Prestin and the cochlear amplifier

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.

In vivo impedance of the gerbil cochlear partition at auditory frequencies

The specific acoustic impedance of the cochlear partition was measured from 4 to 20 kHz in the basal turn of the gerbil cochlea, where the best frequency is approximately 40 kHz. The acoustic impedance was found as the ratio of driving pressure to velocity response. It is the physical attribute that governs cochlear mechanics and has never before been directly measured, to our knowledge. The basilar membrane velocity was measured through the transparent round window membrane. Simultaneously, the intracochlear pressure was measured close to the stapes and quite close to the cochlear partition. The impedance phase was close to -90 degrees and the magnitude decreased with frequency, consistent with stiffness-dominated impedance. The resistive component of the impedance was relatively small. Usually the resistance was negative at frequencies below 8 kHz; this unexpected finding might be due to other vibration modes within the cochlear partition.

Cysteine mutagenesis reveals transmembrane residues associated with charge translocation in prestin

The solute carrier transmembrane protein prestin (SLC26A5) drives an active electromechanical transduction process in cochlear outer hair cells that increases hearing sensitivity and frequency discrimination in mammals. A large intramembraneous charge movement, the nonlinear capacitance (NLC), is the electrical signature of prestin function. The transmembrane domain (TMD) helices and residues involved in the intramembrane charge displacement remain unknown. We have performed cysteine-scanning mutagenesis with serine or valine replacement to investigate the importance of cysteine residues to prestin structure and function. The distribution of oligomeric states and membrane abundance of prestin was also probed to investigate whether cysteine residues participate in prestin oligomerization and/or NLC. Our results reveal that 1) Cys-196 (TMD 4) and Cys-415 (TMD 10) do not tolerate serine replacement, and thus maintaining hydrophobicity at these locations is important for the mechanism of charge movement; 2) Cys-260 (TMD 6) and Cys-381 (TMD 9) tolerate serine replacement and are probably water-exposed; and 3) if disulfide bonds are present, they do not serve a functional role as measured via NLC. These novel findings are consistent with a recent structural model, which proposes that prestin contains an occluded aqueous pore, and we posit that the orientations of transmembrane domain helices 4 and 10 are essential for proper prestin function.

An adaptive model simulating the somatic motility and the active hair bundle motion of the OHC

The outer hair cells (OHC) of the mammalian inner ear change the sensitivity and frequency selectivity of the filtering system of the cochlea using two kinds of mechanical activity: the somatic motility and the active hair bundle motion. We designed a non-linear adaptive model of the OHC employing both mechanisms of the mechanical activity. The modeling results show that the high sensitivity and frequency selectivity of the filtering system of the cochlea depend on the somatic motility of the OHC. However, both mechanisms of mechanical activity are involved in the adaptation to sound intensity and efferent-synaptic influence. The fast (alternating) component (AC) of the mechanical-electrical transduction signal controls the motor protein prestin and fast changes in axial length of the cell. The slow (direct) component (DC) appearing at high signal intensity affects the axial stiffness, the cell length and the position of the hair bundle. The efferent influence is realized by the same mechanism.

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.

Structural rearrangements of the motor protein prestin revealed by fluorescence resonance energy transfer

Prestin is a membrane protein expressed in the outer hair cells (OHCs) in the cochlea that is essential for hearing. This unique motor protein transduces a change in membrane potential into a considerable mechanical force, which leads to a cell length change in the OHC. The nonlinear capacitance in cells expressing prestin is recognized to reflect the voltage-dependent conformational change of prestin, of which its precise nature remains unknown. In the present work, we aimed to detect the conformational changes of prestin by a fluorescence resonance energy transfer (FRET)-based technique. We heterologously expressed prestin labeled with fluorophores at the COOH- or NH(2)-terminus in human embryonic kidney-293T cells, and monitored FRET changes on depolarization-inducing high KCl application. We detected a significant decrease in intersubunit FRET both between the COOH-termini and between the COOH- and NH(2)-termini. A similar FRET decrease was observed when membrane potential was directly and precisely controlled by simultaneous patch clamp. Changes in FRET were suppressed by either of two treatments known to abolish nonlinear capacitance, V499G/Y501H mutation and sodium salicylate. Our results are consistent with significant movements in the COOH-terminal domain of prestin upon change in membrane potential, providing the first dynamic information on its molecular rearrangements.

