Postnatal development of membrane specialisations of gerbil outer hair cells

Cochlear Fluid Spaces and Structures of the Gerbil High-Frequency Region Measured Using Optical Coherence Tomography (OCT)

Since it has been difficult to directly observe the morphology of the living cochlea, our ability to infer the mechanical functioning of the living ear has been limited. Nearly all our knowledge about cochlear morphology comes from postmortem tissue that was fixed and processed using procedures that possibly distort the structures and fluid spaces of the organ of Corti. In this study, optical coherence tomography was employed to obtain volumetric images of the high-frequency hook region of the gerbil cochlea, as viewed through the round window, with far better resolution capability than had been possible before. The anatomical structures and fluid spaces of the organ of Corti were segmented and quantified in vivo and over a 90-min postmortem period. We find that the arcuate-zone and pectinate-zone widths change very little postmortem. The volume of the scala tympani between the round-window membrane and basilar membrane and the volume of the inner spiral sulcus decrease in the first 60-min postmortem. While textbook drawings of the mammalian organ of Corti and cortilymph prominently depict the tunnel of Corti, the outer tunnel is typically missing. This is likely because textbook drawings are typically made from images obtained by histological methods. Here, we show that the outer tunnel is nearly twice as big as the tunnel of Corti or the space of Nuel. This larger outer tunnel fluid space could have a substantial, little-appreciated effect on cochlear micromechanics. We speculate that the outer tunnel forms a resonant structure that may affect reticular-lamina motion.

Dimensions of a Living Cochlear Hair Bundle

The hair bundle is the mechanosensory organelle of hair cells that detects mechanical stimuli caused by sounds, head motions, and fluid flows. Each hair bundle is an assembly of cellular-protrusions called stereocilia, which differ in height to form a staircase. Stereocilia have different heights, widths, and separations in different species, sensory organs, positions within an organ, hair-cell types, and even within a single hair bundle. The dimensions of the stereociliary assembly dictate how the hair bundle responds to stimuli. These hair-bundle properties have been measured previously only to a limited degree. In particular, mammalian data are either incomplete, lack control for age or position within an organ, or have artifacts owing to fixation or dehydration. Here, we provide a complete set of measurements for postnatal day (P) 11 C57BL/6J mouse apical inner hair cells (IHCs) obtained from living tissue, tissue mildly-fixed for fluorescent imaging, or tissue strongly fixed and dehydrated for scanning electronic microscopy (SEM). We found that hair bundles mildly-fixed for fluorescence had the same dimensions as living hair bundles, whereas SEM-prepared hair bundles shrank uniformly in stereociliary heights, widths, and separations. By determining the shrinkage factors, we imputed live dimensions from SEM that were too small to observe optically. Accordingly, we created the first complete blueprint of a living IHC hair bundle. We show that SEM-prepared measurements strongly affect calculations of a bundle’s mechanical properties – overestimating stereociliary deflection stiffness and underestimating the fluid coupling between stereocilia. The methods of measurement, the data, and the consequences we describe illustrate the high levels of accuracy and precision required to understand hair-bundle mechanotransduction.

Prestin-Dependence of Outer Hair Cell Survival and Partial Rescue of Outer Hair Cell Loss in PrestinV499G/Y501H Knockin Mice

A knockin (KI) mouse expressing mutated prestin^V499G/Y501H (499 prestin) was created to study cochlear amplification. Recordings from isolated outer hair cells (OHC) in this mutant showed vastly reduced electromotility and, as a consequence, reduced hearing sensitivity. Although 499 prestin OHCs were normal in stiffness and longer than OHCs lacking prestin, accelerated OHC death was unexpectedly observed relative to that documented in prestin knockout (KO) mice. These observations imply an additional role of prestin in OHC maintenance besides its known requirement for mammalian cochlear amplification. In order to gain mechanistic insights into prestin-associated OHC loss, we implemented several interventions to improve survival. First, 499 prestin KI’s were backcrossed to Bak KO mice, which lack the mitochondrial pro-apoptotic gene Bak. Because oxidative stress is implicated in OHC death, another group of 499 prestin KI mice was fed the antioxidant diet, Protandim. 499 KI mice were also backcrossed onto the FVB murine strain, which retains excellent high-frequency hearing well into adulthood, to reduce the compounding effect of age-related hearing loss associated with the original 499 prestin KIs. Finally, a compound heterozygous (chet) mouse expressing one copy of 499 prestin and one copy of KO prestin was also created to reduce quantities of 499 prestin protein. Results show reduction in OHC death in chets, and in 499 prestin KIs on the FVB background, but only a slight improvement in OHC survival for mice receiving Protandim. We also report that improved OHC survival in 499 prestin KIs had little effect on hearing phenotype, reaffirming the original contention about the essential role of prestin’s motor function in cochlear amplification.

