Lateral wall protein content mediates alterations in cochlear outer hair cell mechanics before and after hearing onset

Prestin derived OHC surface area reduction underlies age-related rescaling of frequency place coding

Outer hair cells (OHC) are key to the mammalian cochlear amplifier, powered by the lateral membrane protein Prestin. In this study, we explored age-related OHC changes and how the changes affected hearing in mouse. OHC nonlinear membrane capacitance measurements revealed that, starting upon completion of postnatal auditory development, a continuous reduction of total Prestin in OHCs accompanied by a significant reduction in their cell surface area. Prestin's density is unaffected by Prestin level drop over the whole age range tested, suggesting that the OHC size reduction is Prestin-dependent. Stereocilia length in aged OHCs remained unchanged but the first row stereocilia on the aged inner hair cells (IHCs) were elongated. Distortion product otoacoustic emission (DPOAE) group delays became longer with aging, suggesting an apical shift in vibration on basilar membrane. Acoustic lesion experiments revealed an apical shift in damage place in old cochleae accompanied by a shallower progression in synaptic damage over a wider frequency range that was indicative of a broader frequency filter. Overall, these findings suggest that in aging cochlea, a shift in frequency place coding could occur due to the changes in cochlear active and passive mechanics. This article is part of the Special Issue Outer hair cell Edited by Joseph Santos-Sacchi and Kumar Navaratnam.

Spatiotemporal Developmental Upregulation of Prestin Correlates With the Severity and Location of Cyclodextrin-Induced Outer Hair Cell Loss and Hearing Loss

2-Hyroxypropyl-beta-cyclodextrin (HPβCD) is being used to treat Niemann-Pick C1, a fatal neurodegenerative disease caused by abnormal cholesterol metabolism. HPβCD slows disease progression, but unfortunately causes severe, rapid onset hearing loss by destroying the outer hair cells (OHC). HPβCD-induced damage is believed to be related to the expression of prestin in OHCs. Because prestin is postnatally upregulated from the cochlear base toward the apex, we hypothesized that HPβCD ototoxicity would spread from the high-frequency base toward the low-frequency apex of the cochlea. Consistent with this hypothesis, cochlear hearing impairments and OHC loss rapidly spread from the high-frequency base toward the low-frequency apex of the cochlea when HPβCD administration shifted from postnatal day 3 (P3) to P28. HPβCD-induced histopathologies were initially confined to the OHCs, but between 4- and 6-weeks post-treatment, there was an unexpected, rapid and massive expansion of the lesion to include most inner hair cells (IHC), pillar cells (PC), peripheral auditory nerve fibers, and spiral ganglion neurons at location where OHCs were missing. The magnitude and spatial extent of HPβCD-induced OHC death was tightly correlated with the postnatal day when HPβCD was administered which coincided with the spatiotemporal upregulation of prestin in OHCs. A second, massive wave of degeneration involving IHCs, PC, auditory nerve fibers and spiral ganglion neurons abruptly emerged 4–6 weeks post-HPβCD treatment. This secondary wave of degeneration combined with the initial OHC loss results in a profound, irreversible hearing loss.

Membrane prestin expression correlates with the magnitude of prestin-associated charge movement

Full expression of electromotility, generation of non-linear capacitance (NLC), and high-acuity mammalian hearing require prestin function in the lateral wall of cochlear outer hair cells (OHCs). Estimates of the number of prestin molecules in the OHC membrane vary, and a consensus has not emerged about the correlation between prestin expression and prestin-associated charge movement in the OHC. Using an inducible prestin-expressing cell line, we demonstrate that the charge density, but not the voltage at peak capacitance, directly correlates with the amount of prestin in the plasma membrane. This correlation is evident in studies involving a controlled increase of prestin expression with time after induction and inducer dose-response. Conversely, membrane prestin levels and charge density gradually decline together following the reduction of prestin levels from a steady state by removal of the inducer. Thus, charge density directly correlates with the level of membrane prestin expression, whereas changing membrane levels of prestin have no effect on the voltage at peak capacitance in this inducible prestin-expressing cell line.

