Organization of microtubules in cochlear hair cells

Microtubule and auditory function - an underestimated connection

The organ of Corti, located in the cochlea within the inner ear is the receptor organ for hearing. It converts auditory signals into neuronal action potentials that are transmitted to the brain for further processing. The mature organ of Corti consists of a variety of highly differentiated sensory cells that fulfil unique tasks in the processing of auditory signals. The actin and microtubule cytoskeleton play essential function in hearing, however so far, more attention has been paid to the role of actin. Microtubules play important roles in maintaining cellular structure and intracellular transport in virtually all eukaryotic cells. Their functions are controlled by interactions with a large variety of microtubule-associated proteins (MAPs) and molecular motors. Current advances show that tubulin posttranslational modifications, as well as tubulin isotypes could play key roles in modulating microtubule properties and functions in cells. These mechanisms could have various effects on the stability and functions of microtubules in the highly specialised cells of the cochlea. Here, we review the current understanding of the role of microtubule-regulating mechanisms in the function of the cochlea and their implications for hearing, which highlights the importance of microtubules in the field of hearing research.

Vesicle traffic in the outer hair cell

The plasma-membrane marker FM1-43 was employed to reveal the relative significance of different types of endocytic and transcytic mechanisms in outer hair cells (OHCs) of the guinea-pig cochlea. A double-barrel local perfusion system was used to label independently the apical or synaptic pole of the isolated OHC to study mechanisms of vesicle uptake at the poles and of vesicle trafficking along and across the cell. Treatment with an inhibitor of macropino- and phagocytosis, phenylarsine oxide, or of clathrin-mediated endocytic activity, concanavalin A, significantly reduced the dye uptake at both the apical and the synaptic poles, indicating the presence of both clathrin-independent and clathrin-mediated processes at both poles. However, measurement of uptake speed in the presence of the inhibitors suggested that clathrin-independent processes contribute more extensively to endocytosis at the basal pole than the apical pole. Treatment with an inhibitor of myosin VI, 2,4,6-triiodophenol, significantly delayed both the apicobasal and the basoapical fluorescence signals. However, treatment with an inhibitor of kinesin, monastrol, or of dynein, ciliobrevin D, significantly delayed the signals only in the basoapical direction. The myosinVI inhibitor, but neither the kinesin nor dynein inhibitors, significantly delayed the signals to the subsurface cisternae. That is, myosin VI carries vesicles in both longitudinal directions as well as radially to the subsurface cisternae, whereas kinesin and dynein participate primarily in basoapical trafficking. This fundamental information is essential for elucidating recycling mechanisms of specific proteins involved in establishing, controlling and maintaining the electromechanical action of OHCs and, therefore, is vital for understanding auditory perception.

Molecular Assembly and Structural Plasticity of Sensory Ribbon Synapses-A Presynaptic Perspective

In the mammalian cochlea, specialized ribbon-type synapses between sensory inner hair cells (IHCs) and postsynaptic spiral ganglion neurons ensure the temporal precision and indefatigability of synaptic sound encoding. These high-through-put synapses are presynaptically characterized by an electron-dense projection-the synaptic ribbon-which provides structural scaffolding and tethers a large pool of synaptic vesicles. While advances have been made in recent years in deciphering the molecular anatomy and function of these specialized active zones, the developmental assembly of this presynaptic interaction hub remains largely elusive. In this review, we discuss the dynamic nature of IHC (pre-) synaptogenesis and highlight molecular key players as well as the transport pathways underlying this process. Since developmental assembly appears to be a highly dynamic process, we further ask if this structural plasticity might be maintained into adulthood, how this may influence the functional properties of a given IHC synapse and how such plasticity could be regulated on the molecular level. To do so, we take a closer look at other ribbon-bearing systems, such as retinal photoreceptors and pinealocytes and aim to infer conserved mechanisms that may mediate these phenomena.

Different patterns of endocytosis in cochlear inner and outer hair cells of mice

Precise and efficient endocytosis is critical for sustained neurotransmission during continuous neuronal activity. Endocytosis is a prerequisite for maintaining the auditory function. However, the differences between the patterns of endocytosis in cochlear inner hair cells (IHCs) and outer hair cells (OHCs) remain unclear. Both IHCs and OHCs were obtained from adult C57 mice. Patterns of endocytosis in cells were estimated by analyzing the uptake of FM1-43, a fluorescent. The observations were made using live confocal imaging, fluorescence intensities were calculated statistically. Results revealed the details about following phenomenon, i) sites of entry: the FM1-43 dye was found to enter IHC at the apical area initially, the additional sites of entry were then found at basolateral membrane of the cells, The entry of the dye into OHCs initially appeared to be occurring around whole apical membranes area, which then diffused towards the other membrane surface of the cells, ii) capacity of endocytosis: fluorescence intensity in IHCs showed significantly higher than that of OHCs (P<0.01). We have found different patterns of endocytosis between IHCs and OHCs, this indicated functional distinctions between them. Moreover, FM1-43 dye can be potentially used as an indicator of the functional loss or repair of cochlear hair cells.

Inflammation up-regulates cochlear expression of TRPV1 to potentiate drug-induced hearing loss

Aminoglycoside antibiotics are essential for treating life-threatening bacterial infections, despite the risk of lifelong hearing loss. Infections induce inflammation and up-regulate expression of candidate aminoglycoside-permeant cation channels, including transient receptor potential vanilloid-1 (TRPV1). Heterologous expression of TRPV1 facilitated cellular uptake of (fluorescently tagged) gentamicin that was enhanced by agonists, and diminished by antagonists, of TRPV1. Cochlear TRPV1 was immunolocalized near the apical membranes of sensory hair cells, adjacent supporting cells, and marginal cells in the stria vascularis. Exposure to immunostimulatory lipopolysaccharides, to simulate of bacterial infections, increased cochlear expression of TRPV1 and hair cell uptake of gentamicin. Lipopolysaccharide exposure exacerbated aminoglycoside-induced auditory threshold shifts and loss of cochlear hair cells in wild-type, but not in heterozygous Trpv1+/- or Trpv1 knockout, mice. Thus, TRPV1 facilitates cochlear uptake of aminoglycosides, and bacteriogenic stimulation upregulates TRPV1 expression to exacerbate cochleotoxicity. Furthermore, loss-of-function polymorphisms in Trpv1 can protect against immunogenic exacerbation of aminoglycoside-induced cochleotoxicity.