Prestin at year 14: progress and prospect

Prestin, the motor protein of cochlear outer hair cells, was identified 14 years ago. Prestin-based outer hair cell motility is responsible for the exquisite sensitivity and frequency selectivity seen in the mammalian cochlea. Prestin is the 5th member of an eleven-member membrane transporter superfamily of SLC26A proteins. Unlike its paralogs, which are capable of transporting anions across the cell membrane, prestin primarily functions as a motor protein with unique capability of performing direct and reciprocal electromechanical conversion on microsecond time scale. Significant progress in the understanding of its structure and the molecular mechanism has been made in recent years using electrophysiological, biochemical, comparative genomics, structural bioinformatics, molecular dynamics simulation, site-directed mutagenesis and domain-swapping techniques. This article reviews recent advances of the structural and functional properties of prestin with focus on the areas that are critical but still controversial in understanding the molecular mechanism of how prestin works: The structural domains for voltage sensing and interaction with anions and for conformational change. Future research directions and potential application of prestin are also discussed. This article is part of a Special Issue entitled <Annual Reviews 2014>.

Gipc1 has a dual role in Vangl2 trafficking and hair bundle integrity in the inner ear

Vangl2 is one of the central proteins controlling the establishment of planar cell polarity in multiple tissues of different species. Previous studies suggest that the localization of the Vangl2 protein to specific intracellular microdomains is crucial for its function. However, the molecular mechanisms that control Vangl2 trafficking within a cell are largely unknown. Here, we identify Gipc1 (GAIP C-terminus interacting protein 1) as a new interactor for Vangl2, and we show that a myosin VI-Gipc1 protein complex can regulate Vangl2 traffic in heterologous cells. Furthermore, we show that in the cochlea of MyoVI mutant mice, Vangl2 presence at the membrane is increased, and that a disruption of Gipc1 function in hair cells leads to maturation defects, including defects in hair bundle orientation and integrity. Finally, stimulated emission depletion microscopy and overexpression of GFP-Vangl2 show an enrichment of Vangl2 on the supporting cell side, adjacent to the proximal membrane of hair cells. Altogether, these results indicate a broad role for Gipc1 in the development of both stereociliary bundles and cell polarization, and suggest that the strong asymmetry of Vangl2 observed in early postnatal cochlear epithelium is mostly a 'tissue' polarity readout.

The ultrastructural distribution of prestin in outer hair cells: a post-embedding immunogold investigation of low-frequency and high-frequency regions of the rat cochlea

Outer hair cells (OHCs) of the mammalian cochlea besides being sensory receptors also generate force to amplify sound-induced displacements of the basilar membrane thus enhancing auditory sensitivity and frequency selectivity. This force generation is attributable to the voltage-dependent contractility of the OHCs underpinned by the motile protein, prestin. Prestin is located in the basolateral wall of OHCs and is thought to alter its conformation in response to changes in membrane potential. The precise ultrastructural distribution of prestin was determined using post-embedding immunogold labelling and the density of the labelling was compared in low-frequency and high-frequency regions of the cochlea. The labelling was confined to the basolateral plasma membrane in hearing rats but declined towards the base of the cells below the nucleus. In pre-hearing animals, prestin labelling was lower in the membrane and also occurred in the cytoplasm, presumably reflecting its production during development. The densities of labelling in low-frequency and high-frequency regions of the cochlea were similar. Non-linear capacitance, thought to reflect charge movements during conformational changes in prestin, was measured in OHCs in isolated cochlear coils of hearing animals. The OHC non-linear capacitance in the same regions assayed in the immunolabelling was also similar in both the apex and base, with charge densities of 10,000/microm(2) expressed relative to the lateral membrane area. The results suggest that prestin density, and by implication force production, is similar in low-frequency and high-frequency OHCs.

