Active hair bundle movements in auditory hair cells

The origin of mechanical harmonic distortion within the organ of Corti in living gerbil cochleae

Although auditory harmonic distortion has been demonstrated psychophysically in humans and electrophysiologically in experimental animals, the cellular origin of the mechanical harmonic distortion remains unclear. To demonstrate the outer hair cell-generated harmonics within the organ of Corti, we measured sub-nanometer vibrations of the reticular lamina from the apical ends of the outer hair cells in living gerbil cochleae using a custom-built heterodyne low-coherence interferometer. The harmonics in the reticular lamina vibration are significantly larger and have broader spectra and shorter latencies than those in the basilar membrane vibration. The latency of the second harmonic is significantly greater than that of the fundamental at low stimulus frequencies. These data indicate that the mechanical harmonics are generated by the outer hair cells over a broad cochlear region and propagate from the generation sites to their own best-frequency locations.

Electrophysiological assessment and pharmacological treatment of blast-induced tinnitus

Tinnitus, the phantom perception of sound, often occurs as a clinical sequela of auditory traumas. In an effort to develop an objective test and therapeutic approach for tinnitus, the present study was performed in blast-exposed rats and focused on measurements of auditory brainstem responses (ABRs), prepulse inhibition of the acoustic startle response, and presynaptic ribbon densities on cochlear inner hair cells (IHCs). Although the exact mechanism is unknown, the “central gain theory” posits that tinnitus is a perceptual indicator of abnormal increases in the gain (or neural amplification) of the central auditory system to compensate for peripheral loss of sensory input from the cochlea. Our data from vehicle-treated rats supports this rationale; namely, blast-induced cochlear synaptopathy correlated with imbalanced elevations in the ratio of centrally-derived ABR wave V amplitudes to peripherally-derived wave I amplitudes, resulting in behavioral evidence of tinnitus. Logistic regression modeling demonstrated that the ABR wave V/I amplitude ratio served as a reliable metric for objectively identifying tinnitus. Furthermore, histopathological examinations in blast-exposed rats revealed tinnitus-related changes in the expression patterns of key plasticity factors in the central auditory pathway, including chronic loss of Arc/Arg3.1 mobilization. Using a formulation of N-acetylcysteine (NAC) and disodium 2,4-disulfophenyl-N-tert-butylnitrone (HPN-07) as a therapeutic for addressing blast-induced neurodegeneration, we measured a significant treatment effect on preservation or restoration of IHC ribbon synapses, normalization of ABR wave V/I amplitude ratios, and reduced behavioral evidence of tinnitus in blast-exposed rats, all of which accorded with mitigated histopathological evidence of tinnitus-related neuropathy and maladaptive neuroplasticity.

Two-tone distortion in reticular lamina vibration of the living cochlea

It has been demonstrated that isolated auditory sensory cells, outer hair cells, can generate distortion products at low frequencies. It remains unknown, however, whether or not motile outer hair cells are able to generate two-tone distortion at high frequencies in living cochleae under the mechanical loads caused by surounding tissues and fluids. By measuring sub-nanometer vibration directly from the apical ends of outer hair cells using a custom-built heterodyne low-coherence interferometer, here we show outer hair cell-generated two-tone distortion in reticular lamina motion in the living cochlea. Reticular-lamina distortion is significantly greater and occurs at a broader frequency range than that of the basilar membrane. Contrary to expectations, our results indicate that motile outer hair cells are capable of generating two-tone distortion in vivo not only at the locations tuned to primary tones but also at a broad region basal to these locations.

Ren et al. used an in house heterodyne low-coherence interferometer to measure sub-nanometer vibrations, a proxy for distortion products, in living cochleae of gerbils. They were able to locate the generation source of the outer hair cell in the reticular lamina versus the basilar membrane in vivo.

Stiffness and tension gradients of the hair cell's tip-link complex in the mammalian cochlea

Sound analysis by the cochlea relies on frequency tuning of mechanosensory hair cells along a tonotopic axis. To clarify the underlying biophysical mechanism, we have investigated the micromechanical properties of the hair cell's mechanoreceptive hair bundle within the apical half of the rat cochlea. We studied both inner and outer hair cells, which send nervous signals to the brain and amplify cochlear vibrations, respectively. We find that tonotopy is associated with gradients of stiffness and resting mechanical tension, with steeper gradients for outer hair cells, emphasizing the division of labor between the two hair-cell types. We demonstrate that tension in the tip links that convey force to the mechano-electrical transduction channels increases at reduced Ca2+. Finally, we reveal gradients in stiffness and tension at the level of a single tip link. We conclude that mechanical gradients of the tip-link complex may help specify the characteristic frequency of the hair cell.

Frequency doubling by active in vivo motility of mechanosensory neurons in the mosquito ear

Across vertebrate and invertebrate species, nonlinear active mechanisms are employed to increase the sensitivity and acuity of hearing. In mosquitoes, the antennal hearing organs are known to use active force feedback to enhance auditory acuity to female generated sounds. This sophisticated form of signal processing involves active nonlinear events that are proposed to rely on the motile properties of mechanoreceptor neurons. The fundamental physical mechanism for active auditory mechanics is theorized to rely on a synchronization of motile neurons, with a characteristic frequency doubling of the force generated by an ensemble of motile mechanoreceptors. There is however no direct biomechanical evidence at the mechanoreceptor level, hindering further understanding of the fundamental mechanisms of sensitive hearing. Here, using in situ and in vivo atomic force microscopy, we measure and characterize the mechanical response of mechanosensory neuron units during forced oscillations of the hearing organ. Mechanoreceptor responses exhibit the hallmark of nonlinear feedback for force generation, with movements at twice the stimulus frequency, associated with auditory amplification. Simultaneous electrophysiological recordings exhibit similar response features, notably a frequency doubling of the firing rate. This evidence points to the nature of the mechanism, whereby active hearing in mosquitoes emerges from the double-frequency response of the auditory neurons. These results open up the opportunity to directly investigate active cellular mechanics in auditory systems, and they also reveal a pathway to study the nanoscale biomechanics and its dynamics of cells beyond the sense of hearing.