DNA sequence analysis of SLC26A5, encoding prestin, in a patient-control cohort: identification of fourteen novel DNA sequence variations

BACKGROUND

Prestin, encoded by the gene SLC26A5, is a transmembrane protein of the cochlear outer hair cell (OHC). Prestin is required for the somatic electromotile activity of OHCs, which is absent in OHCs and causes severe hearing impairment in mice lacking prestin. In humans, the role of sequence variations in SLC26A5 in hearing loss is less clear. Although prestin is expected to be required for functional human OHCs, the clinical significance of reported putative mutant alleles in humans is uncertain.

METHODOLOGY/PRINCIPAL FINDINGS

To explore the hypothesis that SLC26A5 may act as a modifier gene, affecting the severity of hearing loss caused by an independent etiology, a patient-control cohort was screened for DNA sequence variations in SLC26A5 using sequencing and allele specific methods. Patients in this study carried known pathogenic or controversial sequence variations in GJB2, encoding Connexin 26, or confirmed or suspected sequence variations in SLC26A5; controls included four ethnic populations. Twenty-three different DNA sequence variations in SLC26A5, 14 of which are novel, were observed: 4 novel sequence variations were found exclusively among patients; 7 novel sequence variations were found exclusively among controls; and, 12 sequence variations, 3 of which are novel, were found in both patients and controls. Twenty-one of the 23 DNA sequence variations were located in non-coding regions of SLC26A5. Two coding sequence variations, both novel, were observed only in patients and predict a silent change, p.S434S, and an amino acid substitution, p.I663V. In silico analysis of the p.I663V amino acid variation suggested this variant might be benign. Using Fisher's exact test, no statistically significant difference was observed between patients and controls in the frequency of the identified DNA sequence variations. Haplotype analysis using HaploView 4.0 software revealed the same predominant haplotype in patients and controls and derived haplotype blocks in the patient-control cohort similar to those generated from the International HapMap Project.

CONCLUSIONS/SIGNIFICANCE

Although these data fail to support a hypothesis that SLC26A5 acts as a modifier gene of GJB2-related hearing loss, the sample size is small and investigation of a larger population might be more informative. The 14 novel DNA sequence variations in SLC26A5 reported here will serve as useful research tools for future studies of prestin.

Contralateral Acoustic Stimulation Modulates Low-Frequency Biasing of DPOAE: Efferent Influence on Cochlear Amplifier Operating State?

The mammalian efferent medial olivocochlear system modulates active amplification of low-level sounds in the cochlea. Changes of the cochlear amplifier can be monitored by distortion product otoacoustic emissions (DPOAEs). The quadratic distortion product f2–f1 is known to be sensitive to changes in the operating point of the amplifier transfer function. We investigated the effect of contralateral acoustic stimulation (CAS), known to elicit efferent activity, on DPOAEs in the gerbil. During CAS, a significant increase of the f2–f1 level occurred already at low contralateral noise levels (20 dB SPL), whereas 2f1–f2 was much less affected. The effect strength depended on the CAS level and as shown in experiments with pure tones on the frequency of the contralateral stimulus. In a second approach, we biased the position of the cochlear partition and thus the cochlear amplifier operating point periodically by a ipsilateral low-frequency tone, which resulted in a phase-related amplitude modulation of f2–f1. This modulation pattern was changed considerably during contralateral noise stimulation, in dependence on the noise level. The experimental results were in good agreement with a simple model of distortion product generation and suggest that the olivocochlear efferents might change the operating state of cochlear amplification.