Changes in plasma membrane structure and electromotile properties in prestin deficient outer hair cells

Cochlear outer hair cells (OHCs) rapidly change their length and stiffness when their membrane potential is altered. Prestin, the motor protein for this electromotility, is present along the OHC lateral plasma membrane where there is a high density of intra-membrane protein particles (IMPs). However, it is not known to what extent prestin contributes to this unusual dense population of proteins and overall organization of the membrane to generate the unique electromechanical response of OHCs. We investigated the relationship of prestin with the IMPs, the underlying cortical cytoskeletal lattice, and electromotility in prestin-deficient mice. Using freeze-fracture, we observed a reduction in density and size of the IMPs that correlates with the reduction and absence of prestin in the heterozygous and homozygous mice, respectively. We also observed a reduction or absence of electromotility-related charge density, axial stiffness, and piezoelectric properties of the OHC. A comparison of the charge density with the number of IMPs suggests that prestin forms tetramers in the wild type but is likely to form lower number oligomers in the prestin-deficient OHCs from the heterozygous mice. Interestingly, the characteristic actin-based cortical cytoskeletal lattice that underlies the membrane is absent in the prestin-null OHCs, suggesting that prestin is also required for recruiting or maintaining the cortical cytoskeletal lattice. These results suggest that the majority of the IMPs are indeed prestin and that electrically evoked length and stiffness changes are interrelated and dependent on both prestin and on the cortical actin cytoskeletal lattice of the OHC lateral membrane.

Prestin forms oligomer with four mechanically independent subunits

Prestin is the motor protein of cochlear outer hair cells (OHCs) with the unique capability of performing direct, rapid, and reciprocal electromechanical conversion. Prestin consists of 744 amino acids with a molecular mass of approximately 81.4 kDa. The predicted membrane topology and molecular mass of a single prestin molecule appear inadequate to account for the size of intramembrane particles (IMPs) expressed in the OHC membrane. Although recent biochemical evidence suggests that prestin forms homo-oligomers, most likely as a tetramer, the oligomeric structure of prestin in OHCs remains unclear. We obtained the charge density of prestin in the gerbil OHCs by measuring their nonlinear capacitance (NLC). The average charge density (22,608 microm(-2) measured was four times the average IMP density (5686 microm(-2) reported in the freeze-fracture study. This suggests that each IMP contains four prestin molecules, based on the general notion that each prestin transfers a single elementary charge. We subsequently compared the voltage dependency and the values of slope factor of NLC and somatic motility simultaneously measured from the same OHCs to determine whether NLC and motility are fully coupled and how prestin subunits function within the tetramer. We showed that the voltage dependency and slope factors of NLC and motility were not statistically different, suggesting that NLC and motility are fully coupled. The fact that the slope factor is the same between NLC and motility suggests that each prestin monomer in the tetramer is in parallel, each interacting independently with cytoplasmic or other partners to facilitate the mechanical response.

Expression of prestin and Gata-3,-2,-1 mRNA in the rat organ of Corti during the postnatal period and in culture

Based on observations that mutations of GATA-3 are responsible for the HDR-syndrome (hypoparathyroidism, deafness, renal defects) and that GATA-transcription factors have an important role to play in inner ear development, we hypothesized that these transcription factors may be involved in regulatory changes of prestin transcription. To prove this, we examined in parallel the expression of mRNA of prestin and Gata-3,-2 and Gata-1 in the organ of Corti during early postnatal development of rats and in organotypic cultures. Remarkable relations are observed between prestin and Gata-3,-2 expression in organ of Corti preparations in vivo and in vitro: (i) Gata-3,-2 expression display similar apical-basal gradients as prestin mRNA levels. (ii) The prestin expression increases between postnatal day two and postnatal day eight by a factor of about four in the apical and middle segments and by a factor of two in the basal part. Highly significant Pearson correlation coefficients were observed between Gata-3,-2 mRNA and prestin levels when the data were evaluated by regression analyses. (iii) Parallel changes of prestin mRNA and Gata-3,-2 mRNA levels were observed in response to thyroid hormone and to gemfibrozil application. These observations suggest a regulatory role played by the Gata-3,-2 transcription factors in prestin expression.