Two-Dimensional Cochlear Micromechanics Measured In Vivo Demonstrate Radial Tuning within the Mouse Organ of Corti

The exquisite sensitivity and frequency discrimination of mammalian hearing underlie the ability to understand complex speech in noise. This requires force generation by cochlear outer hair cells (OHCs) to amplify the basilar membrane traveling wave; however, it is unclear how amplification is achieved with sharp frequency tuning. Here we investigated the origin of tuning by measuring sound-induced 2-D vibrations within the mouse organ of Corti in vivo Our goal was to determine the transfer function relating the radial shear between the structures that deflect the OHC bundle, the tectorial membrane and reticular lamina, to the transverse motion of the basilar membrane. We found that, after normalizing their responses to the vibration of the basilar membrane, the radial vibrations of the tectorial membrane and reticular lamina were tuned. The radial tuning peaked at a higher frequency than transverse basilar membrane tuning in the passive, postmortem condition. The radial tuning was similar in dead mice, indicating that this reflected passive, not active, mechanics. These findings were exaggerated in Tecta(C1509G/C1509G) mice, where the tectorial membrane is detached from OHC stereocilia, arguing that the tuning of radial vibrations within the hair cell epithelium is distinct from tectorial membrane tuning. Together, these results reveal a passive, frequency-dependent contribution to cochlear filtering that is independent of basilar membrane filtering. These data argue that passive mechanics within the organ of Corti sharpen frequency selectivity by defining which OHCs enhance the vibration of the basilar membrane, thereby tuning the gain of cochlear amplification.

SIGNIFICANCE STATEMENT

Outer hair cells amplify the traveling wave within the mammalian cochlea. The resultant gain and frequency sharpening are necessary for speech discrimination, particularly in the presence of background noise. Here we measured the 2-D motion of the organ of Corti in mice and found that the structures that stimulate the outer hair cell stereocilia, the tectorial membrane and reticular lamina, were sharply tuned in the radial direction. Radial tuning was similar in dead mice and in mice lacking a tectorial membrane. This suggests that radial tuning comes from passive mechanics within the hair cell epithelium, and that these mechanics, at least in part, may tune the gain of cochlear amplification.

Mechanosensory hair cells express two molecularly distinct mechanotransduction channels

Auditory hair cells contain mechanotransduction channels that rapidly open in response to sound-induced vibrations. Surprisingly, we report here that auditory hair cells contain two molecularly distinct mechanotransduction channels. One ion channel is activated by sound and is responsible for sensory transduction. This sensory transduction channel is expressed in hair-cell stereocilia and previous studies show that its activity is affected by mutations in the genes encoding the transmembrane proteins TMHS/LHFPL5, TMIE and TMC1/2. We show here that the second ion channel is expressed at the apical surface of hair cells and contains the Piezo2 protein. The activity of the Piezo2-dependent channel is controlled by the intracellular Ca²⁺ concentration and can be recorded following disruption of the sensory transduction machinery or more generally by disruption of the sensory epithelium. We thus conclude that hair cells express two molecularly and functionally distinct mechanotransduction channels with different subcellular distribution.

Hair cell force generation does not amplify or tune vibrations within the chicken basilar papilla

Frequency tuning within the auditory papilla of most non-mammalian species is electrical, deriving from ion-channel resonance within their sensory hair cells. In contrast, tuning within the mammalian cochlea is mechanical, stemming from active mechanisms within outer hair cells that amplify the basilar membrane travelling wave. Interestingly, hair cells in the avian basilar papilla demonstrate both electrical resonance and force-generation, making it unclear which mechanism creates sharp frequency tuning. Here, we measured sound-induced vibrations within the apical half of the chicken basilar papilla in vivo and found broadly-tuned travelling waves that were not amplified. However, distortion products were found in live but not dead chickens. These findings support the idea that avian hair cells do produce force, but that their effects on vibration are small and do not sharpen tuning. Therefore, frequency tuning within the apical avian basilar papilla is not mechanical, and likely derives from hair cell electrical resonance.

The avian auditory papilla has many similarities to the mammalian cochlea but whether force generation by hair cells amplifies the travelling wave, as it does in mammals, remains unknown. Here the authors show that the chicken basilar papilla does not have a ‘cochlear amplifier' and that sharp frequency tuning does not derive from mechanical vibrations.

Feeling force: physical and physiological principles enabling sensory mechanotransduction

Organisms as diverse as microbes, roundworms, insects, and mammals detect and respond to applied force. In animals, this ability depends on ionotropic force receptors, known as mechanoelectrical transduction (MeT) channels, that are expressed by specialized mechanoreceptor cells embedded in diverse tissues and distributed throughout the body. These cells mediate hearing, touch, and proprioception and play a crucial role in regulating organ function. Here, we attempt to integrate knowledge about the architecture of mechanoreceptor cells and their sensory organs with principles of cell mechanics, and we consider how engulfing tissues contribute to mechanical filtering. We address progress in the quest to identify the proteins that form MeT channels and to understand how these channels are gated. For clarity and convenience, we focus on sensory mechanobiology in nematodes, fruit flies, and mice. These themes are emphasized: asymmetric responses to applied forces, which may reflect anisotropy of the structure and mechanics of sensory mechanoreceptor cells, and proteins that function as MeT channels, which appear to have emerged many times through evolution.

Gating of Acoustic Transducer Channels Is Shaped by Biomechanical Filter Processes

Mechanoelectrical transduction of acoustic signals is the fundamental process for hearing in all ears across the animal kingdom. Here, we performed in vivo laser-vibrometric and electrophysiological measurements at the transduction site in an insect ear (Mecopoda elongata) to relate the biomechanical tonotopy along the hearing organ to the frequency tuning of the corresponding sensory cells. Our mechanical and electrophysiological map revealed a biomechanical filter process that considerably sharpens the neuronal response. We demonstrate that the channel gating, which acts on chordotonal stretch receptor neurons, is based on a mechanical directionality of the sound-induced motion. Further, anatomical studies of the transduction site support our finding of a stimulus-relevant tilt. In conclusion, we were able to show, in an insect ear, that directionality of channel gating considerably sharpens the neuronal frequency selectivity at the peripheral level and have identified a mechanism that enhances frequency discrimination in tonotopically organized ears.