Ion imaging in the cochlear hair cells

Regulation of important cellular functions via signaling pathways is a fundamental property of the cell. Intracellular Ca(2+) is probably a best known second messenger in cell biology. In mechanosensory cells of the inner ear, the hair cells, intracellular Ca(2+) participates in a variety of functions including mechano-electrical transduction, synaptic transmission, and efferent regulation of the outer hair cells, one of two types of hair cells in the mammalian cochlea. The outer hair cells are responsible for the amplification of sound-induced vibrations within the cochlea, which determines the sensitivity of mammalian hearing. Besides Ca(2+), another intracellular ion, Cl(-) may have very specific function in the same outer hair cells. Intracellular Cl(-) is required for the motor function of prestin, a unique plasma membrane molecular motor of these cells. The goal of this article is to review practical aspects of the techniques suitable for imaging of Ca(2+) and Cl(-) in live mammalian cochlear hair cells.

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.

Sensitive response to low-frequency cochlear distortion products in the auditory midbrain

During auditory stimulation with several frequency components, distortion products (DPs) are generated as byproduct of nonlinear cochlear amplification. After generated, DP energy is reemitted into the ear channel where it can be measured as DP otoacoustic emission (DPOAE), and it also induces an excitatory response at cochlear places related to the DP frequencies. We measured responses of 91 inferior colliculus (IC) neurons in the gerbil during two-tone stimulation with frequencies well above the unit's receptive field but adequate to generate a distinct distortion product (f2-f1 or 2f1-f2) at the unit's characteristic frequency (CF). Neuronal responses to DPs could be accounted for by the simultaneously measured DPOAEs for DP frequencies >1.3 kHz. For DP frequencies <1.3 kHz (n = 25), there was a discrepancy between intracochlear DP magnitude and DPOAE level, and most neurons responded as if the intracochlear DP level was significantly higher than the DPOAE level in the ear channel. In 12% of those low-frequency neurons, responses to the DPs could be elicited even if the stimulus tone levels were below the threshold level of the neuron at CF. High intracochlear f2-f1 and 2f1-f2 DP-levels were verified by cancellation of the neuronal DP response with a third phase-adjusted tone stimulus at the DP frequency. A frequency-specific reduction of middle ear gain at low frequencies is possibly involved in the reduction of DPOAE level. The results indicate that pitch-related properties of complex stimuli may be produced partially by high intracochlear f2-f1 distortion levels.

Primary processes in sensory cells: current advances

In the course of evolution, the strong and unremitting selective pressure on sensory performance has driven the acuity of sensory organs to its physical limits. As a consequence, the study of primary sensory processes illustrates impressively how far a physiological function can be improved if the survival of a species depends on it. Sensory cells that detect single-photons, single molecules, mechanical motions on a nanometer scale, or incredibly small fluctuations of electromagnetic fields have fascinated physiologists for a long time. It is a great challenge to understand the primary sensory processes on a molecular level. This review points out some important recent developments in the search for primary processes in sensory cells that mediate touch perception, hearing, vision, taste, olfaction, as well as the analysis of light polarization and the orientation in the Earth's magnetic field. The data are screened for common transduction strategies and common transduction molecules, an aspect that may be helpful for researchers in the field.

Hearing mechanics: a fly in your ear

Recent work suggests that the auditory organ of Drosophila may serve as an excellent model system for understanding the complex mechanical signal processing that takes place in sensory hair cells of the vertebrate inner ear.

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.

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.

Reciprocal synapses between outer hair cells and their afferent terminals: evidence for a local neural network in the mammalian cochlea

Cochlear outer hair cells (OHCs) serve both as sensory receptors and biological motors. Their sensory function is poorly understood because their afferent innervation, the type-II spiral ganglion cell, has small unmyelinated axons and constitutes only 5% of the cochlear nerve. Reciprocal synapses between OHCs and their type-II terminals, consisting of paired afferent and efferent specialization, have been described in the primate cochlea. Here, we use serial and semi-serial-section transmission electron microscopy to quantify the nature and number of synaptic interactions in the OHC area of adult cats. Reciprocal synapses were found in all OHC rows and all cochlear frequency regions. They were more common among third-row OHCs and in the apical half of the cochlea, where 86% of synapses were reciprocal. The relative frequency of reciprocal synapses was unchanged following surgical transection of the olivocochlear bundle in one cat, confirming that reciprocal synapses were not formed by efferent fibers. In the normal ear, axo-dendritic synapses between olivocochlear terminals and type-II terminals and/or dendrites were as common as synapses between olivocochlear terminals and OHCs, especially in the first row, where, on average, almost 30 such synapses were seen in the region under a single OHC. The results suggest that a complex local neuronal circuitry in the OHC area, formed by the dendrites of type-II neurons and modulated by the olivocochlear system, may be a fundamental property of the mammalian cochlea, rather than a curiosity of the primate ear. This network may mediate local feedback control of, and bidirectional communication among, OHCs throughout the cochlear spiral.