Atomic force microscopy in studies of the cochlea

The high sensitivity of mammalian hearing is achieved by amplification of the motion of the cochlear partition. The origin of this cochlear amplification is the elongation and contraction of outer hair cells (OHCs) in response to acoustical stimulation. This motility is made possible by a membrane protein embedded in the lateral membrane of OHCs. The gene of this protein has been identified and termed prestin. We, herein, present a method for observation by atomic force microscopy (AFM) of prestin expressed in the Chinese hamster ovary (CHO) cell plasma membrane. To obtain a stable sample for AFM imaging in liquid, we used as an example in the protocol provide here, CHO cells transfected with prestin or FLAG-tagged prestin, and untransfected CHO cells. The cells attached to a substrate were subjected to ultrasonic waves generated from a sonicator probe so that the inside-out plasma membranes remained on the substrate. Prestin was immunostained with mouse anti-FLAG primary antibody and FITC-conjugated goat anti-mouse IgG secondary antibody. The lipid of the plasma membrane was labeled with fluorescence probes. The cytoplasmic faces of the cells were then observed in liquid by the tapping mode of AFM at low and high magnifications. More particle-like structures 8-12 nm in diameter were observed in the plasma membranes of the prestin-transfected CHO cells than in those of the untransfected CHO cells. Since the difference between these two types of cells is due to the existence of prestin, such particle-like structures in the prestin-transfected CHO cells are possibly constituted by prestin.

Prestin up-regulation in chronic salicylate (aspirin) administration: an implication of functional dependence of prestin expression

Salicylate (aspirin) can reversibly eliminate outer hair cell (OHC) electromotility to induce hearing loss. Prestin is the OHC electromotility motor protein. Here we report that, consistency with increase in distortion product otoacoustic emission, long-term administration of salicylate can increase prestin expression and OHC electromotility. The prestin expression at the mRNA and protein levels was increased by three- to four-fold. In contrast to the acute inhibition, the OHC electromotility associated charge density was also increased by 18%. This incremental increase was reversible. After cessation of salicylate administration, the prestin expression returned to normal. We also found that long-term administration of salicylate did not alter cyclooxygenase (Cox) II expression but down-regulated NF-kappaB and increased nuclear transcription factors c-fos and egr-1. The data suggest that prestin expression in vivo is dynamically up-regulated to increase OHC electromotility in long-term administration of salicylate via the Cox-II-independent pathways.

Cochlear outer hair cell motility

Normal hearing depends on sound amplification within the mammalian cochlea. The amplification, without which the auditory system is effectively deaf, can be traced to the correct functioning of a group of motile sensory hair cells, the outer hair cells of the cochlea. Acting like motor cells, outer hair cells produce forces that are driven by graded changes in membrane potential. The forces depend on the presence of a motor protein in the lateral membrane of the cells. This protein, known as prestin, is a member of a transporter superfamily SLC26. The functional and structural properties of prestin are described in this review. Whether outer hair cell motility might account for sound amplification at all frequencies is also a critical question and is reviewed here.

Lateral wall protein content mediates alterations in cochlear outer hair cell mechanics before and after hearing onset