Remodeling of the Inner Hair Cell Microtubule Meshwork in a Mouse Model of Auditory Neuropathy AUNA1

Auditory neuropathy 1 (AUNA1) is a form of human deafness resulting from a point mutation in the 5′ untranslated region of the Diaphanous homolog 3 (DIAPH3) gene. Notably, the DIAPH3 mutation leads to the overexpression of the DIAPH3 protein, a formin family member involved in cytoskeleton dynamics. Through study of diap3-overexpressing transgenic (Tg) mice, we examine in further detail the anatomical, functional, and molecular mechanisms underlying AUNA1. We identify diap3 as a component of the hair cells apical pole in wild-type mice. In the diap3-overexpressing Tg mice, which show a progressive threshold shift associated with a defect in inner hair cells (IHCs), the neurotransmitter release and potassium conductances are not affected. Strikingly, the overexpression of diap3 results in a selective and early-onset alteration of the IHC cuticular plate. Molecular dissection of the apical components revealed that the microtubule meshwork first undergoes aberrant targeting into the cuticular plate of Tg IHCs, followed by collapse of the stereociliary bundle, with eventual loss of the IHC capacity to transmit incoming auditory stimuli.

Relating structure and function of inner hair cell ribbon synapses

In the mammalian cochlea, sound is encoded at synapses between inner hair cells (IHCs) and type I spiral ganglion neurons (SGNs). Each SGN receives input from a single IHC ribbon-type active zone (AZ) and yet SGNs indefatigably spike up to hundreds of Hz to encode acoustic stimuli with submillisecond precision. Accumulating evidence indicates a highly specialized molecular composition and structure of the presynapse, adapted to suit these high functional demands. However, we are only beginning to understand key features such as stimulus-secretion coupling, exocytosis mechanisms, exo-endocytosis coupling, modes of endocytosis and vesicle reformation, as well as replenishment of the readily releasable pool. Relating structure and function has become an important avenue in addressing these points and has been applied to normal and genetically manipulated hair cell synapses. Here, we review some of the exciting new insights gained from recent studies of the molecular anatomy and physiology of IHC ribbon synapses.

Auditory hair cell centrioles undergo confined Brownian motion throughout the developmental migration of the kinocilium

Planar polarization of the forming hair bundle, the mechanosensory antenna of auditory hair cells, depends on the poorly characterized center-to-edge displacement of a primary cilium, the kinocilium, at their apical surface. Taking advantage of the gradient of hair cell differentiation along the cochlea, we reconstituted a map of the kinocilia displacements in the mouse embryonic cochlea. We then developed a cochlear organotypic culture and video-microscopy approach to monitor the movements of the kinocilium basal body (mother centriole) and its daughter centriole, which we analyzed using particle tracking and modeling. We found that both hair cell centrioles undergo confined Brownian movements around their equilibrium positions, under the apparent constraint of a radial restoring force of ∼0.1 pN. This magnitude depended little on centriole position, suggesting nonlinear interactions with constraining, presumably cytoskeletal elements. The only dynamic change observed during the period of kinocilium migration was a doubling of the centrioles' confinement area taking place early in the process. It emerges from these static and dynamic observations that kinocilia migrate gradually in parallel with the organization of hair cells into rows during cochlear neuroepithelium extension. Analysis of the confined motion of hair cell centrioles under normal and pathological conditions should help determine which structures contribute to the restoring force exerting on them.

Marshalin, a microtubule minus-end binding protein, regulates cytoskeletal structure in the organ of Corti

Dramatic structural changes in microtubules (MT) and the assembly of complicated intercellular connections are seen during the development of the cellular matrix of the sense organ for hearing, the organ of Corti. This report examines the expression of marshalin, a minus-end binding protein, during this process of cochlear development. We discovered that marshalin is abundantly expressed in both sensory hair cells and supporting cells. In the adult, prominent marshalin expression is observed in the cuticular plates of hair cells and in the noncentrosomal MT organization centers (MTOC) of Deiters' and pillar cells. Based upon differences in marshalin expression patterns seen in the organ of Corti, we identified eight isoforms ranging from 863 to 1280 amino acids. mRNAs/proteins associated with marshalin's isoforms are detected at different times during development. These isoforms carry various protein-protein interacting domains, including coiled-coil (CC), calponin homology (CH), proline-rich (PR), and MT-binding domains, referred to as CKK. We, therefore, examined membranous organelles and structural changes in the cytoskeleton induced by expressing two of these marshalin isoforms in vitro. Long forms containing CC and PR domains induce thick, spindle-shaped bundles, whereas short isoforms lacking CC and PR induce more slender variants that develop into densely woven networks. Together, these data suggest that marshalin is closely associated with noncentrosomal MTOCs, and may be involved in MT bundle formation in supporting cells. As a scaffolding protein with multiple isoforms, marshalin is capable of modifying cytoskeletal networks, and consequently organelle positioning, through interactions with various protein partners present in different cells.

The microtubule end-binding protein EB2 is a central regulator of microtubule reorganisation in apico-basal epithelial differentiation

Microtubule end-binding (EB) proteins influence microtubule dynamic instability, a process essential for microtubule reorganisation during apico-basal epithelial differentiation. Here we establish for the first time that EB2, but not EB1, expression is critical for initial microtubule reorganisation during apico-basal epithelial differentiation, and that EB2 downregulation promotes bundle formation. EB2 siRNA knockdown during early stages of apico-basal differentiation prevented microtubule reorganisation, while its downregulation at later stages promoted microtubule stability and bundle formation. Interestingly, while EB1 is not essential for microtubule reorganisation its knockdown prevented apico-basal bundle formation and epithelial elongation. EB2 siRNA depletion in undifferentiated epithelial cells induced formation of straight, less dynamic microtubules with EB1 and ACF7 lattice association and co-alignment with actin filaments, a phenotype that could be rescued by formin inhibition. Importantly, in situ inner ear and intestinal crypt epithelial tissue revealed direct correlations between low level of EB2 expression and presence of apico-basal microtubule bundles, which were absent where EB2 was elevated. EB2 is evidently important for initial microtubule reorganisation during epithelial polarisation, while its downregulation facilitates EB1/ACF7 microtubule lattice association, microtubule-actin filament co-alignment and bundle formation. The spatiotemporal expression of EB2 thus dramatically influences microtubule organisation, EB1/ACF7 deployment and epithelial differentiation.