Oncomodulin, an EF-Hand Ca2+ Buffer, Is Critical for Maintaining Cochlear Function in Mice

Oncomodulin (Ocm), a member of the parvalbumin family of calcium binding proteins, is expressed predominantly by cochlear outer hair cells in subcellular regions associated with either mechanoelectric transduction or electromotility. Targeted deletion of Ocm caused progressive cochlear dysfunction. Although sound-evoked responses are normal at 1 month, by 4 months, mutants show only minimal distortion product otoacoustic emissions and 70-80 dB threshold shifts in auditory brainstem responses. Thus, Ocm is not critical for cochlear development but does play an essential role for cochlear function in the adult mouse.

SIGNIFICANCE STATEMENT

Numerous proteins act as buffers, sensors, or pumps to control calcium levels in cochlear hair cells. In the inner ear, EF-hand calcium buffers may play a significant role in hair cell function but have been very difficult to study. Unlike other reports of genetic disruption of EF-hand calcium buffers, deletion of oncomodulin (Ocm), which is predominately found in outer hair cells, leads to a progressive hearing loss after 1 month, suggesting that Ocm critically protects hearing in the mature ear.

Form and function of the mammalian inner ear

The inner ear of mammals consists of the cochlea, which is involved with the sense of hearing, and the vestibule and three semicircular canals, which are involved with the sense of balance. Although different regions of the inner ear contribute to different functions, the bony chambers and membranous ducts are morphologically continuous. The gross anatomy of the cochlea that has been related to auditory physiologies includes overall size of the structure, including volume and total spiral length, development of internal cochlear structures, including the primary and secondary bony laminae, morphology of the spiral nerve ganglion, and the nature of cochlear coiling, including total number of turns completed by the cochlear canal and the relative diameters of the basal and apical turns. The overall sizes, shapes, and orientations of the semicircular canals are related to sensitivity to head rotations and possibly locomotor behaviors. Intraspecific variation, primarily in the shape and orientation of the semicircular canals, may provide additional clues to help us better understand form and function of the inner ear.

Reverse transduction measured in the living cochlea by low-coherence heterodyne interferometry

It is generally believed that the remarkable sensitivity and frequency selectivity of mammalian hearing depend on outer hair cell-generated force, which amplifies sound-induced vibrations inside the cochlea. This 'reverse transduction' force production has never been demonstrated experimentally, however, in the living ear. Here by directly measuring microstructure vibrations inside the cochlear partition using a custom-built interferometer, we demonstrate that electrical stimulation can evoke both fast broadband and slow sharply tuned responses of the reticular lamina, but only a slow tuned response of the basilar membrane. Our results indicate that outer hair cells can generate sufficient force to drive the reticular lamina over all audible frequencies in living cochleae. Contrary to expectations, the cellular force causes a travelling wave rather than an immediate local vibration of the basilar membrane; this travelling wave vibrates in phase with the reticular lamina at the best frequency, and results in maximal vibration at the apical ends of outer hair cells.

Mechanical amplification exhibited by quiescent saccular hair bundles

Spontaneous oscillations exhibited by free-standing hair bundles from the Bullfrog sacculus suggest the existence of an active process that might underlie the exquisite sensitivity of the sacculus to mechanical stimulation. However, this spontaneous activity is suppressed by coupling to an overlying membrane, which applies a large mechanical load on the bundle. How a quiescent hair bundle utilizes its active process is still unknown. We studied the dynamics of motion of individual hair bundles under different offsets in the bundle position, and observed the occurrence of spikes in hair-bundle motion, associated with the generation of active work. These mechanical spikes can be evoked by a sinusoidal stimulus, leading to an amplified movement of the bundle with respect to the passive response. Amplitude gain reached as high as 100-fold at small stimulus amplitudes. Amplification of motion decreased with increasing amplitude of stimulation, ceasing at ∼6–12 pN stimuli. Results from numerical simulations suggest that the adaptation process, mediated by myosin 1c, is not required for the production of mechanical spikes.

Basilar membrane and tectorial membrane stiffness in the CBA/CaJ mouse

The mouse has become an important animal model in understanding cochlear function. Structures, such as the tectorial membrane or hair cells, have been changed by gene manipulation, and the resulting effect on cochlear function has been studied. To contrast those findings, physical properties of the basilar membrane (BM) and tectorial membrane (TM) in mice without gene mutation are of great importance. Using the hemicochlea of CBA/CaJ mice, we have demonstrated that tectorial membrane (TM) and basilar membrane (BM) revealed a stiffness gradient along the cochlea. While a simple spring mass resonator predicts the change in the characteristic frequency of the BM, the spring mass model does not predict the frequency change along the TM. Plateau stiffness values of the TM were 0.6 ± 0.5, 0.2 ± 0.1, and 0.09 ± 0.09 N/m for the basal, middle, and upper turns, respectively. The BM plateau stiffness values were 3.7 ± 2.2, 1.2 ± 1.2, and 0.5 ± 0.5 N/m for the basal, middle, and upper turns, respectively. Estimations of the TM Young's modulus (in kPa) revealed 24.3 ± 25.2 for the basal turns, 5.1 ± 4.5 for the middle turns, and 1.9 ± 1.6 for the apical turns. Young's modulus determined at the BM pectinate zone was 76.8 ± 72, 23.9 ± 30.6, and 9.4 ± 6.2 kPa for the basal, middle, and apical turns, respectively. The reported stiffness values of the CBA/CaJ mouse TM and BM provide basic data for the physical properties of its organ of Corti.

Modelling cochlear mechanics

The cochlea plays a crucial role in mammal hearing. The basic function of the cochlea is to map sounds of different frequencies onto corresponding characteristic positions on the basilar membrane (BM). Sounds enter the fluid-filled cochlea and cause deflection of the BM due to pressure differences between the cochlear fluid chambers. These deflections travel along the cochlea, increasing in amplitude, until a frequency-dependent characteristic position and then decay away rapidly. The hair cells can detect these deflections and encode them as neural signals. Modelling the mechanics of the cochlea is of help in interpreting experimental observations and also can provide predictions of the results of experiments that cannot currently be performed due to technical limitations. This paper focuses on reviewing the numerical modelling of the mechanical and electrical processes in the cochlea, which include fluid coupling, micromechanics, the cochlear amplifier, nonlinearity, and electrical coupling.

