Mechanotransduction by hair cells: models, molecules, and mechanisms

Abstract

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.

Figure 1. Anatomy of the Hair Bundle and the Transduction Apparatus

(A) Hair bundle of an isolated bullfrog hair cell, labeled with phalloidin to highlight F-actin. (B) Key hair-bundle structures overlaid on the image from (A). Hair bundles consist of actin-rich steroecilia and a microtubule-based kinocilium, not visible in (A). The kinocilium is not required for mechanotransuction and absent in mature cochlear hair cells. (C) Key molecules of the hair bundle. Protocadherin 15 (PCDH15) and cadherin 23 (CDH23) form kinociliary links between the kinocilium and the longest stereocilia, as well as the tip links that connect stereocilia. The very large G protein-coupled receptor 1 (VLGR1) and usherin are localized at the base of stereocilia, where they are thought to form ankle links. Ankle links are present in vestibular hair cells and transiently during development in mammalian auditory hair cells; because they lose the kinocilium, mammalian auditory hair cells also lose their kinocilial links. Myosin 6 (MYO6) is highly concentrated in the cuticular plate at the apical hair cells surface but is also localized to stereocilia. MYO7A is expressed throughout stereocilia and, in some auditory and vestibular epithelia, is enriched at ankle links. (D) Transmission electron micrograph of a stereocilia pair showing a single tip link. Image courtesy of R.A. Jacobs and A.J. Hudspeth. (E) Features of the tip link and its anchor points overlaid on image from (D). (F) Key molecules associated with the tip link. Note that MYO15A and whirlin extend beyond the lower tip-link density (LTLD), as they localize near the ends of all stereocilia actin filaments.

Figure 2. Structures of Key Hair-Bundle Proteins

The domain structure of molecules discussed in the Review is indicated. Abbreviations are as follows: CC, coiled-coil domain; FERM, protein 4.1-ezrin-radixin-moesin domain; IQ, calmodulin-binding IQ domain; MyTH4, myosin tail homology 4 domain; PDZ, PSD95/SAP90-Discs large-zonula occludens-1 domain; PST, proline, serine, threonine-rich domain; PRO, proline-rich domain; SH3, src homology 3 domain.

Figure 3. Transduction-Channel Gating Mechanisms

(A and B) Tether model. Here, the transduction channel binds directly to protocadherin 15 (PCDH15) and to the tether, presumably by strong interactions. Green fill indicates ion flux as channels open. (C and D) Membrane-tension model. By contrast, the transduction channel is not attached to PCDH15 and instead feels lateral membrane tension, elevated by bundle deflection.

Figure 4. Slow Adaptation

(A) Hair bundle at rest. Components and colors are the same as in Figure 3. R1–R3 indicate stereocilia rows from tallest to shortest. (B) Stimulated bundle. Channels in R2 and R3 both open as gating-spring tension rises. (C) Post-adaptation bundle. The upper tip-link density (UTLD) and myosin 1C (MYO1C), considered together to be the adaptation motor, slip down the cytoskeleton to relieve gating-spring tension. Because the motor’s rate is accelerated 5-fold by Ca²⁺, the motor in R2 slips farther than that in R1. Consequentially, the R3 channel closes but the R2 channel does not.

Figure 5. Diffusion of Ca²⁺ from Transduction Channels to Adaptation Sites

Although the distance from the transduction channel to the fast-adaptation site is very short, suggesting it is close to the tip link, the distance from the channel to the site regulating channel conductance at rest and slow adaptation is much greater. Thus it is plausible that the slow-adaptation motor could be regulated by Ca²⁺ diffusing from the very top of a stereocilium down to the upper end of a tip link. (A) Transmission electron micrograph of a turtle cochlea hair cell showing two tip links. For distance calibration, stereocilia are ~400 nm in diameter and tip links are ~150 nm in length. Image courtesy of C. Hackney and R. Fettiplace. (B) Image from (A) overlaid with outlines of key structures. The circle of radius 35 nm indicates the distance Ca²⁺ diffuses to influence fast adaptation (Ricci et al., 1998; Wu et al., 1999). The circle of radius 200 nm indicates the distance Ca²⁺ diffuses to influence channel conductance at rest (Ricci et al., 1998) and slow adaptation (Wu et al., 1999). In addition, the density of gold immunolabeling for myosin 1C (MYO1C) for frog saccular stereocilia is indicated; units are gold particles per µm² (Garcia et al., 1998). Because the distance from the upper end of a tip link to the top of the stereocilium is shorter in turtle cochlear hair cells than in frog saccular hair cells, the plotted MYO1C distribution extends beyond the top of the stereocilium.