Specialized outer hair cells (OHCs) housed within the mammalian cochlea exhibit active, nonlinear, mechanical responses to auditory stimulation termed electromotility. The extraordinary frequency resolution capacity of the cochlea requires an exquisitely equilibrated mechanical system of sensory and supporting cells. OHC electromotile length change, stiffness, and force generation are responsible for a 100-fold increase in hearing sensitivity by augmenting vibrational input to non-motile sensory inner hair cells. Characterization of OHC mechanics is crucial for understanding and ultimately preventing permanent functional deficits due to overstimulation or as a consequence of various cochlear pathologies. The OHCs' major structural assembly is a highly-specialized lateral wall. The lateral wall consists of three structures; a plasma membrane highly-enriched with the motor-protein prestin, an actin-spectrin cortical lattice, and one or more layers of subsurface cisternae. Technical difficulties in independently manipulating each lateral wall constituent have constrained previous attempts to analyze the determinants of OHCs' mechanical properties. Temporal separations in the accumulation of each lateral wall constituent during postnatal development permit associations between lateral wall structure and OHC mechanics. We compared developing and adult gerbil OHC axial stiffness using calibrated glass fibers. Alterations in each lateral wall component and OHC stiffness were correlated as a function of age. Reduced F-actin labeling was correlated with reduced OHC stiffness before hearing onset. Prestin incorporation into the PM was correlated with increased OHC stiffness at hearing onset. Our data indicate lateral wall F-actin and prestin are the primary determinants of OHC mechanical properties before and after hearing onset, respectively.

Developmental expression of the outer hair cell motor prestin in the mouse

The development of motor protein activity in the lateral membrane of the mouse outer hair cell (OHC) from postnatal day 5 (P5) to P18 was investigated under whole-cell voltage clamp. Voltage-dependent, nonlinear capacitance (C (v)), which represents the conformational fluctuations of the motor molecule, progressively increased during development. At P12, the onset of hearing in the mouse, C (v) was about 70% of the mature level. C (v) saturated at P18 when hearing shows full maturation. On the other hand, C (lin), which represents the membrane area of the OHC, showed a relatively small increase with development, reaching steady state at P10. This early maturation of linear capacitance is further supported by morphological estimates of surface area during development. These results, in light of recent prestin knockout experiments and our results with quantitative polymerase chain reaction, suggest that, rather than the incorporation of new motors into the lateral membrane after P10, molecular motors mature to augment nonlinear capacitance. Thus, current estimates of motor protein density based on charge movement may be exaggerated. A corresponding indicator of motor maturation, the motor's operating voltage midpoint, V (pkcm), tended to shift to depolarized potentials during postnatal development, although it was unstable prior to P10. However, after P14, V (pkcm) reached a steady-state level near -67 mV, suggesting that intrinsic membrane tension or intracellular chloride, each of which can modulate V (pkcm), may mature at P14. These developmental data significantly alter our understanding of the cellular mechanisms that control cochlear amplification and provide a foundation for future analysis of genetic modifications of mouse auditory development.

Imaging by atomic force microscopy of the plasma membrane of prestin-transfected Chinese hamster ovary cells

The high sensitivity of mammalian hearing is achieved by amplification of the motion of the cochlear partition. This cochlear amplification is thought to be generated by the elongation and contraction of outer hair cells (OHCs) in response to acoustical stimulation. This motility is made possible by a membrane protein embedded in the lateral membrane of OHCs. Although a fructose transporter, GLUT-5, was initially proposed to be this protein, a later study identified the gene of the motor protein distributed throughout the OHC plasma membrane. This protein has been named "prestin." However, although previous morphological studies by electron microscopy and atomic force microscopy (AFM) found the lateral wall of OHCs to be covered with 10-nm particles, believed to be motor proteins, it is unknown whether such particles consist only of prestin or are a complex of GLUT-5 and prestin molecules. To determine if the 10-nm particles are indeed constituted only of prestin, plasma membranes of prestin-transfected and untransfected Chinese hamster ovary (CHO) cells, which do not express GLUT-5, were observed by AFM. First, the cells attached to a substrate were sonicated so that only the plasma membrane remained on the substrate. The cytoplasmic face of the cell was observed by the tapping mode of the AFM in liquid. As a result, particle-like structures were recognized on the plasma membranes of both the prestin-transfected and untransfected CHO cells. Comparison of the difference in the frequency distribution of these structures between those two cells showed approximately 75% of the particle-like structures with a diameter of 8-12 nm in the prestin-transfected CHO cells to be possibly constituted only by prestin molecules. Our data suggest that the densely packed 10-nm particles observed on the OHC lateral wall are likely to be constituted only of prestin molecules.

