High frequency force generation in outer hair cells from the mammalian ear

Spontaneous otoacoustic emissions from free-standing stereovillar bundles of ten species of lizard with small papillae

Spontaneous otoacoustic emissions (SOAE) were measured in 10 lizard species from the families Iguanidae, Agamidae and Anguidae. The typical feature of these papillae is that the hair cells in the higher-frequency papillar regions that produce SOAE are not covered by a tectorial structure. The number of hair cells in the species used here was between 58 and 292 per ear. SOAE could be measured from all species, but some of their characteristics varied with papillar anatomy. Thus very small papillae produced fewer and smaller SOAE than larger papillae.

Effects of cyclic nucleotides on the function of prestin

Outer hair cells (OHCs) in the mammalian organ of Corti display electromotility, which is thought to provide the local active mechanical amplification of the cochlear response. Prestin is the key molecule responsible for OHC electromotility. Several compounds, including cGMP, have been shown to influence OHC electromotility. There are two potential cAMP/cGMP-dependent protein kinase phosphorylation sites on prestin. Whether these sites are involved in cGMP-dependent reactions is as yet unknown. In this study, prestin cDNA was transiently transfected into TSA 201 cells. Cells that expressed prestin were selected to measure non-linear capacitance (NLC), a signature of outer hair cell motility. We applied cGMP and cAMP analogues and a protein kinase G (PKG) antagonist to the cells. Furthermore, nine mutations at putative phosphorylation sites of prestin were produced. The neutral amino acid alanine replaced serine/threonine at phosphorylation sites to change the conserved phosphorylation motif in order to mimic the dephosphorylated state of prestin, whereas replacement with the negatively charged aspartic acid mimicked the phosphorylated state. The properties of such modified prestin-expressing cells were examined, through measurement of NLC and with confocal microscopy. Our data demonstrate that cGMP is significantly more influential than cAMP in modifying the non-linear, voltage-dependent charge displacement in prestin-transfected cells. The electrical properties of the single and double mutations further indicate a possible interaction between the two PKG target sites. One of these sites may influence the membrane targeting process of prestin. Finally, a new topology map of prestin is proposed.

High-resolution three-dimensional imaging of the lateral plasma membrane of cochlear outer hair cells by atomic force microscopy

The outer hair cells (OHCs) from the mammalian organ of Corti are assumed to enhance the sensitivity and the selectivity of the cochlea via an electromotile response to sound stimulation. These OHC mechanical changes feed energy back into the cochlea before completion of the transduction process by inner hair cells. OHC electromotility is thought to depend on specific transmembrane motor proteins. Electron microscopy has been used previously to image the OHC lateral plasma membrane, where voltage sensors and motors are located. A very specific and regular organization of membrane particles has been described, together with an equally specific submembraneous meshwork of cytoskeleton anchored to the plasma membrane. To confirm and extend these observations, we have used, for the first time on the OHC lateral wall, atomic force microscopy (AFM). As a result of an improved tapping mode technique as well as the unique ultrastructural organization of the OHC plasma membrane, we have obtained high-resolution three-dimensional (3D) images of a markedly enhanced quality, allowing high-resolution 3D imaging. Tapping-mode AFM confirmed the presence of regularly aligned particles (presumably transmembrane proteins) on both faces of the OHC plasma membrane. It also revealed the presence of markedly different membrane domains, smooth and undulating. The differences between these zones probably are due to local differences in cytoskeleton-membrane interactions. Moreover, 3D reconstructions allowed us to distinguish between globular and pore-like particles, a distinction that may be of great functional significance.

Ultrastructural localisation of spectrin in sensory and supporting cells of guinea-pig organ of Corti

Spectrin is a cytoskeletal protein found in the cortex of many cell types. It is known to occur in cochlear outer hair cells (OHCs) with previous immunoelectron microscopical studies showing that it is located in the cuticular plate and the cortical lattice. The latter is a network of filaments associated with the lateral plasma membrane that is thought to play a role in OHC motility. Spectrin has also been found in inner hair cells (IHCs) and supporting cells using immunofluorescent techniques, but its ultrastructural distribution in these cells has not yet been described. This has, therefore, been investigated using a monoclonal antibody to alpha-spectrin in conjunction with pre- and post-embedding immunogold labelling for transmission electron microscopy. Labelling was found in a meshwork of filaments beneath the plasma membranes of both IHCs and supporting cells and, in pillar cells, close to microtubule/microfilament arrays. It was also found in association with the stereocilia of OHCs and IHCs and, as expected, in the cortical lattice and cuticular plate of OHCs. Thus, spectrin is a general component of cytoskeletal structures involved in maintaining the specialised cell shapes in the organ of Corti and may contribute to the mechanical properties of all the cell types examined.