Activity-dependent regulation of prestin expression in mouse outer hair cells

Prestin is a membrane protein necessary for outer hair cell (OHC) electromotility and normal hearing. Its regulatory mechanisms are unknown. Several mouse models of hearing loss demonstrate increased prestin, inspiring us to investigate how hearing loss might feedback onto OHCs. To test whether centrally mediated feedback regulates prestin, we developed a novel model of inner hair cell loss. Injection of diphtheria toxin (DT) into adult CBA mice produced significant loss of inner hair cells without affecting OHCs. Thus, DT-injected mice were deaf because they had no afferent auditory input despite OHCs continuing to receive normal auditory mechanical stimulation and having normal function. Patch-clamp experiments demonstrated no change in OHC prestin, indicating that loss of information transfer centrally did not alter prestin expression. To test whether local mechanical feedback regulates prestin, we used Tecta(C1509G) mice, where the tectorial membrane is malformed and only some OHCs are stimulated. OHCs connected to the tectorial membrane had normal prestin levels, whereas OHCs not connected to the tectorial membrane had elevated prestin levels, supporting an activity-dependent model. To test whether the endocochlear potential was necessary for prestin regulation, we studied Tecta(C1509G) mice at different developmental ages. OHCs not connected to the tectorial membrane had lower than normal prestin levels before the onset of the endocochlear potential and higher than normal prestin levels after the onset of the endocochlear potential. Taken together, these data indicate that OHC prestin levels are regulated through local feedback that requires mechanoelectrical transduction currents. This adaptation may serve to compensate for variations in the local mechanical environment.

Structure and mechanics of supporting cells in the guinea pig organ of Corti

The mechanical properties of the mammalian organ of Corti determine its sensitivity to sound frequency and intensity, and the structure of supporting cells changes progressively with frequency along the cochlea. From the apex (low frequency) to the base (high frequency) of the guinea pig cochlea inner pillar cells decrease in length incrementally from 75–55 µm whilst the number of axial microtubules increases from 1,300–2,100. The respective values for outer pillar cells are 120–65 µm and 1,500–3,000. This correlates with a progressive decrease in the length of the outer hair cells from >100 µm to 20 µm. Deiters'cell bodies vary from 60–50 µm long with relatively little change in microtubule number. Their phalangeal processes reflect the lengths of outer hair cells but their microtubule numbers do not change systematically. Correlations between cell length, microtubule number and cochlear location are poor below 1 kHz. Cell stiffness was estimated from direct mechanical measurements made previously from isolated inner and outer pillar cells. We estimate that between 200 Hz and 20 kHz axial stiffness, bending stiffness and buckling limits increase, respectively,∼3, 6 and 4 fold for outer pillar cells, ∼2, 3 and 2.5 fold for inner pillar cells and ∼7, 20 and 24 fold for the phalangeal processes of Deiters'cells. There was little change in the Deiters'cell bodies for any parameter. Compensating for effective cell length the pillar cells are likely to be considerably stiffer than Deiters'cells with buckling limits 10–40 times greater. These data show a clear relationship between cell mechanics and frequency. However, measurements from single cells alone are insufficient and they must be combined with more accurate details of how the multicellular architecture influences the mechanical properties of the whole organ.

Over half the hair cells in the mouse utricle first appear after birth, with significant numbers originating from early postnatal mitotic production in peripheral and striolar growth zones