Recovery of otoacoustic emissions after high-level noise exposure in the American bullfrog

The American bullfrog (Rana catesbeiana) has an amphibian papilla (AP) that senses airborne, low-frequency sound and generates distortion product otoacoustic emissions (DPOAEs) similar to other vertebrate species. Although ranid frogs are typically found in noisy environments, the effects of noise on the AP have not been studied. First, we determined the noise levels that diminished DPOAE at 2f1-f2 using an f2 stimulus level at 80 dB SPL and that also produced morphological damage of the sensory epithelium. Second, we compared DPOAE (2f1-f2) responses with histopathologic changes occurring in bullfrogs after noise exposure. Consistent morphological damage, such as fragmented hair cells and missing bundles, as well as elimination of DPOAE responses were seen only after very high-level (>150 dB SPL) sound exposures. The morphological response of hair cells to noise differed along the mediolateral AP axis: medial hair cells were sensitive to noise and lateral hair cells were relatively insensitive to noise. Renewed or repaired hair cells were not observed until 9 days post-exposure. Following noise exposure, DPOAE responses disappeared within 24 h and then recovered to normal pre-exposure levels within 3-4 days. Our results suggest that DPOAEs in the bullfrog are sensitive to the initial period of hair cell damage. After noise-induced damage, the bullfrog AP has functional recovery mechanisms that do not depend on substantial hair cell regeneration or repair. Thus, the bullfrog auditory system might serve as an interesting model for investigation of ways to prevent noise damage.

Active cochlear amplification is dependent on supporting cell gap junctions

Mammalian hearing relies upon active cochlear mechanics, which arises from outer hair cell (OHC) electromotility and hair bundle movement, to amplify acoustic stimulations increasing hearing sensitivity and frequency selectivity. Here we describe the novel finding that gap junctions between cochlear supporting cells also have a critical role in active cochlear amplification in vivo. We find that targeted-deletion of connexin26 (Cx26) in Deiters cells (DCs) and outer pillar cells (OPCs), which constrain OHCs standing on the basilar membrane, causes a leftward shift in OHC electromotility towards hyperpolarization, and reduces active cochlear amplification with hearing loss. Coincident with large reduction in distortion product otoacoustic emission (DPOAE) and severe hearing loss at high frequencies, the shift is larger in shorter OHCs. Our study demonstrates that active cochlear amplification in vivo is dependent on supporting cell gap junctions. These new findings also show that Cx26 deficiency can reduce active cochlear amplification to induce hearing loss.

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.

Parallel evolution of auditory genes for echolocation in bats and toothed whales

The ability of bats and toothed whales to echolocate is a remarkable case of convergent evolution. Previous genetic studies have documented parallel evolution of nucleotide sequences in Prestin and KCNQ4, both of which are associated with voltage motility during the cochlear amplification of signals. Echolocation involves complex mechanisms. The most important factors include cochlear amplification, nerve transmission, and signal re-coding. Herein, we screen three genes that play different roles in this auditory system. Cadherin 23 (Cdh23) and its ligand, protocadherin 15 (Pcdh15), are essential for bundling motility in the sensory hair. Otoferlin (Otof) responds to nerve signal transmission in the auditory inner hair cell. Signals of parallel evolution occur in all three genes in the three groups of echolocators—two groups of bats (Yangochiroptera and Rhinolophoidea) plus the dolphin. Significant signals of positive selection also occur in Cdh23 in the Rhinolophoidea and dolphin, and Pcdh15 in Yangochiroptera. In addition, adult echolocating bats have higher levels of Otof expression in the auditory cortex than do their embryos and non-echolocation bats. Cdh23 and Pcdh15 encode the upper and lower parts of tip-links, and both genes show signals of convergent evolution and positive selection in echolocators, implying that they may co-evolve to optimize cochlear amplification. Convergent evolution and expression patterns of Otof suggest the potential role of nerve and brain in echolocation. Our synthesis of gene sequence and gene expression analyses reveals that positive selection, parallel evolution, and perhaps co-evolution and gene expression affect multiple hearing genes that play different roles in audition, including voltage and bundle motility in cochlear amplification, nerve transmission, and brain function.

The convergent origins of laryngeal echolocation in two groups of bats (Yangochiroptera and Rhinolophoidea) and toothed whales have long been a focus of interest for biologists. We screened three candidate genes—Cdh23, Pcdh15, and Otof—involved in different steps in the echolocation system. Signals of parallel evolution occurred in all three genes in the three groups of echolocators. Cdh23 and Pcdh15 constitute part of the mechanical link within the hair bundle of the ear. Both genes showed signals of both convergent evolution and positive selection, which implied they may have co-evolved to optimize cochlear amplification. Further, three lines of evidence suggest that Otof plays an important role in the transmission of signals in the brain during echolocation. First, the gene is more highly expressed in the auditory cortex of the brain in echolocating adult bats than in other cerebral cortexes. Second, this expression is higher in adult echolocating female bats than in their embryos, which do not use echolocation. Third, echolocators also have a higher level of expression in their cerebral cortexes than do non-echolocating bats. Taken with other evidence, the independent origins of echolocation involve the same genes that have evolved in precisely identical ways.

Probing the Xenopus laevis inner ear transcriptome for biological function

Background

The senses of hearing and balance depend upon mechanoreception, a process that originates in the inner ear and shares features across species. Amphibians have been widely used for physiological studies of mechanotransduction by sensory hair cells. In contrast, much less is known of the genetic basis of auditory and vestibular function in this class of animals. Among amphibians, the genus Xenopus is a well-characterized genetic and developmental model that offers unique opportunities for inner ear research because of the amphibian capacity for tissue and organ regeneration. For these reasons, we implemented a functional genomics approach as a means to undertake a large-scale analysis of the Xenopus laevis inner ear transcriptome through microarray analysis.