Localization of proteins associated with the outer hair cell plasma membrane in the gerbil cochlea

There is substantial evidence that the motility of mammalian outer hair cells is generated close to or within the plasma membrane. Several analogies between the outer hair cell cortical lattice and the membrane-related cytoskeleton of erythrocytes have been noted. In erythrocytes a member of the anion exchanger protein family, AE1, also known as Band 3, is involved in membrane-cytoskeleton linkage via Protein 4.1. In the following paper, the presence of these two proteins in gerbilline outer hair cells is confirmed by western blot. Furthermore, co-localization of these two proteins was detected in the lateral wall of outer hair cells by immunofluorescence and postembedding electron immunohistochemistry. Band 3 is restricted to this region, whereas Protein 4.1 has a somewhat more dispersed distribution. Thus, the structure of these sensory receptor cells may result from an adaptation of a strategy used by other motile cells. The proteins investigated likely have a support function and may comprise "pillars" seen between the lateral plasma membrane and the cytoskeleton in micrographs of outer hair cells. The possibility that Band 3 comprises "protein particles" seen in the lateral plasma membrane, or maybe directly involved in the voltage-dependent force generation in outer hair cells, is also discussed.

Development of the organ of Corti in horseshoe bats: scanning and transmission electron microscopy

The late prenatal and early postnatal development of the organ of Corti were studied in the horseshoe bat (Rhinolophus rouxi) by using scanning and transmission electron microscopy. Arrangements and dimensions of stereocilia bundles, together with their contacts with the tectorial membrane, were found to be adult-like shortly before birth, and thus before the biological onset of hearing (3-5 days after birth). During the first postnatal week, there were baso-apical gradients in disappearing kinocilia on inner hair cells (IHC), microvillis of supporting cells, and marginal pillars. The lower basal cochlear turn was mature with respect to these regressing structures at 3 days after birth, the apical turn at 10 days after birth. At birth, cytodifferentiation was found to be completed, and the tunnel of Corti and innermost spaces of Nuel had opened. The ultrastructure of IHCs was not markedly different from that at later ages. In outer hair cells (OHC), the adult-like regular arrangement of a single layer of subsurface cisternae and pillars was seen as soon as protrusions of supporting cells had withdrawn from the lateral wall of OHCs (basal turn at birth and throughout the cochlea 2 days after birth). Numerous efferent endings contacted the somata of IHCs up to the second postnatal week. Since the medial olivocochlear system is absent in horseshoe bats, the adult-like innervation pattern of OHCs was established at the biological onset of hearing. During the first 2 postnatal weeks, the cytoskeleton of pillar and Deiters cells, and the specialized Deiters cups developed. The organ of Corti appeared adult-like at 14 days, apart from the persistence of a reduced tympanic cover layer attached to the basilar membrane. Morphological data support physiological findings that the first broadly tuned auditory responses arise from the basal turn. The distinct low to high frequency gradient in development of sensitivity during the first 2 postnatal weeks of the horseshoe bat was not, however, matched by morphological gradients, and it would appear that the development of the cytoskeleton of supporting cells contributed to the establishment of tuning in the auditory fovea. Adult-like morphology of the organ of Corti coincided with the emergence of sharply tuned responses from the auditory fovea, but there was no clear-cut correlate for the shift in tuned foveal frequency representation that occurred during the following 3 weeks.

Immunological identification of candidate proteins involved in regulating active shape changes of outer hair cells

By employing immunological methods, it has been demonstrated that myosin, myosin light chain (MLC) and myosin light chain kinase (MLCK) proteins in outer hair cells (OHC) are immunologically different from isoforms in platelets, smooth muscle and heart muscle, and are probably more related to isoforms found in red blood cells (RBC). Moreover, proteins related to band 3 protein (b3p) and protein 4.1 (p 4.1), ankyrin as well as fodrin and spectrin, but not glycophorin, have been identified in isolated OHCs. Both OHCs and RBC differ from other motile non-muscle cells in their lack of smooth muscle isoforms of actin, their common high levels of spectrin-, ankyrin- and band 3-like proteins, as well as the expression of the 80 kDa protein 4.1 isoform. The data support the notion that motility of OHC may be based upon regulation of the b3p/p 4.1/ankyrin complex, and thus may be reminiscent to the active shape changes in RBC.