Murine autoimmune hearing loss mediated by CD4+ T cells specific for β-tubulin

Autoimmune inner ear disease is described as progressive, bilateral although asymmetric, sensorineural hearing loss and can be improved by immunosuppressive therapy. We showed that the inner ear autoantigen β-tubulin is capable of inducing experimental autoimmune hearing loss (EAHL) in mice. Immunization of BALB/c mice with β-tubulin resulted in hair cell loss and hearing loss, effects that were not seen in animals immunized with control peptide. Moreover, the EAHL model showed that β-tubulin responsiveness involved CD4(+) T cells producing IFN-γ, and T cell mediation of EAHL was determined by significantly increased auditory brainstem response after adoptive transfer of β-tubulin-activated CD4(+) T cells into naive BALB/c recipients. The potential mechanisms responsible for the observed pathology of EAHL can be attributed to decreased frequency and impaired suppressive function of regulatory T cells. Our study suggests that EAHL may be a T cell-mediated organ-specific autoimmune disorder of the inner ear.

Membrane traffic in outer hair cells of the adult mammalian cochlea

Outer hair cells (OHCs), the sensory-motor cells responsible for the extraordinary frequency selectivity and dynamic range of the cochlea, rapidly endocytose membrane and protein at their apical surface. Endocytosis and transcytosis in isolated OHCs from the mature guinea-pig cochlea were investigated using the amphipathic membrane probe FM1-43. We observed membrane transport from the apical surface to both the basolateral wall and the subnuclear pole. By double-labelling with DiOC6, a stain for endoplasmic reticulum, and aspiration of the plasma membrane, we showed that the basolateral target was the subsurface cisternae. The fluorescent signal was about three times weaker at the basal than at the apical pole. The speed of vesicle transport to the subnuclear pole was approximately 0.4 microm/s. Changing extracellular Ca2+ concentration from 25 microM to 2 mM accelerated rapid endocytosis. Extracellular application of BAPTA-AM (25 microM), an intracellular Ca2+ chelator, and TFP (20 microM), a specific inhibitor of calmodulin, reduced endocytic activity, as did depolarization of the whole cell. The presence of extracellular Cd2+ (200 microM), a Ca2+-channel blocker, had no effect on the voltage dependence of endocytosis at the apical pole, and inhibited the voltage dependence at the subnuclear pole. These results suggest that rapid endocytosis is a Ca2+/calmodulin-dependent process, with extracellular Ca2+ entering through voltage-gated Ca2+ channels at the basal pole. The two distinct destinations of endocytosed membrane are consistent with the functional polarization of the OHC, with the basolateral wall being dedicated to electromechanical transduction and the subnuclear pole being dedicated to electrochemical transduction processes.

Initial characterization of kinocilin, a protein of the hair cell kinocilium

A subtracted library prepared from vestibular sensory areas [Nat. Genet. 26 (2000) 51] was used to identify a 960bp murine transcript preferentially expressed in the inner ear and testis. The cDNA predicts a basic 124aa protein that does not share any significant sequence homology with known proteins. Immunofluorescence and immunoelectron microscopy revealed that the protein is located mainly in the kinocilium of sensory cells in the inner ear. The protein was thus named kinocilin. In the mouse, kinocilin is first detected in the kinocilia of vestibular and auditory hair cells at embryonic days 14.5, and 18.5, respectively. In the mature vestibular hair cells, kinocilin is still present in the kinocilium. As the auditory hair cells begin to lose the kinocilium during postnatal development, kinocilin becomes distributed in an annular pattern at the apex of these cells, where it co-localizes with the tubulin belt [Hear. Res. 42 (1989) 1]. In mature auditory hair cells, kinocilin is also present at the level of the cuticular plate, at the base of each stereocilium. In addition, as the kinocilium regresses from developing auditory hair cells, kinocilin begins to be expressed by the pillar cells and Deiters cells, that both contain prominent transcellular and apical bundles of microtubules. By contrast, kinocilin was not detected in the supporting cells in the vestibular end organs. The protein is also present in the manchette of the spermatids, a transient structure enriched in interconnected microtubules. We propose that kinocilin has a role in stabilizing dense microtubular networks or in vesicular trafficking.

Fm1-43 reveals membrane recycling in adult inner hair cells of the mammalian cochlea

Neural transmission of complex sounds demands fast and sustained rates of synaptic release from the primary cochlear receptors, the inner hair cells (IHCs). The cells therefore require efficient membrane recycling. Using two-photon imaging of the membrane marker FM1-43 in the intact sensory epithelium within the cochlear bone of the adult guinea pig, we show that IHCs possess fast calcium-dependent membrane uptake at their apical pole. FM1-43 did not permeate through the stereocilial mechanotransducer channel because uptake kinetics were neither changed by the blockers dihydrostreptomycin and d-tubocurarine nor by treatment of the apical membrane with BAPTA, known to disrupt mechanotransduction. Moreover, the fluid phase marker Lucifer Yellow produced a similar labeling pattern to FM1-43, consistent with FM1-43 uptake via endocytosis. We estimate the membrane retrieval rate at approximately 0.5% of the surface area of the cell per second. Labeled membrane was rapidly transported to the base of IHCs by kinesin-dependent trafficking and accumulated in structures that resembled synaptic release sites. Using confocal imaging of FM1-43 in excised strips of the organ of Corti, we show that the time constants of fluorescence decay at the basolateral pole of IHCs and apical endocytosis were increased after depolarization of IHCs with 40 mm potassium, a stimulus that triggers calcium influx and increases synaptic release. Blocking calcium channels with either cadmium or nimodipine during depolarization abolished the rate increase of apical endocytosis. We suggest that IHCs use fast calcium-dependent apical endocytosis for activity-associated replenishment of synaptic membrane.

Pronounced infracuticular endocytosis in mammalian outer hair cells

Endocytosis in cochlear hair cells was investigated by staining with the vital fluorescent dye FM 1-43, that partitions reversibly into membranes and is trapped in vesicles during endocytosis. The temporal development and spatial distribution of FM 1-43 induced fluorescence was investigated using confocal laser-scanning microscopy. FM 1-43 rapidly and intensely stained cochlear hair cells, leaving the supporting cells unstained. For short application (0.2-30 s), only the infracuticular region of outer hair cells (OHCs) was labeled, whereas for long application (30-60 s), the OHCs were also labeled in the infranuclear zone and along a central strand extending from the infracuticular zone down to the nucleus, as well as along the entire cell membrane. Except for the cell membrane, the infracuticular zone, directly below the cuticular plate, showed the most rapid and intense staining, and in most cases staining was spherically shaped with a diameter of 3-7 microm. Localization and size of this infracuticular staining coincided with Hensen's body, a specialized variant of the endoplasmic reticulum. In contrast to the OHCs, apical fluorescence of inner hair cells presented a homogeneous distribution. When OHCs were incubated in FM 1-43 for longer than 1 min, many points of contact between the central strand, the infracuticular zone and the lateral cell membrane were observed. Since Hensen's bodies are a specialty of OHCs and the fluorescent staining pattern of these cells was unique, it is proposed that Hensen's body is involved in the turnover of OHC-specific proteins, such as those involved in the molecular machinery of the motor action of the plasma membrane.