Analysis of the oligomeric structure of the motor protein prestin

Prestin, a member of the solute carrier family 26, is expressed in the basolateral membrane of outer hair cells. This protein provides the molecular basis for outer hair cell somatic electromotility, which is crucial for the frequency selectivity and sensitivity of mammalian hearing. It has long been known that there are abundantly expressed approximately 11-nM protein particles present in the basolateral membrane. These particles were hypothesized to be the motor proteins that drive electromotility. Because the calculated size of a prestin monomer is too small to form an approximately 11-nM particle, the possibility of prestin oligomerization was examined. We investigated possible quaternary structures of prestin by lithium dodecyl sulfate-PAGE, perfluoro-octanoate-PAGE, a membrane-based yeast two-hybrid system, and chemical cross-linking experiments. Prestin, obtained from different host or native cells, is resistant to dissociation by lithium dodecyl sulfate and behaves as a stable oligomer on lithium dodecyl sulfate-PAGE. In the membrane-based yeast two-hybrid system, homo-oligomeric interactions between prestin-bait/prestin-prey suggest that prestin molecules can associate with each other. Chemical cross-linking experiments, perfluoro-octanoate-PAGE/Western blot, and affinity purification experiments all indicate that prestin exists as a higher order oligomer, such as a tetramer, in prestin-expressing yeast, mammalian cell lines and native outer hair cells. Our data from experiments using hydrophobic and hydrophilic reducing reagents suggest that the prestin dimer is connected by a disulfide bond embedded in the prestin hydrophobic core. This stable dimer may act as the building block for producing the higher order oligomers that form the approximately 11-nM particles in the outer hair cell's basolateral membrane.

Low density of membrane particles in auditory hair cells of lizards and birds suggests an absence of somatic motility

Hair cells are the mechanoreceptive cells of the vertebrate lateral line and inner ear. In addition to their sensory function, hair cells display motility and thus themselves generate mechanical energy, which is thought to enhance sensitivity. Two principal cellular mechanism are known that can mediate hair-cell motility in vitro. One of these is based on voltage-dependent changes of an intramembrane protein and has so far been demonstrated only in outer hair cells of the mammalian cochlea. Correlated with this, the cell membranes of outer hair cells carry an extreme density of embedded particles, as revealed by freeze fracturing. The present study explored the possibility of membrane-based motility in hair cells of nonmammals, by determining their density of intramembrane particles. Replicas of freeze-fractured membrane were prepared from auditory hair cells of a lizard, the Tokay gecko, and a bird, the barn owl. These species were chosen because of independent evidence for active cochlear mechanics, in the form of spontaneous otoacoustic emissions. For quantitative comparison, mammalian inner and outer hair cells, as well as vestibular hair, cells were reevaluated. Lizard and bird hair cells displayed median densities of 2,360 and 1,880 intramembrane particles/microm2, respectively. This was not significantly different from the densities in vestibular and mammalian inner hair cells; however, it was about half the density in of mammalian outer hair cells. This suggests that nonmammalian hair cells do not possess high densities of motor protein in their membranes and are thus unlikely to be capable of somatic motility.

The molecular architecture of the inner ear

The inner ear is structurally complex. A molecular description of its architecture is now emerging from the use of contemporary methods of cell and molecular biology, and from studies of ontogenetic development. With the application of clinical and molecular genetics, it has now become possible to identify genes associated with inherited, non-syndromic deafness and balance dysfunction in humans and in mice. This work is providing new insights into how the tissues of the inner ear are built to perform their tasks, and into the pathogenesis of a range of inner ear disorders.

Myosin VI, a new force in clathrin mediated endocytosis

The integrity of the actin cytoskeleton and associated motor proteins are essential for the efficient functioning of clathrin mediated endocytosis at least in polarised cells. Myosin VI, the only motor protein so far identified that moves towards the minus end of actin filaments, is the first motor protein to be shown to associate with clathrin coated pits/vesicles at the plasma membrane and to modulate clathrin mediated endocytosis. Recent kinetic studies suggest that myosin VI may move processively along actin filaments providing clues about its functions in the cell. The possible role(s) of myosin VI in the sequential steps involved in receptor mediated endocytosis are discussed.

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.