Evolutionary insights into the unique electromotility motor of mammalian outer hair cells

Prestin (SLC26A5) is the molecular motor responsible for cochlear amplification by mammalian cochlea outer hair cells and has the unique combined properties of energy-independent motility, voltage sensitivity, and speed of cellular shape change. The ion transporter capability, typical of SLC26A members, was exchanged for electromotility function and is a newly derived feature of the therian cochlea. A putative minimal essential motif for the electromotility motor (meEM) was identified through the amalgamation of comparative genomic, evolution, and structural diversification approaches. Comparisons were done among nonmammalian vertebrates, eutherian mammalian species, and the opossum and platypus. The opossum and platypus SLC26A5 proteins were comparable to the eutherian consensus sequence. Suggested from the point-accepted mutation analysis, the meEM motif spans all the transmembrane segments and represented residues 66-503. Within the eutherian clade, the meEM was highly conserved with a substitution frequency of only 39/7497 (0.5%) residues, compared with 5.7% in SLC26A4 and 12.8% in SLC26A6 genes. Clade-specific substitutions were not observed and there was no sequence correlation with low or high hearing frequency specialists. We were able to identify that within the highly conserved meEM motif two regions, which are unique to all therian species, appear to be the most derived features in the SLC26A5 peptide.

Theoretical conditions for high-frequency hair bundle oscillations in auditory hair cells

Substantial evidence exists for spontaneous oscillations of hair cell stereociliary bundles in the lower vertebrate inner ear. Since the oscillations are larger than expected from Brownian motion, they must result from an active process in the stereociliary bundle suggested to underlie amplification of the sensory input as well as spontaneous otoacoustic emissions. However, their low frequency (<100 Hz) makes them unsuitable for amplification in birds and mammals that hear up to 5 kHz or higher. To examine the possibility of high-frequency oscillations, we used a finite-element model of the outer hair cell bundle incorporating previously measured mechanical parameters. Bundle motion was assumed to activate mechanotransducer channels according to the gating spring hypothesis, and the channels were regulated adaptively by Ca(2+) binding. The model generated oscillations of freestanding bundles at 4 kHz whose sharpness of tuning depended on the mechanotransducer channel number and location, and the Ca(2+) concentration. Entrainment of the oscillations by external stimuli was used to demonstrate nonlinear amplification. The oscillation frequency depended on channel parameters and was increased to 23 kHz principally by accelerating Ca(2+) binding kinetics. Spontaneous oscillations persisted, becoming very narrow-band, when the hair bundle was loaded with a tectorial membrane mass.

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.

A role for membrane potential in regulating GPCRs?

G-protein-coupled receptors (GPCRs) have ubiquitous roles in transducing extracellular signals into cellular responses. Therefore, the concept that members of this superfamily of surface proteins are directly modulated by changes in membrane voltage could have widespread consequences for cell signalling. Although several studies have indicated that GPCRs can be voltage dependent, particularly P2Y(1) receptors in the non-excitable megakaryocyte, the evidence has been mostly indirect. Recent work on muscarinic receptors has stimulated substantial interest in this field by reporting the first voltage-dependent charge movements for a GPCR. An underlying mechanism is proposed whereby a voltage-induced conformational change in the receptor alters its ability to couple to the G protein and thereby influences its affinity for an agonist. We discuss the strength of the evidence behind this hypothesis and include suggestions for future work. We also describe other examples in which direct voltage control of GPCRs can account for effects of membrane potential on downstream signals and highlight the possible physiological consequences of this phenomenon.