Many non-mammalian vertebrates produce hair cells throughout life and recover from hearing and balance deficits through regeneration. In contrast, embryonic production of hair cells declines sharply in mammals where deficits from hair cell losses are typically permanent. Hair cell density estimates recently suggested that the vestibular organs of mice continue to add hair cells after birth, so we undertook comprehensive counting in murine utricles at different ages. The counts show that 51% of the hair cells in adults arise during the 2 weeks after birth. Immature hair cells are most common near the neonatal macula's peripheral edge and striola, where anti-Ki-67 labels cycling nuclei in zones that appear to contain niches for supporting-cell-like stem cells. In vivo lineage tracing in a novel reporter mouse where tamoxifen-inducible supporting cell-specific Cre expression switched tdTomato fluorescence to eGFP fluorescence showed that proteolipid-protein-1-expressing supporting cells are an important source of the new hair cells. To assess the contributions of postnatal cell divisions, we gave mice an injection of BrdU or EdU on the day of birth. The labels were restricted to supporting cells 1 day later, but by 12 days, 31% of the labeled nuclei were in myosin-VIIA-positive hair cells. Thus, hair cell populations in neonatal mouse utricles grow appreciably through two processes: the progressive differentiation of cells generated before birth and the differentiation of new cells arising from divisions of progenitors that progress through S phase soon after birth. Subsequent declines in these processes coincide with maturational changes that appear unique to mammalian supporting cells.

Blocking caspase-3-dependent pathway preserves hair cells from salicylate-induced apoptosis in the guinea pig cochlea

In the present study, we aim to explore whether the caspase-3-dependent pathway is involved in the apoptotic cell death that occurs in the hair cells (HCs) of guinea pig cochlea following a salicylate treatment. Guinea pigs received sodium salicylate (Na-SA), at a dose of 200 mg·kg(-1)·d(-1) i.p., as a vehicle for 5 consecutive days. In some experiments, N-benzyloxycarbonyl-Asp-Glu-Val-Asp-fluoromethylketone (zDEVD-FMK), a specific apoptosis inhibitor, was directly applied into the cochlea via the round window niche (RWN) prior to salicylate treatment for determination of caspase-3 activation. Alterations in auditory function were evaluated with auditory brainstem responses (ABR) thresholds. Caspase-3 activity was determined by measuring the proteolytic cleavage product of caspase-3 (N-terminated peptide substrate). DNA fragmentation within the nuclei was examined with a terminal deoxynucleotidyl transferase-mediated dUTP-biotin nick end-labeling (TUNEL) method. Ultrastructure variation in the target cell was assessed by electron microscopy (EM). Salicylate treatment initiated an obvious elevation in ABR thresholds with a maximum average shift of 60 dB sound pressure level (SPL), and caused significant apoptosis in both inner (IHCs) and outer (OHCs) hair cells resulted from an evident increasing in immunoreactivity to caspase-3 protease. Transmission electron microscopy (TEM) displayed chromatin condensation and nucleus margination accompanied by cell body shrinkage in the OHCs, but not in the IHCs. Scanning electron microscopy (SEM) showed breakdown, fusion, and loss in the stereociliary bundles at the apex of OHCs rather than IHCs. zDEVD-FMK pretreatment prior to salicylate injection substantially attenuated an expression of the apoptotic protease and protected HCs against apoptotic death, followed by a moderate relief in the thresholds of ABR, an alleviation in the submicroscopic structure was also identified. In particular, disorientation and insertion in the hair bundles at the apex of OHCs was exhibited though no classic apoptotic change found. The above changes were either prevented or significantly attenuated by zDEVD-FMK. These findings indicate that salicylate could damage cochlear hair cells via inducing apoptosis associated with caspase-3 activation.

Oncomodulin identifies different hair cell types in the mammalian inner ear

The tight regulation of Ca(2+) is essential for inner ear function, and yet the role of Ca(2+) binding proteins (CaBPs) remains elusive. By using immunofluorescence and reverse transcriptase-polymerase chain reaction (RT-PCR), we investigated the expression of oncomodulin (Ocm), a member of the parvalbumin family, relative to other EF-hand CaBPs in cochlear and vestibular organs in the mouse. In the mouse cochlea, Ocm is found only in outer hair cells and is localized preferentially to the basolateral outer hair cell membrane and to the base of the hair bundle. Developmentally, Ocm immunoreactivity begins as early as postnatal day (P) 2 and shows preferential localization to the basolateral membrane and hair bundle after P8. Unlike the cochlea, Ocm expression is substantially reduced in vestibular tissues at older adult ages. In vestibular organs, Ocm is found in type I striolar or central hair cells, and has a more diffuse subcellular localization throughout the hair cell body. Additionally, Ocm immunoreactivity in vestibular hair cells is present as early as E18 and is not obviously affected by mutations that cause a disruption of hair bundle polarity. We also find Ocm expression in striolar hair cells across mammalian species. These data suggest that Ocm may have distinct functional roles in cochlear and vestibular hair cells.