Keratin filament deployment and cytoskeletal networking in a sensory epithelium that vibrates during hearing

The intricate and spatially precise ways in which keratin intermediate filaments are deployed in certain cochlear epithelial cells, called supporting cells, suggests that these filaments make a micromechanically important contribution to the functional design of the guinea pig organ of Corti. Filament arrays that include keratins 8, 18, and 19 are confined mainly to regions close to the ends of large transcellular microtubule bundles in supporting cells. These cells and their microtubule bundles link sensory hair cells to a specialized basement membrane that vibrates during hearing. The keratin filament arrays apparently help anchor the ends of the microtubule bundles to cell surfaces. Filaments are concentrated at the apices and bases of most cells that contact hair cells. Substantial arrays of adherens junctions link the apices of these cells. Hence, keratin filaments may contribute to a cytoskeletal network that distributes mechanical forces from cell to cell and that coordinates the displacement of neighboring hair cells. However, high concentrations of keratin filaments have not been detected at the apices of one of the supporting cell types, which apparently has a mechanical role that is different from that of the others. Transmission electron microscopy has revealed previously undescribed filament networks at all the locations where the binding of antibodies to keratins is most marked. There is evidence that intercellular linkage of the keratin networks via their association with actin-containing meshworks and adherens junctions is more extensive than linkage provided by desmosomes.

Prestin is the motor protein of cochlear outer hair cells

The outer and inner hair cells of the mammalian cochlea perform different functions. In response to changes in membrane potential, the cylindrical outer hair cell rapidly alters its length and stiffness. These mechanical changes, driven by putative molecular motors, are assumed to produce amplification of vibrations in the cochlea that are transduced by inner hair cells. Here we have identified an abundant complementary DNA from a gene, designated Prestin, which is specifically expressed in outer hair cells. Regions of the encoded protein show moderate sequence similarity to pendrin and related sulphate/anion transport proteins. Voltage-induced shape changes can be elicited in cultured human kidney cells that express prestin. The mechanical response of outer hair cells to voltage change is accompanied by a 'gating current', which is manifested as nonlinear capacitance. We also demonstrate this nonlinear capacitance in transfected kidney cells. We conclude that prestin is the motor protein of the cochlear outer hair cell.

Role of myosin VI in the differentiation of cochlear hair cells

The mouse mutant Snell's waltzer (sv) has an intragenic deletion of the Myo6 gene, which encodes the unconventional myosin molecule myosin VI (K. B. Avraham et al., 1995, Nat. Genet. 11, 369-375). Snell's waltzer mutants exhibit behavioural abnormalities suggestive of an inner ear defect, including lack of responsiveness to sound, hyperactivity, head tossing, and circling. We have investigated the effects of a lack of myosin VI on the development of the sensory hair cells of the cochlea in these mutants. In normal mice, the hair cells sprout microvilli on their upper surface, and some of these grow to form a crescent or V-shaped array of modified microvilli, the stereocilia. In the mutants, early stages of stereocilia development appear to proceed normally because at birth many stereocilia bundles have a normal appearance, but in places there are signs of disorganisation of the bundles. Over the next few days, the stereocilia become progressively more disorganised and fuse together. Practically all hair cells show fused stereocilia by 3 days after birth, and there is extensive stereocilia fusion by 7 days. By 20 days, giant stereocilia are observed on top of the hair cells. At 1 and 3 days after birth, hair cells of mutants and controls take up the membrane dye FM1-43, suggesting that endocytosis occurs in mutant hair cells. One possible model for the fusion is that myosin VI may be involved in anchoring the apical hair cell membrane to the underlying actin-rich cuticular plate, and in the absence of normal myosin VI this apical membrane will tend to pull up between stereocilia, leading to fusion.