Differential expression of beta tubulin isotypes in the adult gerbil cochlea

Tubulin, the principal component of microtubules, exists as two polypeptides, termed alpha and beta. Seven isotypes of beta tubulin are known to exist in mammals. The distributions of four beta tubulin isotypes, beta(I), beta(II), beta(III), and beta(IV), have been examined in the adult cochlea by indirect immunofluorescence using isotype-specific antibodies. In the organ of Corti, outer hair cells contained only beta(I) and beta(IV), while inner hair cells contained only beta(I) and beta(II). Inner and outer pillar cells contained beta(II) and beta(IV), but Deiters cells contained those isotypes plus beta(I). Fine fibers in the inner spiral bundle, tunnel crossing fibers, and outer spiral fibers, probably efferent in character, contained beta(I), beta(II), and beta(III), but not beta(IV). In the spiral ganglion, the somas and axons of neurons contained all four isotypes, and the myelination of ganglion cells also contained beta(I). Fibers of the intraganglionic spiral bundle contained beta(I), beta(II), and beta(III). No antibody labeled the dendritic processes of spiral ganglion neurons. The differences in isotype distribution in organ of Corti and neurons described here are consistent with and support the multi-tubulin hypothesis, which states that tubulin isotypes are expressed specifically in different cell types and may therefore have different functions.

Localization of microtubules containing posttranslationally modified tubulin in cochlear epithelial cells during development

In the adult gerbil inner ear, hair cell microtubules contain predominantly tyrosinated tubulin while supporting cell microtubules contain almost exclusively other isoforms. This cell-type specific segregation of tubulin isoforms is unusual, and in this respect the sensory and supporting cells in this sensory organ differ from other cells observed both in vivo and in vitro. Thus, we hypothesized there must be a shift in the presence and location of tubulin isoforms during development, directly associated with the onset of specialized functions of the cells. We describe the appearance and/or disappearance of tubulin isoforms in sensory hair cells and five different supporting cells (inner and outer pillar cells, Deiters cells, cells of Kölliker's organ, and cells of the tympanic covering layer) during development of the gerbil organ of Corti from birth to 14 days after birth. Tyrosinated tubulin was initially present in all cells and remained predominant in cells that decrease in number (Kölliker's organ and tympanic covering layer) and exhibit active processes such as secretion and motility (sensory cells). Posttranslational modifications occurred in the supporting cells in a time-dependent manner as the number and length of microtubules increased and development proceeded, but the establishment of elongated cell shape and polarity occurred prior to the appearance of acetylation, detyrosination, and polyglutamylation of tubulin. In the pillar and Deiters cells, posttranslational modifications progressed from cell apex to base in the same direction as microtubule elongation. In the pillar cells, posttranslational modifications occurred first at the apical surfaces. In the pillar cells, the appearance of acetylated tubulin was rapidly followed by the appearance of detyrosinated tubulin. In Deiters cells, the appearance of acetylated tubulin preceded the appearance of detyrosinated tubulin by one or more days. At onset of cochlear function, detyrosinated tubulin and acetylated tubulin had achieved their adult-like pattern, but polyglutamylated tubulin had not.

Tubulin expression in the developing and adult gerbil organ of Corti

In the late stages of inner ear development, the relatively undifferentiated cells of Kollicker's organ are transformed into the elaborately specialized cell types of the organ of Corti. Microtubules are prominent features of adult cells in the organ of Corti, particularly supporting cells. To test the possible role of microtubules in organ of Corti development, the microtubule organization in the organ of Corti has been examined using indirect immunofluorescence to beta-tubulin in the developing gerbil cochlea. Tubulin first appears at post-natal day 0 (P0) as filamentous asters in inner hair cells and by P2, asters are also seen in outer hair cells. Tubulin appears at P3 in inner pillar cells in a tooth crown-like figure. By P6, tubulin expression is also evident in outer pillar cells and by P9, it is seen in Deiters cells. Elaboration of microtubules in pillar cells was observed to proceed from the reticular lamina towards the basilar membrane. The pattern of tubulin expression in the apical organ of Corti lags the base by about 3 days until P6, but by P9, apical and basal organ of Corti appear substantially the same.

Calbindin and parvalbumin are early markers of non-mitotically regenerating hair cells in the bullfrog vestibular otolith organs

Earlier studies have demonstrated hair cell regeneration in the absence of cell proliferation, and suggested that supporting cells could phenotypically convert into hair cells following hair cell loss. Because calcium-binding proteins are involved in gene up-regulation, cell growth, and cell differentiation, we wished to determine if these proteins were up-regulated in scar formations and regenerating hair cells following gentamicin treatment. Calbindin and parvalbumin immunolabeling was examined in control or gentamicin-treated (GT) bullfrog saccular and utricular explants cultured for 3 days in amphibian culture medium or amphibian culture medium supplemented with aphidicolin, a blocker of nuclear DNA replication in eukaryotic cells. In control cultures, calbindin and parvalbumin immunolabeled the hair bundles and, less intensely, the cell bodies of mature hair cells. In GT or mitotically-blocked GT (MBGT) cultures, calbindin and parvalbumin immunolabeling was also seen in the hair bundles, cuticular plates, and cell bodies of hair cells with immature hair bundles. Thus, these antigens were useful markers for both normal and regenerating hair cells. Supporting cell immunolabeling was not seen in control cultures nor in the majority of supporting cells in GT cultures. In MBGT cultures, calbindin and parvalbumin immunolabeling was up-regulated in the cytosol of single supporting cells participating in scar formations and in supporting cells with hair cell-like characteristics. These data provide further evidence that non-mitotic hair cell regeneration in cultures can be accomplished by the conversion of supporting cells into hair cells.

