Integrating the biophysical and molecular mechanisms of auditory hair cell mechanotransduction

Abstract

Mechanosensation is a primitive and somewhat ubiquitous sense. At the inner ear, sensory hair cells are refined to enhance sensitivity, dynamic range and frequency selectivity. Thirty years ago, mechanisms of mechanotransduction and adaptation were well accounted for by simple mechanical models that incorporated physiological and morphological properties of hair cells. Molecular and genetic tools, coupled with new optical techniques, are now identifying and localizing specific components of the mechanotransduction machinery. These new findings challenge long-standing theories, and require modification of old and development of new models. Future advances require the integration of molecular and physiological data to causally test these new hypotheses.

Figure 1. Introduction to the organ of Corti

(a) Cross-section of the organ of Corti pointing out the salient features relevant to hearing transduction. Hair bundles on the apical surface of inner hair cells (IHCs) and outer hair cells (OHCs) are bathed in endolymph, whereas the basolateral side of hair cells is bathed in perilymph. (b) Scanning electron microscopy image looking at the apical surface of hair cells with the tectorial membrane removed. IHC and OHC hair bundles are pseudo-coloured orange and red, respectively. Scale bar, 2 μm. (c) Enlargment of a schematic of the hair bundle and hair cell apical surface seen in a. Salient features of stereocilia rows that comprise the hair bundle are indicated. When hair bundles are stimulated, the stereocilia are sheared towards the tallest row of stereocilia, this is also defined as the positive direction of stimulation.

Figure 2. Hair bundles and MET vary between hair bundle types

(a) Three distinct hair bundle morphologies are presented (more subtle variations exist). Some stereocilia (blue) do not have functional mechanotransducer channels, whereas others (pink) have functional channels but no direct means of regulating slow adaptation. Orange stereocilia have both functional channels and can be indirectly modulated by slow adaptation. IHC, inner hair cell; OHC, outer hair cell. (b) A family of MET currents from an inner, outer and turtle auditory hair cell are shown, where the cells were voltage clamped at − 84 mV and the hair bundles deflected with a stiff rod between − 200 and 800 nm (scale bars are the same in all panels). Both fast and slow adaptations are faster in mammalian hair cells, while the slow component of adaptation is more prominent in the turtle (multi-rowed) hair bundle. (c) Plots the current-displacement function for the data in b showing that IHCs are less sensitive than other hair bundles. (d) Adaptation causes a shift in the activation curve as schematically represented here. An activation curve generated about the hair bundle’s resting position is depicted as the solid line, and an activation curve taken from a statically displaced hair bundle is depicted as the dashed line, demonstrating the extended dynamic range. BP, basilar papilla.

Figure 3. Channel gating and the gating spring model

(a) How force is translated to mechanotransducer channels remains to be resolved. At present there are four potential mechanisms, the channel can be tethered (depicted as attached directly to the tip link, although there could be intermediate molecules), internally, externally, both internally and externally, or not tethered at all but simply sensitive to membrane stretch. (b) The upper panel plots the activation curve for the MET channel, whereas the lower panel plots the comparable force-displacement plot. Dashed lines indicate when the channel is fully opened (blue) or fully closed (red). In the force-displacement plot, the reduced slope during the transition from closed to open state in the force-displacement plot underlies the gating spring theory of MET. Depicted in c the gating spring model posits that a gating element moves in series with the spring as the channel opens, thereby reducing tension to the spring transiently, with no required change in spring properties.

Figure 4. Hair bundle proteins

A variety of hair bundle proteins have been identified via genetic and proteomic technologies. These proteins are categorized by general functions into proteins associated with (a) Usher syndrome, (b) stereocilia links, (c) ankle link complex, (d) myosin motors, (e) actin regulation and (f) metabolism and homeostasis. (g) A similar grouping of proteins showing their onset (of detection) and maturation timeline (where applicable) illustrates how the bundle protein composition changes with developmental age. Also included in this timeline plot is the physiological measurements for the onset of MET.

Figure 5. Hair bundle mechanics reveal correlations with channel activation and adaptation

(a,b) Upper panels present a schematic of lower frequency (turtle and frog) and higher frequency (mammalian outer hair cell (OHC)) MET currents in response to flexible fibre stimulation. Middle panels present the corresponding hair bundle movements. In each case, the highlighted red curve represents an adaptation response in the current and its correlated movement. These panels further illustrate the correlation between mechanical movement in time and fast adaptation measured in current, suggesting that there is no time point during these measurements where adaptation is not occurring intimating that gating compliance cannot be temporally separated from adaptation. The coloured lines in the middle panel correlate with the lines on the force-displacement plots shown in the bottom panels showing the time course of changes in the force-displacement plots. Similar responses are observed in both high- and low-frequency animals over time, with the main difference between responses being that the mammalian response is much faster, so measurements may be limited by temporal resolution of the recording system. The dashed black line in the force-displacement plot represents an adapted response when compared with the blue line. The dashed grey line depicts negative stiffness, important because it may belie a hair bundle amplification mechanism and also because it challenges whether the gating compliance change is purely a gating phenomenon. (c) Illustrates a voltage-dependent hair bundle movement associated with the MET channel opening (black arrows) that requires functional tip links but not current flow (‘the flick’). (d) The currently uncharacterized ‘Sag’ response is schematized where strong depolarization can move the bundle away from the tall edge and even overshoot the original baseline position (dashed line), without a corresponding reduction in the MET current. This uncoupling of hair bundle position and MET current is unprecedented and may reflect active movements at the cuticular plate.

Figure 6. Hair bundle and mechanosensitivity develop along similar time frames

(a) Hair bundle development at five distinct stages is illustrated showing how the kinocilium (red), stereocilia (blue) and interciliary links (black) change at each time point. Depicted for apical hair bundles are the tuft stage (P0–P1), the establishment of polarity (P1–P2), the initial height increase (P3–P4), the thickening and pruning (P5–P6) and the final height increase (P7–P9). (b) Corresponding maturation of MET is schematized with responses to negative stimuli away from the tall row (red) and positive towards the tall row to open channels (black). Illustrated are the slow increase in current amplitude, the initial lack of directional sensitivity, and the increase in magnitude and kinetics of adaptation.