Compliance of the hair bundle associated with gating of mechanoelectrical transduction channels in the bullfrog's saccular hair cell

Mechanical stimuli are thought to open the transduction channels of a hair cell by tensing elastic components, the gating springs, that pull directly on the channels. To test this model, we measured the stiffness of hair bundles during mechanical stimulation. A bundle's compliance increased by about 40% at the position where half of the channels opened. This we attribute to conformational changes of transduction channels as they open and close. The magnitude and displacement dependence of the gating compliance provide quantitative information about the molecular basis of mechanoelectrical transduction: the force required to open each channel, the number of transduction channels per hair cell, the stiffness of a gating spring, and the swing of a channel's gate as it opens.     

ATPase activity of myosin in hair bundles of the bullfrog''s sacculus.

Mechanoelectrical transduction by a hair cell displays adaptation, which is thought to occur as myosin-based molecular motors within the mechanically sensitive hair bundle adjust the tension transmitted to transduction channels. To assess the enzymatic capabilities of the myosin isozymes in hair bundles, we examined the actin-dependent ATPase activity of bundles isolated from the bullfrog's sacculus. Separation of 32P-labeled inorganic phosphate from unreacted [gamma-32P]ATP by thin-layer chromatography enabled us to measure the liberation of as little as 0.1 fmol phosphate. To distinguish the Mg(2+)-ATPase activity of myosin isozymes from that of other hair-bundle enzymes, we inhibited the interaction of hair-bundle myosin with actin and determined the reduction in ATPase activity. N-ethylmaleimide (NEM) decreased neither physiologically measured adaptation nor the nucleotide-hydrolytic activity of a 120-kDa protein thought to be myosin 1 beta. The NEM-insensitive, actin-activated ATPase activity of myosin increased from 1.0 fmol x s-1 in 1 mM EGTA to 2.3 fmol x s-1 in 10 microM Ca2+. This activity was largely inhibited by calmidazolium, but was unaffected by the addition of exogenous calmodulin. These results, which indicate that hair bundles contain enzymatically active, Ca(2+)-sensitive myosin molecules, are consistent with the role of Ca2+ in adaptation and with the hypothesis that myosin forms the hair cell's adaptation motor.     

Eps8 regulates hair bundle length and functional maturation of mammalian auditory hair cells

Hair cells of the mammalian cochlea are specialized for the dynamic coding of sound stimuli. The transduction of sound waves into electrical signals depends upon mechanosensitive hair bundles that project from the cell's apical surface. Each stereocilium within a hair bundle is composed of uniformly polarized and tightly packed actin filaments. Several stereociliary proteins have been shown to be associated with hair bundle development and function and are known to cause deafness in mice and humans when mutated. The growth of the stereociliar actin core is dynamically regulated at the actin filament barbed ends in the stereociliary tip. We show that Eps8, a protein with actin binding, bundling, and barbed-end capping activities in other systems, is a novel component of the hair bundle. Eps8 is localized predominantly at the tip of the stereocilia and is essential for their normal elongation and function. Moreover, we have found that Eps8 knockout mice are profoundly deaf and that IHCs, but not OHCs, fail to mature into fully functional sensory receptors. We propose that Eps8 directly regulates stereocilia growth in hair cells and also plays a crucial role in the physiological maturation of mammalian cochlear IHCs. Together, our results indicate that Eps8 is critical in coordinating the development and functionality of mammalian auditory hair cells.     

Adaptation in auditory hair cells

The narrow stimulus limits of hair cell transduction, equivalent to a total excursion of about 100nm at the tip of the hair bundle, demand tight regulation of the mechanical input to ensure that the mechanoelectrical transducer (MET) channels operate in their linear range. This control is provided by multiple components of Ca(2+)-dependent adaptation. A slow mechanism limits the mechanical stimulus through the action of one or more unconventional myosins. There is also a fast, sub-millisecond, Ca(2+) regulation of the MET channel, which can generate resonance and confer tuning on transduction. Changing the conductance or kinetics of the MET channels can vary their resonant frequency. The tuning information conveyed in transduction may combine with the somatic motility of outer hair cells to produce an active process that supplies amplification and augments frequency selectivity in the mammalian cochlea.     