Development of gerbil outer hair cells after the onset of cochlear function: an ultrastructural study

It has recently been proposed that elements which contribute to active cochlear processes develop at the same time (between postnatal day (PND) 12 and 21) as the shift of the place code in the developing gerbil cochlea. Since outer hair cells (OHCs) have been implicated in these processes, we have hypothesized that developing OHCs will exhibit changes in anatomical features that contribute to cochlear maturation. Our results demonstrate that the ultrastructural characteristics of OHCs change after the onset of hearing (PND 12), during the time that cochlear nonlinearities are being established (PND 12-21). Differences are primarily associated with the distribution of cytoplasmic organelles. The subsurface cisternae (SSC), which are thought to be related to the mechanical support of the outer hair cell, to cell motility, and therefore to cochlear mechanics, are present at PND 10 but remain immature, with cisternal layers added during the preweanling period. In immature OHCs, more mitochondria are centrally-located than in mature OHCs. During development mitochondria come to form a continuous row near the innermost leaflet of the SSC. These ultrastructural features undergo rapid change during the maturation of peripheral auditory function.

Fodrin is a constituent of the cortical lattice in outer hair cells of the guinea pig cochlea: immunocytochemical evidence

Localization of fodrin, a membrane skeletal protein, in the outer hair cell of the guinea pig cochlea was examined by immunocytochemical techniques. By immunofluorescence microscopy, fodrin was observed in the cuticular plate, in the infracuticular network and along the lateral wall. By immunoelectron microscopy of ultrathin cryosections, labeling for fodrin along the lateral wall was localized between the cell membrane and the outermost layer of the subsurface cisternae. Furthermore, pre-embedding immunoelectron microscopy of permeabilized specimens showed that most immunogolds for fodrin were on the thin cross-linking component of the cortical lattice. The results indicate that fodrin is a constituent of the cortical lattice which is thought to play an important role in outer hair cell motility.

Microtubule rearrangement and bending during assembly of large curved microtubule bundles in mouse cochlear epithelial cells

Mature inner pillar cells in the mammalian organ of Corti are curved through about 60 degrees, where they arch over adjacent epithelial cells and the apex of an intercellular space called the tunnel of Corti. This report deals with changes in microtubule organization that are associated with cell bending and tunnel formation during morphogenesis of the mouse organ of Corti. A large bundle of up to 3,000 microtubules assembles in each inner pillar cell. Microtubule rearrangement occurs about 5 days after bundle assembly begins. The lumen of each initially straight hollow tube-shaped microtubule bundle is occluded as the bundle becomes more compact and elliptical in cross section. This event anticipates the once-only bending which subsequently occurs between particular levels (about 9-19 microns) below the top of a bundle as it curves into its final shape about 2 days later. Microtubule rearrangement presumably facilitates bending which is effected in the plane of least mechanical resistance parallel to the short axis of a bundle's elliptical cross-sectional profile. Precocious bending of bundles has been induced about 1.5 days in advance of the natural event. Abnormal positioning of these prematurely curved bundles indicates that bending is effected by a contractile mechanism located within bundles rather than being a response to externally applied forces. The potential importance of such microtubule-associated contractions for active modulation of the vibratory response in the cochlea during hearing is considered.

A simple in vitro method for raising monoclonal antibodies to cochlear proteins

Monoclonal antibodies have been produced to mammalian hair cell antigens using a simple in vitro kit. Antigen was crudely prepared from dissected cochlear tissue by detergent extraction. There was no need to purify hair cells. Hybridoma supernatents were screened most efficiently on dissociated cells fixed with acetone. The immunisation method is sensitive to nanograms of antigen and can generate responses to conserved or weak antigens. The kit requires very little previous experience with cell culture and generates monoclonal antibodies within 3-4 weeks. It has overcome a number of problems with production of antibodies to hair cells and it should prove to be a very valuable tool in many laboratories.