Results

Microarray analysis uncovered genes within the X. laevis inner ear transcriptome associated with inner ear function and impairment in other organisms, thereby supporting the inclusion of Xenopus in cross-species genetic studies of the inner ear. The use of gene categories (inner ear tissue; deafness; ion channels; ion transporters; transcription factors) facilitated the assignment of functional significance to probe set identifiers. We enhanced the biological relevance of our microarray data by using a variety of curation approaches to increase the annotation of the Affymetrix GeneChip® Xenopus laevis Genome array. In addition, annotation analysis revealed the prevalence of inner ear transcripts represented by probe set identifiers that lack functional characterization.

Conclusions

We identified an abundance of targets for genetic analysis of auditory and vestibular function. The orthologues to human genes with known inner ear function and the highly expressed transcripts that lack annotation are particularly interesting candidates for future analyses. We used informatics approaches to impart biologically relevant information to the Xenopus inner ear transcriptome, thereby addressing the impediment imposed by insufficient gene annotation. These findings heighten the relevance of Xenopus as a model organism for genetic investigations of inner ear organogenesis, morphogenesis, and regeneration.

Spontaneous voltage oscillations and response dynamics of a Hodgkin-Huxley type model of sensory hair cells

We employ a Hodgkin-Huxley type model of basolateral ionic currents in bullfrog saccular hair cells to study the genesis of spontaneous voltage oscillations and their role in shaping the response of the hair cell to external mechanical stimuli. Consistent with recent experimental reports, we find that the spontaneous dynamics of the model can be categorized using conductance parameters of calcium activated potassium, inward rectifier potassium, and mechano-electrical transduction ionic currents. The model is demonstrated to exhibit a broad spectrum of autonomous rhythmic activity, including periodic and quasiperiodic oscillations with two independent frequencies as well as various regular and chaotic bursting patterns. Complex patterns of spontaneous oscillations in the model emerge at small values of the conductance of Ca(2+) activated potassium currents. These patterns are significantly affected by thermal fluctuations of the mechano-electrical transduction current. We show that self-sustained regular voltage oscillations lead to enhanced and sharply tuned sensitivity of the hair cell to weak mechanical periodic stimuli. While regimes of chaotic oscillations are argued to result in poor tuning to sinusoidal driving, chaotically oscillating cells do provide a high sensitivity to low-frequency variations of external stimuli.

Measurement of cochlear power gain in the sensitive gerbil ear

The extraordinary sensitivity of the mammalian ear is commonly attributed to the cochlear amplifier, a cellular process thought to locally boost responses of the cochlear partition to soft sounds. However, cochlear power gain has not been measured directly. Here we use a scanning laser interferometer to determine the volume displacement and volume velocity of the cochlear partition by measuring its transverse vibration along and across the partition. We show the transverse displacement at the peak-response location can be >1,000 times greater than the displacement of the stapes, whereas the volume displacement of an area centred at this location is approximately tenfold greater than that of the stapes. Using the volume velocity and cochlear-fluid impedance, we discover that power at the peak-response area is >100-fold greater than that at the stapes. These results demonstrate experimentally that the cochlea amplifies soft sounds, offering insight into the mechanism responsible for the cochlear sensitivity.

Localization of the cochlear amplifier in living sensitive ears

Background

To detect soft sounds, the mammalian cochlea increases its sensitivity by amplifying incoming sounds up to one thousand times. Although the cochlear amplifier is thought to be a local cellular process at an area basal to the response peak on the spiral basilar membrane, its location has not been demonstrated experimentally.

Methodology and Principal Findings

Using a sensitive laser interferometer to measure sub-nanometer vibrations at two locations along the basilar membrane in sensitive gerbil cochleae, here we show that the cochlea can boost soft sound-induced vibrations as much as 50 dB/mm at an area proximal to the response peak on the basilar membrane. The observed amplification works maximally at low sound levels and at frequencies immediately below the peak-response frequency of the measured apical location. The amplification decreases more than 65 dB/mm as sound levels increases.

Conclusions and Significance

We conclude that the cochlea amplifier resides at a small longitudinal region basal to the response peak in the sensitive cochlea. These data provides critical information for advancing our knowledge on cochlear mechanisms responsible for the remarkable hearing sensitivity, frequency selectivity and dynamic range.

Cortical tonotopic map plasticity and behavior

Central topographic representations of sensory epithelia have a genetic basis, but are refined by patterns of afferent input and by behavioral demands. Here we review such experience-driven map development and plasticity, focusing on the auditory system, and giving particular consideration to its adaptive value and to the putative mechanisms involved. Recent data have challenged the widely held notion that only the developing auditory brain can be influenced by changes to the prevailing acoustic environment, unless those changes convey information of behavioral relevance. Specifically, it has been shown that persistent exposure of adult animals to random, bandlimited, moderately loud sounds can lead to a reorganization of auditory cortex not unlike that following restricted hearing loss. The mature auditory brain is thus more plastic than previously supposed, with potentially troubling consequences for those working or living in noisy environments, even at exposure levels considerably below those presently considered just-acceptable.

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 interplay between active hair bundle motility and electromotility in the cochlea

The cochlear amplifier is a nonlinear active process providing the mammalian ear with its extraordinary sensitivity, large dynamic range and sharp frequency tuning. While there is much evidence that amplification results from active force generation by mechanosensory hair cells, there is debate about the cellular processes behind nonlinear amplification. Outer hair cell electromotility has been suggested to underlie the cochlear amplifier. However, it has been shown in frog and turtle that spontaneous movements of hair bundles endow them with a nonlinear response with increased sensitivity that could be the basis of amplification. The present work shows that the properties of the cochlear amplifier could be understood as resulting from the combination of both hair bundle motility and electromotility in an integrated system that couples these processes through the geometric arrangement of hair cells embedded in the cochlear partition. In this scenario, the cochlear partition can become a dynamic oscillator which in the vicinity of a Hopf bifurcation exhibits all the key properties of the cochlear amplifier. The oscillatory behavior and the nonlinearity are provided by active hair bundles. Electromotility is largely linear but produces an additional feedback that allows hair bundle movements to couple to basilar membrane vibrations.

Review series: The cell biology of hearing

Mammals have an astonishing ability to sense and discriminate sounds of different frequencies and intensities. Fundamental for this process are mechanosensory hair cells in the inner ear that convert sound-induced vibrations into electrical signals. The study of genes that are linked to deafness has provided insights into the cell biological mechanisms that control hair cell development and their function as mechanosensors.