Sensory transduction: the 'swarm intelligence' of auditory hair bundles

In vertebrate hair cells, the hair bundle is responsible for the conversion of mechanical vibrations into electrical signals. In a combined experimental and computational tour de force, a group of researchers now presents a quantitative model that explains how the bundle's specific microarchitecture gives rise to its exquisite mechanosensory properties.     

The actions of calcium on hair bundle mechanics in mammalian cochlear hair cells

Sound stimuli excite cochlear hair cells by vibration of each hair bundle, which opens mechanotransducer (MT) channels. We have measured hair-bundle mechanics in isolated rat cochleas by stimulation with flexible glass fibers and simultaneous recording of the MT current. Both inner and outer hair-cell bundles exhibited force-displacement relationships with a nonlinearity that reflects a time-dependent reduction in stiffness. The nonlinearity was abolished, and hair-bundle stiffness increased, by maneuvers that diminished calcium influx through the MT channels: lowering extracellular calcium, blocking the MT current with dihydrostreptomycin, or depolarizing to positive potentials. To simulate the effects of Ca²⁺, we constructed a finite-element model of the outer hair cell bundle that incorporates the gating-spring hypothesis for MT channel activation. Four calcium ions were assumed to bind to the MT channel, making it harder to open, and, in addition, Ca²⁺ was posited to cause either a channel release or a decrease in the gating-spring stiffness. Both mechanisms produced Ca²⁺ effects on adaptation and bundle mechanics comparable to those measured experimentally. We suggest that fast adaptation and force generation by the hair bundle may stem from the action of Ca²⁺ on the channel complex and do not necessarily require the direct involvement of a myosin motor. The significance of these results for cochlear transduction and amplification are discussed.     

K+ and Ca++ currents in hair cells isolated from the semicircular canals of the frog]

Hair cells of the inner ear are endowed with different types of ionic channels. To characterize voltage- and ion-dependent channels in vestibular hair cells, experiments were performed in enzymatically isolated hair cells of frog semicircular canals by using the whole-cell configuration of the patch-clamp technique. A large outward current, identified as a K+ current, was recorded when 132 mM KCl were present in the pipette filling solution. It could be dissected pharmacologically into three different components. The first component, which was transient and selectively blocked by 10 mM external 4AP, is most likely an IA-type current. The second one, sensitive to 20 mM external TEA, might be a delayed rectifier K+ current, while the third component insensitive to TEA and showing faster activation time course has been interpreted as a K+ current of IKCa-type. After blocking the outward current by substituting Cs+ for K+ and adding 20 mM TEA to the internal solution, a sustained inward current, identified as a Ca++ current, could be recorded. This current did not inactivate, and was blocked by Cd++ more effectively than Ni++, thus suggesting the presence of Ca++ channels similar to the neuronal "L" channels. Since both K+ and Ca++ channels were recruited at potentials near the resting level, it is suggested that they are involved in the modulation of the resting as well as the evoked transmitter release from the basal pole of the hair cells.     

Nonlinear mechanical responses of mouse cochlear hair bundles.

The stiffness of sensory hair bundles of both inner (IHC) and outer (OHC) hair cells was measured with calibrated silica fibres in mouse cochlear cultures to test the hypothesis that the mechanical properties of the hair bundle reflect processes underlying mechanotransduction. For OHCs, the displacement of the hair bundle relaxed with time constants of 6 ms for displacements which open transducer channels and 4 ms for displacements which close the channels. The corresponding values of the time constants for IHCs were 10 ms and 8 ms, respectively. A displacement-dependent change in the stiffness of the hair bundle was not observed when the bundle was displaced orthogonally to the direction of excitation. The stiffness of the hair bundle as a function of nanometre displacements from the resting position was remarkably nonlinear. The stiffness declined to a minimum from the resting stiffness by about 12% for OHCs and 20% for IHCs when the hair bundle was displaced by about 20 nm in the excitatory direction, and it increased by a similar amount when the bundle was displaced by 20 nm in the inhibitory direction. The displacement at which the stiffness reached a minimum was within the most sensitive region of the hair-cell transducer function (receptor potential as a function of hair-bundle displacement), and the displacement at which the stiffness reached a maximum was at the point of saturation of the transducer function in the inhibitory direction. The nonlinear displacement-dependent compliance change is reversibly abolished, and the time constant of relaxation of the bundle for excitatory displacements is reversibly reduced, when mechanotransduction is blocked by the addition of either neomycin sulphate or cobalt chloride to the solution bathing the hair cells. The displacement-dependent compliance change was not apparently reduced when the receptor potential was attenuated through the substitution of sodium in the bathing solution with a less permeant cation, tetraethylammonium. These findings suggest that the nonlinear mechanical properties of the hair bundle are associated with aspects of the hair-cell mechanotransducer process. The mechanical properties of the hair bundle are discussed in relation to the 'gating-spring' hypothesis of hair-cell transduction.     