The dimensions and composition of stereociliary rootlets in mammalian cochlear hair cells: comparison between high- and low-frequency cells and evidence for a connection to the lateral membrane

The sensory bundle of vertebrate cochlear hair cells consists of actin-containing stereocilia that are thought to bend at their ankle during mechanical stimulation. Stereocilia have dense rootlets that extend through the ankle region to anchor them into the cuticular plate. Because this region may be important in bundle stiffness and durability during prolonged stimulation at high frequencies, we investigated the structure and dimensions of rootlets relative to the stereocilia in apical (low-frequency) and basal (high-frequency) regions of rodent cochleae using light and electron microscopy. Their composition was investigated using postembedding immunogold labeling of tropomyosin, spectrin, beta-actin, gamma-actin, espin, and prestin. The rootlets have a thick central core that widens at the ankle, and are embedded in a filamentous meshwork in the cuticular plate. Within a particular frequency region, rootlet length correlates with stereociliary height but between regions it changes disproportionately; apical stereocilia are, thus, approximately twice the height of basal stereocilia in equivalent rows, but rootlet lengths increase much less. Some rootlets contact the tight junctions that underlie the ends of the bundle. Rootlets contain spectrin, tropomyosin, and beta- and gamma-actin, but espin was not detected; spectrin is also evident near the apical and junctional membranes, whereas prestin is confined to the basolateral membrane below the junctions. These data suggest that rootlets strengthen the ankle region to provide durability and may contact with the lateral wall either to give additional anchoring of the stereocilia or to provide a route for interactions between the bundle and the lateral wall.

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.

Calcium- and otoferlin-dependent exocytosis by immature outer hair cells

Immature cochlear outer hair cells (OHCs) make transient synaptic contacts (ribbon synapses) with type I afferent nerve fibers, but direct evidence of synaptic vesicle exocytosis is still missing. We thus investigated calcium-dependent exocytosis in murine OHCs at postnatal day 2 (P2)-P3, a developmental stage when calcium current maximum amplitude was the highest. By using time-resolved patch-clamp capacitance measurements, we show that voltage step activation of L-type calcium channels triggers fast membrane capacitance increase. Capacitance increase displayed two kinetic components, which are likely to reflect two functionally distinct pools of synaptic vesicles, a readily releasable pool (RRP; tau = 79 ms) and a slowly releasable pool (tau = 870 ms). The RRP size and maximal release rate were estimated at approximately 1200 vesicles and approximately 15,000 vesicles/s, respectively. In addition, we found a linear relationship between capacitance increase and calcium influx, like in mature inner hair cells (IHCs). These results give strong support to the existence of efficient calcium-dependent neurotransmitter release in immature OHCs. Moreover, we show that immature OHCs, just like immature IHCs, are able to produce regenerative calcium-dependent action potentials that could trigger synaptic exocytosis in vivo. Finally, the evoked membrane capacitance increases were abolished in P2-P3 OHCs from mutant Otof-/- mice defective for otoferlin, despite normal calcium currents. We conclude that otoferlin, the putative major calcium sensor at IHC ribbon synapses, is essential to synaptic exocytosis in immature OHCs too.

Physiologically inspired signal preprocessing for auditory prostheses: Insights from the electro-motility of the OHC

We designed a non-linear functional model of the outer hair cell (OHC) functioning in the filtering system of the cochlea and then isolated from it two second-order structures, one employing the mechanism of the somatic motility and the other the hair bundle motion of the OHC. The investigation of these circuits showed that the main mechanism increasing the sensitivity and frequency selectivity of the filtering system is the somatic motility. The mechanism of the active hair bundle motion appeared less suitable for realization of the band-pass filtering structures due to the dependence of the sensitivity, natural frequency and selectivity on the signal intensity. We combined three second-order filtering structures employing the mechanism of the somatic motility and the lateral inhibition to form a parallel-type filtering channel of the sixth order with the frequency characteristics of the Butterworth-type and Gaussian-type. The investigation of these channels showed that the Gaussian-type channel has the advantage over the Butterworth-type channel. It is more suitable for realization of a filter bank with common lateral circuits and has less distorted frequency characteristic in the nonlinear mode.

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.

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.

