Review series: The cell biology of hearing

Abstract

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.

Figure 1.

The auditory sense organ. (A) Diagram of the auditory sense organ highlighting the snail-shaped cochlea. (B) Diagram of the organ of Corti. (C) Scanning electron micrographs of hair bundles in the cochlea after removal of the tectorial membrane. Three rows of OHCs are shown at the left, one row of IHCs at the right. (D) Higher magnification view of OHCs. Bars, 5 µm.

Figure 2.

Hair bundle development and structure. (A) Diagram of sequential stages of hair bundle development. At the onset, the apical hair cell surface contains microvilli and one kinocilium. The microvilli grow in length. The kinocilium moves to the lateral edge of the hair cell. Some microvilli elongate to form stereocilia of graded heights. (B) Cross section through a hair bundle and apical hair cell surface indicating the kinocilium, stereocilia, and cuticular plate. Some of the linkages in hair bundles are highlighted.

Figure 3.

Hair bundle proteins. (A) Domain structure of proteins discussed in the text. Abbreviations: 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; ADF, actin-depolymerization factor; AR, ankyrin-like repeat; PR, proline-rich peptide; ABS, F-actin–binding site; WH, WASP homology 2 domain; ABM, actin bundling module; EC, extracellular cadherin repeat; CalX-β, Ca²⁺-binding calcium enhancer β modules; Lam, Laminin GL or NT domain; EAR/EPTP, putative β-propeller folding domain; EGF, laminin-type epidermal growth factor–like domain; FN3, fibronectin type 3 repeat. (B) Diagram of two stereocilia indicating the distribution of some of the molecules discussed in the text.

Figure 4.

Hair bundles and mechanotransduction. Model of transduction and adaptation. Deflection of hair bundles in the direction of the longest stereocilia leads to the opening of transduction channels at the lower ends of tip links. Ca²⁺ enters the transduction channel and binds to the channel or a side near the channel and leads to channel closure (fast adaptation). The adaptation motor at the upper end of tip links subsequently detaches from the actin cytoskeleton and slides down the stereocilium, leading to release of tension in the transduction machinery (slipping phase of slow adaptation). Next, the motor complex climbs up the stereocilium, reestablishing tension (climbing phase of slow adaptation). (B) Molecular components of the mechanotransduction complex in stereocilia.