The calcium-activated potassium channels of turtle hair cells

A major factor determining the electrical resonant frequency of turtle cochlear hair cells is the time course of the Ca-activated K current (Art, J. J., and R. Fettiplace. 1987. Journal of Physiology. 385:207- 242). We have examined the notion that this time course is dictated by the K channel kinetics by recording single Ca-activated K channels in inside-out patches from isolated cells. A hair cell's resonant frequency was estimated from its known correlation with the dimensions of the hair bundle. All cells possess BK channels with a similar unit conductance of approximately 320 pS but with different mean open times of 0.25-12 ms. The time constant of relaxation of the average single- channel current at -50 mV in 4 microM Ca varied between cells from 0.4 to 13 ms and was correlated with the hair bundle height. The magnitude and voltage dependence of the time constant agree with the expected behavior of the macroscopic K(Ca) current, whose speed may thus be limited by the channel kinetics. All BK channels had similar sensitivities to Ca which produced half-maximal activation for a concentration of approximately 2 microM at +50 mV and 12 microM at -50 mV. We estimate from the voltage dependence of the whole-cell K(Ca) current that the BK channels may be fully activated at -35 mV by a rise in intracellular Ca to 50 microM. BK channels were occasionally observed to switch between slow and fast gating modes which raises the possibility that the range of kinetics of BK channels observed in different hair cells reflects a common channel protein whose kinetics are regulated by an unidentified intracellular factor. Membrane patches also contained 30 pS SK channels which were approximately 5 times more Ca-sensitive than BK channels at -50 mV. The SK channels may underlie the inhibitory synaptic potential produced in hair cells by efferent stimulation.     

The hair cell's transduction channel

The elusive transduction channel is the key player in mechanical transduction by the sensory hair cells of the inner ear. Multiple factors have thwarted molecular identification of this channel, including the lack of a definitive pharmacological signature, the paucity of hair cells, and the uniqueness of their transduction mechanism. At present, we are forced to speculate as to the transduction channel's identity; functional characteristics suggest, however, that it may well belong to transient receptor potential superfamily of ion channels.     

Putative immunolocalization of the mechanoelectrical transduction channels in mammalian cochlear hair cells.

Hair cells bear an apical bundle of stereocilia arranged in serried rows. Deflection of the bundle controls the opening and closing of mechanoelectrical transduction channels, thereby altering the conductance across the apical plasma membrane. Two locations for these channels have been proposed in the bundle, either near the bases of the stereocilia or towards their tips. One hypothesis that is consistent with the latter possibility suggests that fine extracellular filaments, which run between the tips of the shorter stereocilia and the sides of the taller stereocilia behind, operate the channels. Determining the precise position of the channels is essential to test this hypothesis. We have therefore attempted to localize them immunocytochemically. Because hair-cell transduction is amiloride sensitive, the channels may have an amiloride-binding site associated with them. We have therefore used a polyclonal antibody raised against another amiloride-sensitive ion channel to hunt for them. This antibody recognizes a 62-64 kDa band in immunoblots of cochlear tissue, and produces discrete labelling in the hair bundle. This is most concentrated just below the tips of the shorter stereocilia, coinciding with a region of specialization in the closely apposed membranes of the short and tall stereocilia but not with either end of the tip link.     