Somatic motility and hair bundle mechanics, are both necessary for cochlear amplification?

Hearing organs have evolved to detect sounds across several orders of magnitude of both intensity and frequency. Detection limits are at the atomic level despite the energy associated with sound being limited thermodynamically. Several mechanisms have evolved to account for the remarkable frequency selectivity, dynamic range, and sensitivity of these various hearing organs, together termed the active process or cochlear amplifier. Similarities between hearing organs of disparate species provides insight into the factors driving the development of the cochlear amplifier. These properties include: a tonotopic map, the emergence of a two hair cell system, the separation of efferent and afferent innervations, the role of the tectorial membrane, and the shift from intrinsic tuning and amplification to a more end organ driven process. Two major contributors to the active process are hair bundle mechanics and outer hair cell electromotility, the former present in all hair cell organs tested, the latter only present in mammalian cochlear outer hair cells. Both of these processes have advantages and disadvantages, and how these processes interact to generate the active process in the mammalian system is highly disputed. A hypothesis is put forth suggesting that hair bundle mechanics provides amplification and filtering in most hair cells, while in mammalian cochlea, outer hair cell motility provides the amplification on a cycle by cycle basis driven by the hair bundle that provides frequency selectivity (in concert with the tectorial membrane) and compressive nonlinearity. Separating components of the active process may provide additional sites for regulation of this process.

Prestin and the cholinergic receptor of hair cells: positively-selected proteins in mammals

The hair cells of the vertebrate inner ear posses active mechanical processes to amplify their inputs. The stereocilia bundle of various vertebrate animals can produce active movements. Though standard stereocilia-based mechanisms to promote amplification persist in mammals, an additional radically different mechanism evolved: the so-called somatic electromotility which refers to the elongation/contraction of the outer hair cells' (OHC) cylindrical cell body in response to membrane voltage changes. Somatic electromotility in OHCs, as the basis for cochlear amplification, is a mammalian novelty and it is largely dependent upon the properties of the unique motor protein prestin. We review recent literature which has demonstrated that although the gene encoding prestin is present in all vertebrate species, mammalian prestin has been under positive selective pressure to acquire motor properties, probably rendering it fit to serve somatic motility in outer hair cells. Moreover, we discuss data which indicates that a modified α10 nicotinic cholinergic receptor subunit has co-evolved in mammals, most likely to give the auditory feedback system the capability to control somatic electromotility.

How are the inner hair cells and auditory nerve fibers activated without the mediation of the outer hair cells and the cochlear amplifier?

The present study was designed to assess whether, in the presence of a depression of the cochlear amplifier i.e. a sensorineural hearing loss (SNHL), the inner hair cells (IHCs) require the presence of a normal endocochlear potential for transduction. An SNHL was induced by injecting salicylic acid (which binds to the motor protein prestin in the outer hair cells), and then furosemide (which depresses the endocochlear potential) was injected. Furosemide did not cause an additional elevation of the threshold of the auditory nerve brainstem evoked response (ABR) over that induced by the salicylic acid injection. Exposure to noise was also used to induce a SNHL in other mice, and then furosemide was injected. Here too furosemide did not cause an additional ABR threshold elevation over that induced by the noise. These results show that the IHCs (and the auditory nerve) can be excited in the presence of a SNHL (i.e. without the cochlear amplifier) and in the absence of an endocochlear potential. Possible mechanisms of excitation in such a state are discussed.

Mechanotransduction by hair cells: models, molecules, and mechanisms

Mechanotransduction, the transformation of mechanical force into an electrical signal, allows living organisms to hear, register movement and gravity, detect touch, and sense changes in cell volume and shape. Hair cells in the inner ear are specialized mechanoreceptor cells that detect sound and head movement. The mechanotransduction machinery of hair cells is extraordinarily sensitive and responds to minute physical displacements on a submillisecond timescale. The recent discovery of several molecular constituents of the mechanotransduction machinery of hair cells provides a new framework for the interpretation of biophysical data and necessitates revision of prevailing models of mechanotransduction.

Amplifying effect of a release mechanism for fast adaptation in the hair bundle

A "release" mechanism, which has been experimentally observed as the fast component in the hair bundle's response to mechanical stimulation, appears similar to common mechanical relaxation with a damping effect. This observation is puzzling because such a response is expected to have an amplifying role in the mechanoelectrical transduction process in hair cells. Here it is shown that a release mechanism can indeed have a role in amplification, if it is associated with negative stiffness due to the gating of the mechonoelectric transducer channel.

Power efficiency of outer hair cell somatic electromotility

Cochlear outer hair cells (OHCs) are fast biological motors that serve to enhance the vibration of the organ of Corti and increase the sensitivity of the inner ear to sound. Exactly how OHCs produce useful mechanical power at auditory frequencies, given their intrinsic biophysical properties, has been a subject of considerable debate. To address this we formulated a mathematical model of the OHC based on first principles and analyzed the power conversion efficiency in the frequency domain. The model includes a mixture-composite constitutive model of the active lateral wall and spatially distributed electro-mechanical fields. The analysis predicts that: 1) the peak power efficiency is likely to be tuned to a specific frequency, dependent upon OHC length, and this tuning may contribute to the place principle and frequency selectivity in the cochlea; 2) the OHC power output can be detuned and attenuated by increasing the basal conductance of the cell, a parameter likely controlled by the brain via the efferent system; and 3) power output efficiency is limited by mechanical properties of the load, thus suggesting that impedance of the organ of Corti may be matched regionally to the OHC. The high power efficiency, tuning, and efferent control of outer hair cells are the direct result of biophysical properties of the cells, thus providing the physical basis for the remarkable sensitivity and selectivity of hearing.