Unconventional myosins in inner-ear sensory epithelia

To understand how cells differentially use the dozens of myosin isozymes present in each genome, we examined the distribution of four unconventional myosin isozymes in the inner ear, a tissue that is particularly reliant on actin-rich structures and unconventional myosin isozymes. Of the four isozymes, each from a different class, three are expressed in the hair cells of amphibia and mammals. In stereocilia, constructed of cross-linked F-actin filaments, myosin-Ibeta is found mostly near stereociliary tips, myosin-VI is largely absent, and myosin-VIIa colocalizes with crosslinks that connect adjacent stereocilia. In the cuticular plate, a meshwork of actin filaments, myosin-Ibeta is excluded, myosin-VI is concentrated, and modest amounts of myosin-VIIa are present. These three myosin isozymes are excluded from other actin-rich domains, including the circumferential actin belt and the cortical actin network. A member of a fourth class, myosin-V, is not expressed in hair cells but is present at high levels in afferent nerve cells that innervate hair cells. Substantial amounts of myosins-Ibeta, -VI, and -VIIa are located in a pericuticular necklace that is largely free of F-actin, squeezed between (but not associated with) actin of the cuticular plate and the circumferential belt. Our localization results suggest specific functions for three hair-cell myosin isozymes. As suggested previously, myosin-Ibeta probably plays a role in adaptation; concentration of myosin-VI in cuticular plates and association with stereociliary rootlets suggest that this isozyme participates in rigidly anchoring stereocilia; and finally, colocalization with cross-links between adjacent stereocilia indicates that myosin-VIIa is required for the structural integrity of hair bundles.

Cytoskeletal and calcium-binding proteins in the mammalian organ of Corti: cell type-specific proteins displaying longitudinal and radial gradients

Whole mounts and tissue sections of the organ of Corti from two representative mammalian species, the Mongolian gerbil (Meriones unguiculatus) and the guinea pig (Cavea porcellus) were probed with antibodies to cytoskeletal and calcium-binding proteins (actin, tubulin, including post-translational modifications, spectrin, fimbrin, calmodulin, parvalbumin, calbindin, S-100 and calretinin). All of the proteins tested were expressed in both species. New findings include the following. Actin is present in large accumulations in cell bodies of the Deiters cells under the outer hair cells (OHC), as well as in the filament networks previously described. These accumulations are more prominent in the apical turns. Tubulin is present in sensory cells in the tyrosinated (more dynamic) form, while tubulin in the supporting cells is post-translationally modified, indicating greater stability. Fimbrin, present in the stereocilia of both IHCs and OHCs, is similar to the isoform of fimbrin found in the epithelial cells of the intestine (fimbrin-I), which implies that actin bundling by fimbrin is reduced in the presence of increased calcium. Parvalbumin appears to be an IHC-specific calcium-binding protein in the gerbil as well as in the guinea pig; labeling displays a longitudinal gradient, with hair cells at the apex staining intensely and hair cells at the base staining weakly. Calbindin displays a similar longitudinal gradient, with staining intense in the IHCs and OHCs at the apex and weak to absent in the base. In the middle turns of the guinea pig cochlea, OHCs in the first row near the pillar cells lose immunoreactivity to calbindin before those in the second and third rows. Calmodulin is found throughout the whole cochlea in the IHCs and OHCs in the stereocilia, cuticular plate, and cell body. Calretinin is present in IHCs and Deiters cells in both species, as well as the tectal cell (modified Hensen cell) in the gerbil. S-100 is a supporting cell-specific calcium-binding protein which has not been localized in the sensory cells of these two species. The supporting cells containing S-100 include the inner border, inner phalangeal, pillar, Deiters, tectal (in gerbil) and Hensen cells, where labeling displays a longitudinal gradient decreasing in intensity towards the apex (opposite to what has been seen with labeling for other proteins in the cochlea).

Post-translational modifications of tubulin suggest that dynamic microtubules are present in sensory cells and stable microtubules are present in supporting cells of the mammalian cochlea

Post-translational modifications to tubulin in the sensory and supporting cells of the cochlea were studied using antibodies specific to the tyrosinated, detyrosinated, acetylated and polyglutamylated isoforms. In the sensory cells, microtubules which label intensely with antibodies to tyrosinated tubulin are found in networks within the cytoplasm. Microtubules which label with antibodies to detyrosinated tubulin and polyglutamylated tubulin, but not acetylated tubulin, form a small component of the microtubules found in the cytoplasm only in the region below the cuticular plate. Microtubules in the supporting cells (inner and outer pillar cells and Deiters cells) are arranged in bundles and contain little tyrosinated tubulin. They are composed instead of predominantly post-translationally modified isoforms which include detyrosinated, acetylated and polyglutamylated tubulin. The findings suggest that microtubules in the sensory cells form dynamic structures, since microtubules that undergo cyclic polymerization and depolymerization predominantly contain tubulin that has not yet had its carboxy-terminal tyrosine residue removed. The presence of microtubules in the supporting cells in which the tubulin has been polymerized into microtubules long enough to be post-translationally modified, provides evidence that these microtubules are stable, long-lived and could contribute to the structural support of the sensory organ of Corti.

The kinocilium of auditory hair cells and evidence for its morphogenetic role during the regeneration of stereocilia and cuticular plates

Auditory hair cells that survive mechanical injury in culture begin their recovery by reforming the kinocilium. This study is based on cultures of the organ of Corti of newborn mice and two control animals. The axonemal patterns were examined in 165 kinocilia in cross-section. In the immature and regenerating kinocilium, one of the normally peripheral doublets is frequently located inward, forming the modified 8 + 1 (double) form; the distribution of the remaining microtubules is irregular. As the cell matures, the 9 + 0 form predominates. Overall, 34-61% of auditory kinocilia consist of 9 + 0 microtubules. The 9 + 2 (single) form, previously thought to characterize the organelle, occurs only in about 3-14%, whereas the remaining population comprises the modified 8 + 1 (double) form. Normally, the kinocilium lasts only about 10 postnatal days; however, post-traumatic hair cells reform their kinocilia regardless of age. Concomitant with the regrowth of the kinocilium, the basal body and its cilium take a central location in the cuticular plate, stereocilia regrow, and the cytoplasmic area adjacent to the basal body displays pericentriolar fibrous densities, growth vesicles, and microtubules, all surrounded by actin filaments. Pericentriolar bodies nucleate microtubules. Involvement of microtubules is seen in the alignment of actin filaments and in the formation of the filamentous matrix of the cuticular plate. We propose that reformation of the kinocilium in recovering post-traumatic hair cells indicates the possible role of its basal body in the morphogenesis and differentiation of cuticular plates and stereocilia.