Prestin-driven cochlear amplification is not limited by the outer hair cell membrane time constant

Outer hair cells (OHCs) provide amplification in the mammalian cochlea using somatic force generation underpinned by voltage-dependent conformational changes of the motor protein prestin. However, prestin must be gated by changes in membrane potential on a cycle-by-cycle basis and the periodic component of the receptor potential may be greatly attenuated by low-pass filtering due to the OHC time constant (τ(m)), questioning the functional relevance of this mechanism. Here, we measured τ(m) from OHCs with a range of characteristic frequencies (CF) and found that, at physiological endolymphatic calcium concentrations, approximately half of the mechanotransducer (MT) channels are opened at rest, depolarizing the membrane potential to near -40 mV. The depolarized resting potential activates a voltage-dependent K+ conductance, thus minimizing τ(m) and expanding the membrane filter so there is little receptor potential attenuation at the cell's CF. These data suggest that minimal τ(m) filtering in vivo ensures optimal activation of prestin.     

Cells resembling hair cells in developing rat olfactory and nasal respiratory epithelia

A hitherto ignored microvillous cell type, distinct from microvillous supporting cells and other microvillous cell types, was encountered in olfactory and respiratory epithelia of nasal turbinates of rat fetuses, near the transition between these two epithelia. The apex of the cell resembles the apices of vestibular hair cells. The cell has a cone-shaped bundle of microvilli, resembling the complex bundle of hair-cell stereocilia, accompanied by a cilium. Therefore we called this cell type the nasal hair cell. Cilium and microvilli seemed adhered. Cell numbers were very low, up to about 5 per turbinate. The cell's appearance is precocious compared to that of olfactory receptor and supporting cells. Also, while the apices of olfactory receptor and supporting cells and of ciliated respiratory cells underwent major morphological maturation during the developmental period from embryonic day 16 to day 21, the apical structures of the nasal hair cell only changed marginally from embryonic day 16, when they were first seen, through to at least embryonic day 21. The cell's location and precociously mature appearance suggests that it plays a special role in the development of nasal epithelia.     

Tip-link integrity and mechanical transduction in vertebrate hair cells.

An attractive hypothesis for hair-cell transduction is that fine, filamentous "tip links" pull directly on mechanically sensitive ion channels located at the tips of the stereocilia. We tested the involvement of tip links in the transduction process by treating bundles with a BAPTA-buffered, low-Ca2+ saline (10(-9) M). BAPTA abolished the transduction current in a few hundred milliseconds. BAPTA treatment for a few seconds eliminated the tip links observed by either scanning or transmission electron microscopy. BAPTA also eliminated the voltage-dependent movement and caused a positive bundle displacement of 133 nm, in quantitative agreement with a model for regulation of tension. We conclude that tip links convey tension to the transduction channels of hair cells.     

Efferent regulation of hair cells in the turtle cochlea

Intracellular recordings were made from hair cells in the isolated cochlea of the turtle to characterize the inhibition achieved by the cochlea's efferent innervation. A short train of shocks delivered to the efferent axons produced in the hair cells slow hyperpolarizing synaptic potentials which could be reversed by shifting the membrane potential more negative than about -80 mV. Throughout the efferent hyperpolarization, there was a reduction of up to 25-fold in the amplitude of the receptor potential for tones presented at the hair cell's characteristic frequency. Efferent stimulation also was shown to degrade the cell's tuning properties. It is argued that the combined effects of the hyperpolarization and the loss in hair cell sensitivity could account for a threshold elevation of at least 70 dB in the auditory nerve fibres.     

A virtual hair cell, I: addition of gating spring theory into a 3-D bundle mechanical model

We have developed a virtual hair cell that simulates hair cell mechanoelectrical transduction in the turtle utricle. This study combines a full three-dimensional hair bundle mechanical model with a gating spring theory. Previous mathematical models represent the hair bundle with a single degree of freedom system which, we have argued, cannot fully explain hair bundle mechanics. In our computer model, the tip link tension and fast adaptation modulator kinetics determine the opening and closing of each channel independently. We observed the response of individual transduction channels with our presented model. The simulated results showed three features of hair cells in vitro. First, a transient rebound of the bundle tip appeared when fast adaptation dominated the dynamics. Second, the dynamic stiffness of the bundle was minimized when the response-displacement (I-X) curve was steepest. Third, the hair cell showed "polarity", i.e., activation decreased from a peak to zero as the forcing direction rotated from the excitatory to the inhibitory direction.     