Psychophysical evidence of damaged active processing mechanisms in Belgian Waterslager Canaries

Belgian Waterslager canaries (BWC), bred for a distinct low-pitched song, have an inherited high-frequency hearing loss associated with hair cell abnormalities. Hair cells near the abneural edge of the papilla, which receive primarily efferent innervation in normal birds, are among the most severely affected. These cells are thought to support nonlinear active processing in the avian ear, though the mechanisms are poorly understood. Here we present psychophysical evidence that suggests degraded active processing in BWC compared to normal-hearing non-BWC. Critical ratios, psychophysical masking patterns and phase effects on masking by harmonic complexes were measured in BWC and non-BWC using operant conditioning procedures. Critical ratios were much larger in BWC than in non-BWC at high frequencies. Psychophysical tuning curves derived from the masking patterns for BWC were broadened at high frequencies. BWC also showed severely reduced phase effects on masking by harmonic complexes compared to non-BWC. As has been hypothesized previously for hearing-impaired humans, these results are consistent with a loss of active processing mechanisms in BWC.

Comment on "Measuring power production in the mammalian cochlea" [Curr. Biol. 17, 1340 (2007)]

Recently, a paper by Lakashkin et al. (2007) ("Power amplification in the mammalian cochlea," Curr. Biol. 17, 1340-1344) was published on how power can be measured in the mammalian cochlea. The general subject is of current widespread interest, so the question of whether the method used by Lakashkin et al. is valid may be of interest to the readers of this journal. Power generation in the cochlea can account for the extraordinary sensitivity of hearing. Lukashkin et al. claimed to provide a direct proof of cochlear power generation. A first-order spring-dashpot system was used to model the organ of Corti. The power flux direction can be derived from the sign of the phase difference between the force and displacement, which can be presented as a "hysteresis plot." Basilar membrane (BM) vibration near the characteristic frequency (CF) was measured while applying a low-frequency modulation tone together with the CF tone. A force was derived from the modulation profile of the BM CF vibration and when plotted versus the displacement at the modulation frequency, the function had a counterclockwise direction of hysteresis, suggesting power generation. In this letter, we present comments on the analysis in the report: (1) that it is not appropriate to analyze at the modulation frequency to derive the power generation at CF; (2) that the derivation of a force from just the displacement profile is not justified, followed by an alternative interpretation of the experimental data.

Stereocilin-deficient mice reveal the origin of cochlear waveform distortions

Although the cochlea is an amplifier and a remarkably sensitive and finely tuned detector of sounds, it also produces conspicuous mechanical and electrical waveform distortions. These distortions reflect nonlinear mechanical interactions within the cochlea. By allowing one tone to suppress another (masking effect), they contribute to speech intelligibility. Tones can also combine to produce sounds with frequencies not present in the acoustic stimulus. These sounds compose the otoacoustic emissions that are extensively used to screen hearing in newborns. Because both cochlear amplification and distortion originate from the outer hair cells-one of the two types of sensory receptor cells-it has been speculated that they stem from a common mechanism. Here we show that the nonlinearity underlying cochlear waveform distortions relies on the presence of stereocilin, a protein defective in a recessive form of human deafness. Stereocilin was detected in association with horizontal top connectors, lateral links that join adjacent stereocilia within the outer hair cell's hair bundle. These links were absent in stereocilin-null mutant mice, which became progressively deaf. At the onset of hearing, however, their cochlear sensitivity and frequency tuning were almost normal, although masking was much reduced and both acoustic and electrical waveform distortions were completely lacking. From this unique functional situation, we conclude that the main source of cochlear waveform distortions is a deflection-dependent hair bundle stiffness resulting from constraints imposed by the horizontal top connectors, and not from the intrinsic nonlinear behaviour of the mechanoelectrical transducer channel.

Loss of GABAB receptors in cochlear neurons: threshold elevation suggests modulation of outer hair cell function by type II afferent fibers

Despite pharmacological and immunohistochemical evidence for GABA as a neurotransmitter in the olivocochlear efferent bundle, a clear functional role of GABA in the inner ear has not emerged. To explore the role of metabotropic GABA(B) receptors, we characterized the cochlear phenotype of mice with targeted deletion of the GABA(B1) subunit and determined its tissue localization using a mouse line expressing a GFP-tagged GABA(B1) subunit under the endogenous promoter. Immunostaining revealed GABA(B1) expression in both type I and type II ganglion cells and in their synaptic terminals under inner and outer hair cells, respectively. No GABA(B1) expression was observed in hair cells. Mean cochlear thresholds, measured via both auditory brainstem responses and distortion product otoacoustic emissions (DPOAEs), were elevated by approximately 10 dB in GABA(B1)-deficient mice, consistent with outer hair cell dysfunction. Olivocochlear efferent function, assessed via DPOAE suppression during efferent electrical stimulation, was unaffected by GABA(B1) deletion. GABA(B1)-deficient mice showed increased resistance to permanent acoustic injury, with mean threshold shifts approximately 25 dB smaller than wild-types after exposure to 8-16-kHz noise at 100 dB for 2 h. In contrast, there was no vulnerability difference to temporary acoustic injury following exposure to the same noise at 94 dB for 15 min. Our results suggest that GABAergic signaling in type II afferent neurons may be required for normal outer hair cell amplifier function at low sound levels and may also modulate outer hair cell responses to high-level sound.

Cochlear amplification, outer hair cells and prestin

Mechanical amplification of acoustic signals is apparently a common feature of vertebrate auditory organs. In non-mammalian vertebrates amplification is produced by stereociliary processes, related to the mechanotransducer channel complex and probably to the phenomenon of fast adaptation. The extended frequency range of the mammalian cochlea has probably co-evolved with a novel hair cell type, the outer hair cell and its constituent membrane protein, prestin. Cylindrical outer hair cells are motile and their somatic length changes are voltage driven and powered by prestin. One of the central outstanding problems in mammalian cochlear neurobiology is the relation between the two amplification processes.

alphaII-betaV spectrin bridges the plasma membrane and cortical lattice in the lateral wall of the auditory outer hair cells

The sensitivity and frequency selectivity of the mammalian cochlea involves a mechanical amplification process called electromotility, which requires prestin-dependent length changes of the outer hair cell (OHC) lateral wall in response to changes in membrane electric potential. The cortical lattice, the highly organized cytoskeleton underlying the OHC lateral plasma membrane, is made up of F-actin and spectrin. Here, we show that alphaII and two of the five beta-spectrin subunits, betaII and betaV, are present in OHCs. betaII spectrin is restricted to the cuticular plate, a dense apical network of actin filaments, whereas betaV spectrin is concentrated at the cortical lattice. Moreover, we show that alphaII-betaV spectrin directly interacts with F-actin and band 4.1, two components of the OHC cortical lattice. betaV spectrin is progressively recruited into the cortical lattice between postnatal day 2 (P2) and P10 in the mouse, in parallel with prestin membrane insertion, which itself parallels the maturation of cell electromotility. Although betaV spectrin does not directly interact with prestin, we found that addition of lysates derived from mature auditory organs, but not from the brain or liver, enables betaV spectrin-prestin interaction. Using this assay, betaV spectrin, via its PH domain, indirectly interacts with the C-terminal cytodomain of prestin. We conclude that the cortical network involved in the sound-induced electromotility of OHCs contains alphaII-betaV spectrin, and not the conventional alphaII-betaII spectrin.