The ultrastructural distribution of prestin in outer hair cells: a post-embedding immunogold investigation of low-frequency and high-frequency regions of the rat cochlea

Outer hair cells (OHCs) of the mammalian cochlea besides being sensory receptors also generate force to amplify sound-induced displacements of the basilar membrane thus enhancing auditory sensitivity and frequency selectivity. This force generation is attributable to the voltage-dependent contractility of the OHCs underpinned by the motile protein, prestin. Prestin is located in the basolateral wall of OHCs and is thought to alter its conformation in response to changes in membrane potential. The precise ultrastructural distribution of prestin was determined using post-embedding immunogold labelling and the density of the labelling was compared in low-frequency and high-frequency regions of the cochlea. The labelling was confined to the basolateral plasma membrane in hearing rats but declined towards the base of the cells below the nucleus. In pre-hearing animals, prestin labelling was lower in the membrane and also occurred in the cytoplasm, presumably reflecting its production during development. The densities of labelling in low-frequency and high-frequency regions of the cochlea were similar. Non-linear capacitance, thought to reflect charge movements during conformational changes in prestin, was measured in OHCs in isolated cochlear coils of hearing animals. The OHC non-linear capacitance in the same regions assayed in the immunolabelling was also similar in both the apex and base, with charge densities of 10,000/microm(2) expressed relative to the lateral membrane area. The results suggest that prestin density, and by implication force production, is similar in low-frequency and high-frequency OHCs.

Targeting of the hair cell proteins cadherin 23, harmonin, myosin XVa, espin, and prestin in an epithelial cell model

We have developed an advantageous epithelial cell transfection model for examining the targeting, interactions, and mutations of hair cell proteins. When expressed in LLC-PK1-CL4 epithelial cells (CL4 cells), the outer hair cell protein prestin showed faithful domain-specific targeting to the basolateral plasma membrane. We examined the consequences of mutations affecting prestin activity and assigned a targeting role to the cytoplasmic tail. The stereociliary link protein cadherin 23 (Cdh23) was targeted to the plasma membrane of CL4 cell microvilli, the topological equivalent of stereocilia. In cells coexpressing the Cdh23 cytoplasmic binding protein harmonin, a large fraction of harmonin became colocalized with Cdh23 in microvilli. Using this assay and in vitro protein binding assays, we formulated an alternative model for Cdh23-harmonin binding, in which the primary interaction is between the harmonin N-domain and a 35-residue internal peptide in the Cdh23 cytoplasmic tail. Contrary to a previous model, we found no role for the Cdh23 C-terminal PDZ (PSD-95/Dlg/ZO-1)-binding motif and observed that Cdh23 bound similar levels of harmonin with or without the exon 68 peptide. We also examined two proteins involved in stereocilium elongation. The stereociliary actin-bundling protein espin was targeted to CL4 cell microvilli and caused microvillar elongation, whereas espin with the c.2469delGTCA or c.1988delAGAG human deafness mutation showed defects in microvillar targeting and elongation. The unconventional myosin motor myosin XVa accumulated at the tips of espin-elongated microvilli, by analogy to its location in stereocilia, whereas myosin XVa with the c.4351G>A or c.4669A>G human deafness mutation did not, revealing functional deficits in motor activity.