The structural and functional differentiation of hair cells in a lizard's basilar papilla suggests an operational principle of amniote cochleas

The hair cells in the mammalian cochlea are of two distinct types. Inner hair cells are responsible for transducing mechanical stimuli into electrical responses, which they forward to the brain through a copious afferent innervation. Outer hair cells, which are thought to mediate the active process that sensitizes and tunes the cochlea, possess a negligible afferent innervation. For every inner hair cell, there are approximately three outer hair cells, so only one-quarter of the hair cells directly deliver information to the CNS. Although this is a surprising feature for a sensory system, the occurrence of a similar innervation pattern in birds and crocodilians suggests that the arrangement has an adaptive value. Using a lizard with highly developed hearing, the tokay gecko, we demonstrate in the present study that the same principle operates in a third major group of terrestrial animals. We propose that the differentiation of hair cells into signaling and amplifying classes reflects incompatible strategies for the optimization of mechanoelectrical transduction and of an active process based on active hair-bundle motility.

A mechanism for active hearing

The remarkable sensitivity, frequency selectivity, and nonlinearity of the cochlea have been attributed to the putative 'cochlear amplifier', which consumes metabolic energy to amplify the cochlear mechanical response to sounds. Recent studies have demonstrated that outer hair cells actively generate force using somatic electromotility and active hair-bundle motion. However, the expected power gain of the cochlear amplifier has not been demonstrated experimentally, and the measured location of cochlear nonlinearity is inconsistent with the predicted location of the cochlear amplifier. We instead propose a 'cochlear transformer' mechanism to interpret cochlear performance.

Contribution of outer hair cell bending to stereocilium deflection in the cochlea

The outer hair cell (OHC) in the cochlea is believed to actively enhance the cochlear sensitivity and frequency selectivity. Besides the well-known axial length change of the OHC, the bending mode of the OHC may also contribute to the stereocilium deflection. To investigate the contribution of the OHC bending to the stereocilium deflection, and the active process in the cochlea, we develop a simple kinematic model of the organ of Corti, consisting of the reticular lamina, the stereocilia and tectorial membrane. The electrically evoked axial length change and bending of the OHC are simulated, and their contributions to the stereocilium deflection are obtained. At the apical turn of the cochlea, the bending mode of the OHC results in stereocilium deflection comparable to that due to the axisymmetric length change of the OHC. At the basal turn, the contribution of the bending mode to the stereocilium deflection becomes insignificant compared to that of the axisymmetric mode.

Ca2+ signaling in the inner ear

The inner ear contains delicate sensory receptors that have adapted to detect the minutest mechanical disturbances. Ca(2+) ions are implicated in all steps of the transduction process, as well as in its regulation by an impressive ensemble of finely tuned feedback control mechanisms. Recent studies have unveiled some of the key players, but things do not sound quite right yet.

Application of fluorescence recovery after photobleaching to study prestin lateral mobility in the human embryonic kidney cell

The transmembrane protein prestin is crucial to outer hair cell (OHC) electromotility and contributes to the sensitivity and frequency selectivity of mammalian hearing. The molecular mechanisms of electromotility remain unclear, but prestin is purported to function as both a voltage sensor and a molecular motor. Understanding the role of prestin requires characterizing its organization and behavior in the plasma membrane. Fluorescence recovery after photobleaching (FRAP) provides a powerful means to quantitatively study molecular diffusion. However, OHCs are inherently fragile ex vivo, and dynamic studies of prestin require model systems, such as human embryonic kidney (HEK) cells, expressing fluorescently labeled prestin. Utilizing this system, we provide the first direct, quantitative measurement of prestin lateral mobility. The results show remarkably different diffusion behavior for prestin-green fluorescent protein (GFP) as compared to a control protein, human somatostatin receptor 5 (SSTR5). Prestin-GFP FRAP experiments reveal immobile fractions approaching 50%, low effective diffusion coefficients, and recovery times slower than those of SSTR5. Secondary bleaching of a region reveals distinctly different diffusion parameters, which we propose reflect the transient confinement of prestin in the HEK cell. Although uncharacterized, intermolecular interactions between prestin and the membrane and/or cytoskeleton may be important for the unique properties of prestin in electromotile OHCs.