Immunocytochemical comparison of posttranslationally modified forms of tubulin in the vestibular end-organs of the gerbil: tyrosinated, acetylated and polyglutamylated tubulin

Specific antibodies against alpha-tubulin, acetylated alpha-tubulin, tyrosinated alpha-tubulin and polyglutamylated alpha- and beta-tubulin were used to compare the distribution of posttranslationally modified tubulin in the vestibular end-organs of the gerbil. Antibodies to acetylated tubulin labeled a dense network of microtubules in the hair cells and bundles of microtubule in the supporting cells. Nerve fibers within and below the epithelium were weakly labeled. This localization paralleled that seen with antibodies to alpha-tubulin which labeled all microtubules present in the cells. Antibodies to tyrosinated tubulin labeled networks and bundles of microtubules in both hair cells and supporting cells and in addition gave intense, diffuse labeling in the cytoplasm of both cell types. It also labeled the nerve fibers. Antibodies to polyglutamylated tubulin were localized mainly in nerve fibers, and in the calyces the labeled microtubules were found running circumferentially around the type I sensory hair cells. Thus, tyrosinated tubulin was found in the fine networks of microtubules in both the sensory and supporting cells. Acetylated tubulin was found in the dense networks and bundles of microtubules in the sensory and supporting cells, but did not colocalize with polyglutamylated tubulin, which was found predominantly in the nerve fibers. The labeling patterns for the tyrosinated tubulin and posttranslationally modified tubulins in the sensory and supporting cells of the vestibular end organs differ from that seen in the organ of Corti and may reflect differences in the stability of the microtubules and the mechanical properties of the sensory epithelium.

The arrangements of F-actin, tubulin and fodrin in the organ of Corti of the horseshoe bat (Rhinolophus rouxi) and the gerbil (Meriones unguiculatus)

The composition of cytoskeletal elements in hair cells and non-sensory cells was studied in paraformaldehyde fixed cochleae of the horseshoe bat and the gerbil using phallotoxins and antibodies directed against actin, alpha-tubulin and fodrin. In both species, cryostat sections of the organ of Corti were studied using confocal fluorescence microscopy; in the bat, ultrathin sections were investigated using actin-immunoelectron and classical electron microscopy. F-actin was found in stereocilia and cuticular plates of inner and outer hair cells (IHCs and OHCs) of both species. In fixed material from both species, no F-actin staining was detected in the cytoplasm or along the lateral cell membrane of OHCs, whereas in freshly isolated OHCs of the gerbil, a faint F-actin staining was detected along the lateral wall. In the bat, the patterns of F-actin staining were confirmed with actin-immunoelectron microscopy. The alpha-tubulin antibody strongly labeled IHCs of both species. They contained a complex network of microtubules especially in the neck portion. In the bat, OHCs showed no distinct alpha-tubulin reactivity, as would be expected given the scarcity of microtubules observed at the ultrastructural level. In the gerbil, alpha-tubulin reactivity was found throughout the OHC body with highest intensity in the cell apex. In Deiters cells, pillar cells and Boettcher cells of both species, F-actin and microtubules were colocalized at contact zones with the basilar membrane. In Deiters cups, F-actin staining was most pronounced in the basal turn of the bat cochlea. In the gerbil, a distinct baso-apical gradient was found in immunostaining properties and morphology of the Deiters cells. Intense fodrin reactivity was found in the cuticular plates and along the lateral cell membrane of both types of hair cells of the bat. Cytoplasmic fodrin staining was localized within the IHCs of the bat. In the gerbil, intense fodrin staining was only found in cuticular plates of hair cells and staining of the lateral cell membrane of hair cells was faint. A faint fodrin staining was also seen in Deiters cells of both species. The basic arrangement of the cytoskeletal elements in the batś organ of Corti is similar to that of other mammals, however, certain features suggest the presence of subtle differences in micromechanical properties: there is an increased concentration of microtubules in the neck portion of IHCs, an increase in the amount of F-actin within the Deiters cups and a reduced amount of microtubules in the OHCs.

Cytoskeletal proteins in human hair cells

Human cochlear and vestibular hair cells (HCs) were analysed for the three major cytoskeletal types of structures: microfilaments, microtubules, and intermediate filaments and several of their associated proteins. Specific immunomorphological features were identified: the apical surface, the infracuticular rod of F-actin, the cytoplasm and the HC bases and their interactions with adjacent supporting cells. There is morphological evidence for at least three functionally interacting units: the OHC as a cytoskeletal string interacting with the Deiter's cell, being a part of the rigid pancellular protein shell of the organ of Corti; compartmentalization of HCs (surrounded by rigid intermediate filament proteins in the supporting cells); and a calcium-guided short-loop feed-back control of type I HCs. The cytoskeletal composition of all four types of human HCs and their adjacent structures is similar to, although not identical with, corresponding compositions in other mammalian species.

The changing microtubule arrangements in developing hair cells of the chick cochlea

It has been suggested that microtubules in auditory hair cells might be involved in directing the morphological and hence functional polarisation of the sensory hair bundles. The distribution of microtubules was studied in hair cells of the chick cochlea, during the developmental stages when the stereocilia and cuticular plate were being formed. Cochleae were immunofluorescently labelled with antibodies to tubulin at specific stages in development, and hair cell ultrastructure was observed by electron microscopy. We found that the microtubule array changed from a simple symmetrical apical plate with a central kinocilium before the cuticular plate forms, to a ring with the kinocilium to one side when the cuticular plate begins to form, through to a cup-like arrangement below the cuticular plate once the plate has formed. In the earliest stages, no asymmetries were observed in the distribution of the microtubules, suggesting that structures other than the microtubules set up the functional polarisation of the stereociliary bundle.

Structural basis for mechanical transduction in the frog vestibular sensory apparatus: II. The role of microtubules in the organization of the cuticular plate

The actin matrix of the cuticular plate, which supports the sensory stereocilia bundle, is coupled to the axial cytoskeleton of the hair cell through a well defined microtubule columnar framework. A collection of axial microtubules in a columnar organization penetrate deep into the dense actin matrix of the cuticular plate. Each microtubule displays at the end a 300-500 nm long fuzzy cap that enmeshes with the actin matrix of the cuticular plate. The microtubule associated proteins MAP-1A and MAP-1B were localized by confocal immunofluorescence to the point of microtubule insertion in the cuticular plate. These proteins are likely components of the microtubule capping structure and may mediate the interaction of the microtubules with the actin matrix. The structural interaction of the microtubules with the cuticular plate provides important mechanical coupling of the transduction apparatus to the axial cytoskeleton of the hair cell.