Linear and nonlinear processing in hair cells

Mechanosensory hair cells in the ear are exquisitely responsive to minute sensory inputs, nearly to the point of instability. Active mechanisms bias the transduction apparatus and subsequent electrical amplification away from saturation in either the negative or positive direction, to an operating point where the response to small signals is approximately linear. An active force generator coupled directly to the transducer enhances sensitivity and frequency selectivity, and counteracts energy loss to viscous drag. Active electrical amplification further enhances gain and frequency selectivity. In both cases, nonlinear properties may maintain the system close to instability, as evidenced by small spontaneous oscillations, while providing a compressive nonlinearity that increases the cell's operating range. Transmitter release also appears to be frequency selective and biased to operate most effectively near the resting potential. This brief overview will consider the resting stability of hair cells, and their responses to small perturbations that correspond to soft sounds or small accelerations.

Voltage-dependent outer hair cell stereocilia stiffness at acoustic frequencies

The aim of this report is to show the effects of voltage changes on stereocilia stiffness in mammalian outer hair cells (OHCs). With the OHC cuticular plate anchored at a microchamber tip, step voltage commands drove an OHC inside the microchamber to move freely while stereocilia were oscillated at 510 Hz by a constant fluid-jet force. With basolateral OHC depolarized and shortened, the amplitude of stereocilia motion was increased, suggesting a decrease in stereocilia stiffness. Such a decrease in stiffness may serve as an important adjusting factor inside the cochlear amplifying loop.

Medial olivocochlear efferent inhibition of basilar-membrane responses to clicks: evidence for two modes of cochlear mechanical excitation

Conceptualizations of mammalian cochlear mechanics are based on basilar-membrane (BM) traveling waves that scale with frequency along the length of the cochlea, are amplified by outer hair cells (OHCs), and excite inner hair cells and auditory-nerve (AN) fibers in a simple way. However, recent experimental work has shown medial-olivocochlear (MOC) inhibition of AN responses to clicks that do not fit with this picture. To test whether this AN-initial-peak (ANIP) inhibition might result from hitherto unrecognized aspects of the traveling-wave or MOC-evoked inhibition, MOC effects on BM responses to clicks in the basal turns of guinea pig and chinchilla cochleae were measured. MOC stimulation inhibited BM click responses in a time and level dependent manner. Inhibition was not seen during the first half-cycle of the responses, but built up gradually, and ultimately increased the responses' decay rates. MOC stimulation also produced small phase leads in the response wave forms, but had little effect on the instantaneous frequency or the waxing and waning of the responses. These data, plus recent AN data, support the hypothesis that the MOC-evoked inhibitions of the traveling wave and of the ANIP response are separate phenomena, and indicate that the OHCs can affect at least two separate modes of excitation in the mammalian cochlea.

Silencing the cochlear amplifier by immobilizing prestin

Achieving the exquisite sensitivity and frequency selectivity of the mammalian ear requires active amplification of input sound. In this issue of Neuron, Dallos and colleagues demonstrate that the molecular motor prestin, which drives shape changes in the soma of mechanosensory hair cells, underlies mechanical feedback mechanisms for sound amplification in mammals.

Prestin-based outer hair cell motility is necessary for mammalian cochlear amplification

It is a central tenet of cochlear neurobiology that mammalian ears rely on a local, mechanical amplification process for their high sensitivity and sharp frequency selectivity. While it is generally agreed that outer hair cells provide the amplification, two mechanisms have been proposed: stereociliary motility and somatic motility. The latter is driven by the motor protein prestin. Electrophysiological phenotyping of a prestin knockout mouse intimated that somatic motility is the amplifier. However, outer hair cells of knockout mice have significantly altered mechanical properties, making this mouse model unsatisfactory. Here, we study a mouse model without alteration to outer hair cell and organ of Corti mechanics or to mechanoelectric transduction, but with diminished prestin function. These animals have knockout-like behavior, demonstrating that prestin-based electromotility is required for cochlear amplification.

Interplay between TRP channels and the cytoskeleton in health and disease

Transient receptor potential (TRP) channels are a family of cation channels that play a key role in ion homeostasis and cell volume regulation. In addition, TRP channels are considered universal integrators of sensory information required for taste, vision, hearing, touch, temperature, and the detection of mechanical force. Seminal investigations exploring the molecular mechanisms of phototransduction in Drosophila have demonstrated that TRP channels operate within macromolecular complexes closely associated with the cytoskeleton. More recent evidence shows that mammalian TRP channels similarly connect to the cytoskeleton to affect cytoskeletal organization and cell adhesion via ion-transport-dependent and -independent mechanisms. In this review, we discuss new insights into the interplay between TRP channels and the cytoskeleton and provide recent examples of such interactions in different physiological systems.

Cochlear outer hair cell motility

Normal hearing depends on sound amplification within the mammalian cochlea. The amplification, without which the auditory system is effectively deaf, can be traced to the correct functioning of a group of motile sensory hair cells, the outer hair cells of the cochlea. Acting like motor cells, outer hair cells produce forces that are driven by graded changes in membrane potential. The forces depend on the presence of a motor protein in the lateral membrane of the cells. This protein, known as prestin, is a member of a transporter superfamily SLC26. The functional and structural properties of prestin are described in this review. Whether outer hair cell motility might account for sound amplification at all frequencies is also a critical question and is reviewed here.