Making an effort to listen: mechanical amplification in the ear

The inner ear's performance is greatly enhanced by an active process defined by four features: amplification, frequency selectivity, compressive nonlinearity, and spontaneous otoacoustic emission. These characteristics emerge naturally if the mechanoelectrical transduction process operates near a dynamical instability, the Hopf bifurcation, whose mathematical properties account for specific aspects of our hearing. The active process of nonmammalian tetrapods depends upon active hair-bundle motility, which emerges from the interaction of negative hair-bundle stiffness and myosin-based adaptation motors. Taken together, these phenomena explain the four characteristics of the ear's active process. In the high-frequency region of the mammalian cochlea, the active process is dominated instead by the phenomenon of electromotility, in which the cell bodies of outer hair cells extend and contract as the protein prestin alters its membrane surface area in response to changes in membrane potential.

Cochlear mechanics: no shout but a twist in the absence of prestin

Mammalian hearing is boosted by mechanically active auditory receptor cells, the outer hair cells which amplify the actions of incoming sounds. Recent evidence indicates that the molecular motor that drives this amplification, prestin, may do more than boogie.

Functional prestin transduction of immature outer hair cells from normal and prestin-null mice

Prestin is a membrane protein in the outer hair cell (OHC) that has been shown to be essential for electromotility. OHCs from prestin-null mice do not express prestin, do not have a nonlinear capacitance (the electrical signature of electromotility), and are smaller in size than wild-type OHCs. We sought to determine whether prestin-null OHCs can be transduced to incorporate functional prestin protein in a normal fashion. A recombinant helper-dependent adenovirus expressing prestin and green fluorescent protein (HDAd-prestin-GFP) was created and tested in human embryonic kidney cells (HEK cells). Transduced HEK cells demonstrated membrane expression of prestin and nonlinear capacitance. HDAd-prestin-GFP was then applied to cochlear sensory epithelium explants harvested from wild-type and prestin-null mice at postnatal days 2-3, the age at which native prestin is just beginning to become functional in wild-type mice. At postnatal days 4-5, we investigated transduced OHCs for (1) their prestin expression pattern as revealed by immunofluorescence; (2) their cell surface area as measured by linear capacitance; and (3) their prestin function as indicated by nonlinear capacitance. HDAd-prestin-GFP efficiently transduced OHCs of both genotypes and prestin protein localized to the plasma membrane. Whole-cell voltage clamp studies revealed a nonlinear capacitance in transduced wild-type and prestin-null OHCs, but not in non-transduced cells of either genotype. Prestin transduction did not increase the linear capacitance (cell surface area) for either genotype. In peak nonlinear capacitance, voltage at peak nonlinear capacitance, charge density of the nonlinear capacitance, and shape of the voltage-capacitance curves, the transduced cells of the two genotypes resembled each other and previously reported data from adult wild-type mouse OHCs. Thus, prestin introduced into prestin-deficient OHCs segregates normally to the cell membrane and generates a normal nonlinear capacitance, indicative of normal prestin function.

The role of prestin in the generation of electrically evoked otoacoustic emissions in mice

Electrically evoked otoacoustic emissions are sounds emitted from the inner ear when alternating current is injected into the cochlea. Their temporal structure consists of short- and long-delay components and they have been attributed to the motile responses of the sensory-motor outer hair cells of the cochlea. The nature of these motile responses is unresolved and may depend on either somatic motility, hair bundle motility, or both. The short-delay component persists after almost complete elimination of outer hair cells. Outer hair cells are thus not the sole generators of electrically evoked otoacoustic emissions. We used prestin knockout mice, in which the motor protein prestin is absent from the lateral walls of outer hair cells, and Tecta(Delta ENT/Delta ENT) mice, in which the tectorial membrane, a structure with which the hair bundles of outer hair cells normally interact, is vestigial and completely detached from the organ of Corti. The amplitudes and delay spectra of electrically evoked otoacoustic emissions from Tecta(Delta ENT/Delta ENT) and Tecta(+/+) mice are very similar. In comparison with prestin(+/+) mice, however, the short-delay component of the emission in prestin(-/-) mice is dramatically reduced and the long-delay component is completely absent. Emissions are completely suppressed in wild-type and Tecta(Delta ENT/Delta ENT) mice at low stimulus levels, when prestin-based motility is blocked by salicylate. We conclude that near threshold, the emissions are generated by prestin-based somatic motility.