The cuticular plate of the hair cell in relation to morphological gradients of the chicken basilar papilla

The aim of the present study was to provide details on the diversity and morphological gradients in the anatomy of the cuticular plate of hair cells in the chicken basilar papilla. The structure of the cuticular plate, which is mainly made up of a network of actin filaments, may be related to differences in the mechanical demands on the anchorage of the stereovillar bundle. We describe the morphological gradients in the cuticular plates as seen in transverse section for four positions along the basilar papilla. Three different shapes of cuticular plate could be distinguished. In general, cuticular plates in neurally-lying hair cells have their main mass on the neural side of the cells; for abneural cells, the converse is true. The shape of the plates changes gradually across the papilla; symmetrical forms exist. The hair-cell bundle orientation (and thus the preferred direction of stimulation of the bundle), as measured using scanning EM preparations, does not correlate with the shape of the plate in transverse section. The present data confirm the notion developed from other studies that (1) there are no distinct populations of hair cells, (2) there are no linear or monotonic morphological gradients, and (3) the gradients on the papilla are species- and position-specific.

Characterization of cochlear outer hair cell turgor

The cochlear outer hair cell (OHC) is a cylindrical cell with structural features suggestive of a hydraulic skeleton, i.e., an elastic shell with a positive internal pressure. This study characterizes the role of the OHC elevated cytoplasmic pressure in maintaining the cell shape. Intracellular pressure of OHCs from guinea pig is estimated by measuring changes in cell morphology in response to increasing or decreasing osmolarity. Cells collapse when subjected to a continuous increase in osmolarity. Collapse occurs at an average of 8 mosM above the standard medium, suggesting that normal cells have an effective intracellular pressure of 128 mmHg. Fewer cells collapse when exposed to slow rates of osmolarity increase than cells exposed to fast rates of osmolarity increase, although the final change in osmolarity in the perfusion chamber is similar. Furthermore, cells undergo a slow, spontaneous increase in volume on exposure to either no osmolarity change or slow rates of osmolarity increase, suggesting that the cell's internal osmolarity increases in vitro. After volume reduction or elevation, cells do not return to their initial volume.

Glutathione S-transferases in the organ of Corti of the rat: enzymatic activity, subunit composition and immunohistochemical localization

Glutathione S-transferases (GSTs), a family of ubiquitous cytosolic isozymes, catalyze the detoxification of electrophilic substrates with reduced glutathione and participate in intracellular binding and transport of lipophilic substances. This study measured GST activity biochemically in the inner ear of the rat; determined the isozyme profile by Western blotting; and identified, immunohistochemically, the distribution of the mu and pi class GSTs in the organ of Corti. GST enzymatic activity in inner ear tissues ranged from 117 to 348 nmoles glutathione converted/min/mg protein, values somewhat higher than those found in brain (130) and much lower than in liver (1011). Of the GST isoforms, the pi class (identified by antibodies against the Yp subunit) was most prominent, the mu class (Yb1 subunit) clearly evident while the alpha class (Y(a) subunit) was barely detectable on Western blots. Immunocytochemical analysis showed differential distribution of the Yb1 and Yp subunits. The Yb1 subunit was present in the sensory cells, while supporting cells were not specifically stained. At the subcellular level, the isozyme was localized in the apical zones of inner (IHCs) and outer hair cells (OHCs) close to the cuticular plate. The extent of staining, however, varied between OHCs and IHCs. In the OHCs, staining appeared in discrete spots in the apical areas only, whereas in IHCs staining extended further towards the center of the cells. The Yp subunit was mainly localized to Deiters cell processes and pillar cells. Both Yb1 and Yp colocalized with tubulin-specific antibody. The functional significance of GST in the cochlear receptor cells is speculative. However, a role analogous to that in other tissues (detoxification, prostaglandin synthesis) can be assumed. In addition, an association of GST with the microtubule system is possible based on immunohistochemical colocalization with tubulin.

Cytoskeletal polypeptides: cell-type specific markers useful in investigative otorhinolaryngology

In the last decade, it has been established that eukaryotic cells possess a cytoskeleton, i.e. an integrated cytoplasmic network of microfilaments (MFs), microtubules (MTs) and intermediate filaments (IFs). Moreover, certain cell membrane specializations as well as the inner lamina of the nuclear membrane also participate in the cytoskeletal structure. Although this definition of the cytoskeleton is up to date it is obvious that the future course of cell biology will be reflected in a revised definition. While the bulk of structural polypeptides involved were characterized at regular intervals, surprisingly, the function of the cytoskeleton remained largely speculative and is still less precisely defined. The most widely postulated function concerns mechanical support and integration of diverse cellular activities and thus refers to cellular architecture. Briefly, the mechanical function is thought to involve cell movement, adhesive interaction with the extracellular matrix and neighbouring cells, as well as the stabilization of cell shape. The integrative function refers to intracellular movement, i.e. transport and positioning to the appropriate locations of organelles, intracellular particles, RNA and proteins. It has been established from numerous investigations that (certain) cytoskeletal polypeptides provide significant information about the cellular origin and differentiation state. This consideration constitutes the most prominent reflection underlying this review. Furthermore, this appreciation encourages additional efforts to explore these markers in normal and pathological conditions. The first purpose of this review is briefly to summarize our present comprehension of the molecular components of the cytoskeleton, restricted to the filamentous trinity for practical reasons. The second and main aim is to survey the field with respect to otorhinolaryngology-related issues. To the author's knowledge, this has not been dealt with in the past. In bridging this gap in the literature, I hope to provoke additional interest in one of the fastest moving areas of cell biology. A comprehensive review covering the whole cytoskeleton has been covered by Preston et al. (The Cytoskeleton and Cell Motility. Blackie, Glasgow and London, 1990, pp. 7-69, 188-191). Additional information on the participating substructures is provided in the text, inclusive of last year's reviews.

