECTIVES

Similar to other zona pellucida mutations in the alpha-tectorin (TECTA) gene, the p.Y1870C alteration in DFNA8/12 causes prelingual, nonsyndromic, autosomal dominant hearing loss. Here we investigated the effect of p.Y1870C on reverse transduction by audiometric studies in the family.

METHODS

Pure tone audiometry, brainstem evoked response audiometry, the Freiburger test for speech understanding and transient evoked and distortion product otoacoustic emissions were assessed in three available affected members bearing p.Y1870C.

RESULTS

Pure tone audiometry showed U-shaped curves with moderate to severe degrees of hearing impairment confirmed by brainstem evoked response audiometry. Transient evoked and distortion product otoacoustic emissions were completely absent in all affected family members whereas word recognition scores were up to 95%.

CONCLUSIONS

Although the missense p.Y1870C TECTA mutation leads to complete failure of the cochlear amplifier in humans, very high speech perception scores can be achieved with appropriate therapy.

A rare novel mutation in TECTA causes autosomal dominant nonsyndromic hearing loss in a Mongolian family

Background

The genetic basis of autosomal dominant nonsyndromic hearing loss is complex. Genetic factors are responsible for approximately 50% of cases with congenital hearing loss. However, no previous studies have documented the clinical phenotype and genetic basis of autosomal dominant nonsyndromic hearing loss in Mongolians.

Methods

In this study, we performed exon capture sequencing of a Mongolian family with hereditary hearing loss and identified a novel mutation in TECTA gene, which encodes α -tectorin, a major component of the inner ear extracellular matrix that contacts the specialized sensory hair cells.

Results

The novel G → T missense mutation at nucleotide 6016 results in a substitution of amino acid aspartate at 2006 with tyrosine (Asp2006Tyr) in a highly conserved zona pellucida (ZP) domain of α-tectorin. The mutation is not found in control subjects from the same family with normal hearing and a genotype-phenotype correlation is observed.

Conclusion

A novel missense mutation c.6016 G > T (p.Asp2006Tyr) of TECTA gene is a characteristic TECTA-related mutation which causes autosomal dominant nonsyndromic hearing loss. Our result indicated that mutation in TECTA gene is responsible for the hearing loss in this Mongolian family.

A novel mutation in the TECTA gene in a Chinese family with autosomal dominant nonsyndromic hearing loss

TECTA-related deafness can be inherited as autosomal-dominant nonsyndromic deafness (designated DFNA) or as the autosomal-recessive version. The α-tectorin protein, which is encoded by the TECTA gene, is one of the major components of the tectorial membrane in the inner ear. Using targeted DNA capture and massively parallel sequencing (MPS), we screened 42 genes known to be responsible for human deafness in a Chinese family (Family 3187) in which common deafness mutations had been ruled out as the cause, and identified a novel mutation, c.257-262CCTTTC>GCT (p. Ser86Cys; p. Pro88del) in exon 3 of the TECTA gene in the proband and his extended family. All affected individuals in this family had moderate down-sloping hearing loss across all frequencies. To our knowledge, this is the second TECTA mutation identified in Chinese population. This study demonstrates that targeted genomic capture, MPS, and barcode technology might broaden the availability of genetic testing for individuals with undiagnosed DFNA.

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

BACKGROUND

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

OBJECTIVE

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

Three deaf mice: mouse models for TECTA-based human hereditary deafness reveal domain-specific structural phenotypes in the tectorial membrane

Tecta is a modular, non-collagenous protein of the tectorial membrane (TM), an extracellular matrix of the cochlea essential for normal hearing. Missense mutations in Tecta cause dominant forms of non-syndromic deafness and a genotype-phenotype correlation has been reported in humans, with mutations in different Tecta domains causing mid- or high-frequency hearing impairments that are either stable or progressive. Three mutant mice were created as models for human Tecta mutations; the Tecta(L1820F,G1824D/+) mouse for zona pellucida (ZP) domain mutations causing stable mid-frequency hearing loss in a Belgian family, the Tecta(C1837G/+) mouse for a ZP-domain mutation underlying progressive mid-frequency hearing loss in a Spanish family and the Tecta(C1619S/+) mouse for a zonadhesin-like (ZA) domain mutation responsible for progressive, high-frequency hearing loss in a French family. Mutations in the ZP and ZA domains generate distinctly different changes in the structure of the TM. Auditory brainstem response thresholds in the 8-40 kHz range are elevated by 30-40 dB in the ZP-domain mutants, whilst those in the ZA-domain mutant are elevated by 20-30 dB. The phenotypes are stable and no evidence has been found for a progressive deterioration in TM structure or auditory function. Despite elevated auditory thresholds, the Tecta mutant mice all exhibit an enhanced tendency to have audiogenic seizures in response to white noise stimuli at low sound pressure levels (≤84 dB SPL), revealing a previously unrecognised consequence of Tecta mutations. These results, together with those from previous studies, establish an allelic series for Tecta unequivocally demonstrating an association between genotype and phenotype.

Frequency-dependent properties of the tectorial membrane facilitate energy transmission and amplification in the cochlea

The remarkable sensitivity, frequency selectivity, and dynamic range of the mammalian cochlea relies on longitudinal transmission of minuscule amounts of energy as passive, pressure-driven, basilar membrane (BM) traveling waves. These waves are actively amplified at frequency-specific locations by a mechanism that involves interaction between the BM and another extracellular matrix, the tectorial membrane (TM). From mechanical measurements of isolated segments of the TM, we made the important new (to our knowledge) discovery that the stiffness of the TM is reduced when it is mechanically stimulated at physiologically relevant magnitudes and at frequencies below their frequency place in the cochlea. The reduction in stiffness functionally uncouples the TM from the organ of Corti, thereby minimizing energy losses during passive traveling-wave propagation. Stiffening and decreased viscosity of the TM at high stimulus frequencies can potentially facilitate active amplification, especially in the high-frequency, basal turn, where energy loss due to internal friction within the TM is less than in the apex. This prediction is confirmed by neural recordings from several frequency regions of the cochlea.