Ion channels for the mechano-electrical transduction and efferent synapse of the hair cell

Auditory and vestibular information is applied to the hair cell hair bundle as mechanical energy, and is transduced into electrical energy by gating ion channels. The m-e.t. channel has a unitary conductance of 50 pS and a broad selectivity to monovalent cations and to divalent cations. Ca ions are the most permeable through the channel. The angular displacement of the hair bundle is the primary gating factor. Circumstantial evidence indicates the possibility of the direct gating of channels by the membrane deformation itself. The transduction potential activates voltage gated Ca channel and leads to the release of neurotransmitters which activate afferent neurones. Cholinergic muscarinic receptors likely mediate the inhibitory efferent innervation to the hair cell.     

A developmental model for generating frequency maps in the reptilian and avian cochleas.

Hair cells in the turtle cochlea are frequency-tuned by a mechanism involving the combined activation of voltage-sensitive Ca2+ channels and Ca(2+)-activated K+ (KCa) channels. The main determinants of a hair cell's characteristic frequency (Fo) are the KCa channels' density and kinetics, both of which change systematically with location in the cochlea in conjunction with the observed frequency map. We have developed a model based on the differential expression of two KCa channel subunits, which when accompanied by concurrent changes in other properties (e.g., density of Ca2+ channels and inwardly rectifying K+ channels), will generate sharp tuning at frequencies from 40 to 600 Hz. The kinetic properties of the two subunits were derived from previous single-channel analysis, and it was assumed that the subunits (A and B) combine to form five species of tetrameric channel (A4, A3B, A2B2, AB3, and B4) with intermediate kinetics and overlapping distribution. Expression of KCa and other channels was assumed to be regulated by diffusional gradients in either one or two chemicals. The results are consistent with both current- and voltage-clamp data on turtle hair cells, and they show that five channel species are sufficient to produce smooth changes in both Fo and kinetics of the macroscopic KCa current. Other schemes for varying KCa channel kinetics are examined, including one that allows extension of the model to the chick cochlea to produce hair cells with Fo's from 130 to 4000 Hz. A necessary assumption in all models is a gradient in the values of the parameters identified with the cell's cytoplasmic Ca2+ buffer.     

Vertebrate and invertebrate TRPV-like mechanoreceptors

Our senses of touch, hearing, and balance are mediated by mechanosensitive ion channels. In vertebrates, little is known about the molecular composition of these mechanoreceptors, an example of which is the transduction channel of the inner ear's receptor cells, hair cells. Members of the TRP family of ion channels are considered candidates for the vertebrate hair cell's mechanosensitive transduction channel and here we review the evidence for this candidacy. We start by examining the results of genetic screens in invertebrates that identified members of the TRP gene family as core components of mechanoreceptors. In particular, we discuss the Caenorhabditis elegans OSM-9 channel, an invertebrate TRPV channel, and the Drosophila melanogaster TRP channel NOMPC. We then evaluate basic features of TRPV4, a vertebrate member of the TRPV subfamily, which is gated by a variety of physical and chemical stimuli including temperature, osmotic pressure, and ligands. Finally, we compare the characteristics of all discussed mechanoreceptive TRP channels with the biophysical characteristics of hair cell mechanotransduction, speculating about the possible make-up of the elusive inner ear mechanoreceptor.     

Two adaptation processes in auditory hair cells together can provide an active amplifier

The hair cells of the vertebrate inner ear convert mechanical stimuli to electrical signals. Two adaptation mechanisms are known to modify the ionic current flowing through the transduction channels of the hair bundles: a rapid process involves Ca(2+) ions binding to the channels; and a slower adaptation is associated with the movement of myosin motors. We present a mathematical model of the hair cell which demonstrates that the combination of these two mechanisms can produce "self-tuned critical oscillations", i.e., maintain the hair bundle at the threshold of an oscillatory instability. The characteristic frequency depends on the geometry of the bundle and on the Ca(2+) dynamics, but is independent of channel kinetics. Poised on the verge of vibrating, the hair bundle acts as an active amplifier. However, if the hair cell is sufficiently perturbed, other dynamical regimes can occur. These include slow relaxation oscillations which resemble the hair bundle motion observed in some experimental preparations.