Protein gene product 9.5 in the developing cochlea of the rat: cellular distribution and relation to the cochlear cytoskeleton

Protein gene product 9.5 was immunolocalized in the adult and early postnatal (P2-P15) rat cochlea, and its distribution compared with a 200 kDa highly phosphorylated neurofilament subunit (neurofilament 200) and alpha-tubulin. In the adult, Protein gene product 9.5 was expressed exclusively in cochlear nerve fibres and ganglion cells, a small percentage of these (Type II ganglion cells and olivocochlear bundle fibres) being intensely positive for both protein gene product and neurofilament 200. In postnatal development, pillar and Deiters' cells were at first (P2-P15) strongly positive for protein gene product 9.5, and hair cells moderately so. At P2, all nerve fibres and ganglion cells showed co-expression of protein gene product 9.5 and neurofilament 200, but at later stages, the subset of intensely co-labelled neurons appeared, nerve fibres at P7 onwards and ganglion cells from P12. There was no overt correlation between the onset of protein gene product 9.5 and alpha-tubulin expression in any cochlear component. Protein gene product 9.5 expression in ganglion cells was at first (P2 and P7) mainly nuclear, and later also cytoplasmic. It is concluded that there is a clear correlation of high levels of protein gene product 9.5 and neurofilament protein expression, and that protein gene product 9.5 is expressed in some non-neuronal cells of the cochlea during its early development, persisting until after hearing has commenced.

Immunocytochemical localization of 205 kDa microtubule-associated protein (205 kDa MAP) in the guinea pig organ of Corti

We have studied the immunocytochemical localization of microtubule-associated proteins (MAPs) in the guinea pig organ of Corti. Using immunological methods with antibodies against MAP1A, MAP1B, MAP2, tau and 205 kDa MAP, we have identified 205 kDa MAP as a major MAP of the sensory epithelium in the organ of Corti. Immunoperoxidase microscopic study has shown that both cochlear hair cells and supporting cells reacted with anti-205 kDa MAP antibody. Immunoelectron microscopy revealed that 205 kDa MAP was associated with most microtubules in the sensory epithelial cells. It was also associated with the microtubules of bundle structures within supporting cells, suggesting that this MAP might form a part of cross-bridges between microtubules and between microtubules and actin filaments in the bundle structure. In contrast, MAP1A, MAP1B and tau, which are known to be expressed in neuronal tissue, were localized only in nerve fibers in the organ of Corti, not in the sensory epithelium. MAP2, which is known to be localized in dendrites and soma of nerve cells, was not distributed in nerve fibers in the organ of Corti. These results suggest possible roles of the 205 kDa MAP in the formation and maintenance of the highly polarized morphology of the epithelial cells of the organ of Corti, through stabilization and modulation of microtubule networks of these cells.

Postnatal development of the rat organ of Corti. II. Hair cell receptors and their supporting elements

The development of cochlear receptor cells and their supporting elements was studied by means of semi-thin and ultra-thin sections during the first postnatal weeks in the rat. The temporal and spatial patterns of the receptor cell development were investigated between the 4th and 24th days after birth. At approx. ten equidistant positions along the entire cochlear duct length of inner and outer hair cells, width of outer hair cell triad and stereocilia-length of the outer hair cells were quantitatively analyzed. Striking maturational changes take place before the 12th day after birth, that is, when the onset of hearing occurs. These changes are the formation of the tunnel of Corti, of the Nuel spaces, the appearance of filaments within the supporting elements and the change in cell shape of the hair cells. Between 4 days and 20 days after birth the maturation of outer hair cells is characterized by a decrease of organelles in the cytoplasm and establishment of the subsurface cistern. The quantitative analysis revealed a unique developmental pattern of the length of the outer hair cells, the width of the outer hair cell triad and the stereocilia length of the outer hair cells. Shortly after birth these structures have an almost constant size along the whole cochlear duct, but with increasing age the structures shorten at the cochlear base and enlarge at the apex. This pattern results in the establishment of a baso-apical gradient of the above mentioned structures. We assume that this baso-apical gradient is of central importance for the frequency representation.

Actin-binding and microtubule-associated proteins in the organ of Corti

Actin-binding and microtubule-associated proteins regulate microfilament and microtubule number, length, organization and location in cells. In freeze-dried preparations of the guinea pig cochlea, both actin and tubulin are found in the sensory and supporting cells of the organ of Corti. Fodrin (brain spectrin) co-localized with actin in the cuticular plates of both inner and outer hair cells and along the lateral wall of the outer hair cells. Alpha-actinin co-localized with actin in the cuticular plates of the hair cells and in the head and foot plates of the supporting cells. It was also found in the junctional regions between hair cells and supporting cells. Profilin co-localized with actin in the cuticular plates of the sensory hair cells. Myosin was detected only in the cuticular plates of the outer hair cells and in the supporting cells in the region facing endolymph. Gelsolin was found in the region of the nerve fibers. Tubulin is found in microtubules in all cells of the organ of Corti. In supporting cells, microtubules are bundled together with actin microfilaments and tropomyosin, as well as being present as individual microtubules arranged in networks. An intensely stained network of microtubules is found in both outer and inner sensory hair cells. The microtubules in the outer hair cells appear to course throughout the entire length of the cells, and based on their staining with antibodies to the tyrosinated form of tubulin they appear to be more dynamic structures than the microtubules in the supporting cells. The microtubule-associated protein MAP-2 is present only in outer hair cells within the organ of Corti and co-localizes with tubulin in these cells. No other MAPs (1,3,4,5) are present. Tau is found in the nerve fibers below both inner and outer hair cells and in the osseous spiral lamina. It is clear that the actin-binding and microtubule-associated proteins present in the cochlea co-localize with actin and tubulin and that they modulate microfilament and microtubule structure and function in a manner similar to that seen in other cell types. The location of some of these proteins in outer hair cells suggests a role for microfilaments and microtubules in outer hair cell motility.

Concomitant salicylate-induced alterations of outer hair cell subsurface cisternae and electromotility

Isolated cochlear outer hair cells undergo rapid, force-generating length changes in response to electrical stimulation. The cellular mechanism responsible for electromotility and its structural substrate is not yet known. Salicylates reduce and block electromotility in vitro. Therefore, we exposed isolated outer hair cells from the guinea pig cochlea to various doses of sodium salicylate and evaluated both ultrastructural changes and responses to electrical stimulation. Following salicylate superfusion, the subsurface cisternae showed dilatation, vesiculation and a deviation from their normal, unfenestrated, axial orientation below the plasma membrane. These changes were time and dose dependent and reversible over a time course of about 30 min. Electromotility was blocked and showed recovery following the same time course as the salicylate-induced reversible structural changes. These results indicate that intact, unfenestrated subsurface cisternae are required for the optimal generation of electrically-induced motility in mammalian outer hair cells.