Electrokinetic properties of the mammalian tectorial membrane

The tectorial membrane (TM) clearly plays a mechanical role in stimulating cochlear sensory receptors, but the presence of fixed charge in TM constituents suggests that electromechanical properties also may be important. Here, we measure the fixed charge density of the TM and show that this density of fixed charge is sufficient to affect mechanical properties and to generate electrokinetic motions. In particular, alternating currents applied to the middle and marginal zones of isolated TM segments evoke motions at audio frequencies (1-1,000 Hz). Electrically evoked motions are nanometer scaled (∼5-900 nm), decrease with increasing stimulus frequency, and scale linearly over a broad range of electric field amplitudes (0.05-20 kV/m). These findings show that the mammalian TM is highly charged and suggest the importance of a unique TM electrokinetic mechanism.

A mouse model for human deafness DFNB22 reveals that hearing impairment is due to a loss of inner hair cell stimulation

The gene causative for the human nonsyndromic recessive form of deafness DFNB22 encodes otoancorin, a 120-kDa inner ear-specific protein that is expressed on the surface of the spiral limbus in the cochlea. Gene targeting in ES cells was used to create an EGFP knock-in, otoancorin KO (Otoa(EGFP/EGFP)) mouse. In the Otoa(EGFP/EGFP) mouse, the tectorial membrane (TM), a ribbon-like strip of ECM that is normally anchored by one edge to the spiral limbus and lies over the organ of Corti, retains its general form, and remains in close proximity to the organ of Corti, but is detached from the limbal surface. Measurements of cochlear microphonic potentials, distortion product otoacoustic emissions, and basilar membrane motion indicate that the TM remains functionally attached to the electromotile, sensorimotor outer hair cells of the organ of Corti, and that the amplification and frequency tuning of the basilar membrane responses to sounds are almost normal. The compound action potential masker tuning curves, a measure of the tuning of the sensory inner hair cells, are also sharply tuned, but the thresholds of the compound action potentials, a measure of inner hair cell sensitivity, are significantly elevated. These results indicate that the hearing loss in patients with Otoa mutations is caused by a defect in inner hair cell stimulation, and reveal the limbal attachment of the TM plays a critical role in this process.

Mutations of the gene encoding otogelin are a cause of autosomal-recessive nonsyndromic moderate hearing impairment

Already 40 genes have been identified for autosomal-recessive nonsyndromic hearing impairment (arNSHI); however, many more genes are still to be identified. In a Dutch family segregating arNSHI, homozygosity mapping revealed a 2.4 Mb homozygous region on chromosome 11 in p15.1-15.2, which partially overlapped with the previously described DFNB18 locus. However, no putative pathogenic variants were found in USH1C, the gene mutated in DFNB18 hearing impairment. The homozygous region contained 12 additional annotated genes including OTOG, the gene encoding otogelin, a component of the tectorial membrane. It is thought that otogelin contributes to the stability and strength of this membrane through interaction or stabilization of its constituent fibers. The murine orthologous gene was already known to cause hearing loss when defective. Analysis of OTOG in the Dutch family revealed a homozygous 1 bp deletion, c.5508delC, which leads to a shift in the reading frame and a premature stop codon, p.Ala1838ProfsX31. Further screening of 60 unrelated probands from Spanish arNSHI families detected compound heterozygous OTOG mutations in one family, c.6347C>T (p.Pro2116Leu) and c. 6559C>T (p.Arg2187X). The missense mutation p.Pro2116Leu affects a highly conserved residue in the fourth von Willebrand factor type D domain of otogelin. The subjects with OTOG mutations have a moderate hearing impairment, which can be associated with vestibular dysfunction. The flat to shallow "U" or slightly downsloping shaped audiograms closely resembled audiograms of individuals with recessive mutations in the gene encoding α-tectorin, another component of the tectorial membrane. This distinctive phenotype may represent a clue to orientate the molecular diagnosis.

Genetics of hearing and deafness

This article is a review of the genes and genetic disorders that affect hearing in humans and a few selected mouse models of deafness. Genetics is playing an increasingly critical role in the practice of medicine. This is not only in part to the importance that genetic knowledge has on traditional genetic diseases but also in part to the fact that genetic knowledge provides an understanding of the fundamental biological process of most diseases. The proteins coded by the genes related to hearing loss (HL) are involved in many functions in the ear, such as cochlear fluid homeostasis, ionic channels, stereocilia morphology and function, synaptic transmission, gene regulation, and others. Mouse models play a crucial role in understanding of the pathogenesis associated with these genes. Different types of familial HL have been recognized for years; however, in the last two decades, there has been tremendous progress in the discovery of gene mutations that cause deafness. Most of the cases of genetic deafness recognized today are monogenic disorders that can be broadly classified by the mode of inheritance (i.e., autosomal dominant, autosomal recessive, X-linked, and mitochondrial inheritance) and by the presence of associated phenotypic features (i.e., syndromic; and nonsyndromic). In terms of nonsyndromic HL, the chromosomal locations are currently known for ∼ 125 loci (54 for dominant and 71 for recessive deafness), 64 genes have been identified (24 for dominant and 40 for recessive deafness), and there are many more loci for syndromic deafness and X-linked and mitochondrial DNA disorders (http://hereditaryhearingloss.org). Thus, today's clinician must understand the science of medical genetics as this knowledge can lead to more effective disease diagnosis, counseling, treatment, and prevention.

Advantages of a mouse model for human hearing impairment

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