Expression density and functional characteristics of the outer hair cell motor protein are regulated during postnatal development in rat

1. The non-linear capacitance (Cnon-lin) of postnatal outer hair cells (OHCs) of the rat was measured by a patch-clamp lock-in technique. Cnon-lin is thought to result from a membrane protein that provides the molecular basis for the unique electromotility of OHCs by undergoing conformational changes in response to changes in membrane potential (Vm). Protein conformation is coupled to Vm by a charged voltage sensor, which imposes Cnon-lin on the OHC. Cnon-lin was investigated in order to characterize the surface expression and voltage dependence of this motor protein during postnatal development. 2. On the day of birth (P0), Cnon-lin was not detected in OHCs of the basal turn of the cochlea, whilst it was 89 fF in apical OHCs. Cnon-lin increased gradually during postnatal development and reached 2.3 pF (basal turn, P9) and 7.5 pF (apical turn, P14) at the oldest developmental stages covered by our measurements. The density of the protein in the plasma membrane, deduced from non-linear charge movement per membrane area, increased steeply between P6 and P11 and reached steady state (4200 e- microm-2) at about P12. 3. Voltage at peak capacitance (V) shifted with development from hyperpolarized potentials shortly after birth (-88.3 mV, P2) to the depolarized potential characteristic of mature OHCs (-40.8 mV, P14). This developmental difference in V was also observed in outside-out patches immediately after patch excision. During subsequent wash-out V shifted towards the depolarized value found in the adult state, suggesting a direct modulation of the molecular motor. 4. Thus, the density of the motor protein in the plasma membrane and also its voltage dependence change concomitantly in the postnatal period and reach adult characteristics right at the onset of hearing.

A sugar transporter as a candidate for the outer hair cell motor

Forces developed by cochlear outer hair cells (OHCs) are responsible for the sharp tuning that underlies sensitivity and frequency selectivity in the ear. OHCs exhibit a voltage-dependent motility involving a 'motor' protein embedded in the basolateral membrane. The motor has so far resisted molecular identification. Here we provide evidence that it may be related to a fructose transporter. We show that OHCs are able to transport this sugar selectively and that the sugar alters electrical properties of the OHC motor. These data can be combined into an integrated model of a sugar carrier, that makes the novel prediction, demonstrated here, that such 'neutral' transporters can be voltage dependent.

Immunohistochemical localization of Ca2+/Calmodulin-dependent protein kinase IV in outer hair cells

A smooth membrane system consisting of subsurface cisternae (SSC) underlies the lateral plasmalemma of auditory outer hair cells (OHCs). The SSC contain Ca-ATPase and are regarded as an intracellular Ca2+ reservoir like the sarcoplasmic reticulum of myocytes. Recently, it has been demonstrated that Ca-ATPase activity in sarcoplasmic reticulum is regulated by Ca2+/calmodulin-dependent protein kinases (CaM kinases). Here we investigated the presence of CaM kinases in OHCs and their possible association with the SSC. Inner ears collected from adult gerbils and from neonates at 2-day intervals between 0 and 20 days after birth were immunostained with antibodies specific for different CaM kinases. A polyclonal antiserum against CaM kinase IV yielded a strong immunostaining reaction along the lateral wall of OHCs. The staining appeared after the tenth postnatal day and continued into adulthood. No other site in the inner ear, including cochlear inner hair cells and vestibular hair cells, was reactive. The kinase's apparent association with the SSC strongly supports its involvement in intracellular Ca2+ homeostasis and suggests a role in regulating the OHCs' slow motile responses.

Planar polarization of Drosophila and vertebrate epithelia

Our understanding of the actin and microtubule rearrangements that generate planar polarity in Drosophila and in vertebrate epithelia has been extended by recent discoveries. Three different Rho family proteins have been shown to mediate polarization in the wing and the eye of Drosophila. In vertebrates, the importance of myosin VIIa has been uncovered by mutations that cause defects in planar polarization in the ear. Advances in our understanding of the Frizzled pathway, which coordinates planar polarization in Drosophila, are moving the field closer to understanding the links between signal transduction and polarized cytoskeletal reorganization.

Organization of cell junctions and cytoskeleton in the reticular lamina in normal and ototoxically damaged organ of Corti

The reticular lamina creates an ion barrier, withstands mechanical stress in the organ of Corti and is able to maintain its integrity during and after severe hair cell loss. Tight junctions maintain the ionic gradient whereas adherens junctions and the cytoskeleton are responsible for the integrity and mechanical resistance of tissues. In this study we used immunofluorescence and electron microscopy to examine the distribution of proteins of tight junctions (cingulin), adherens junctions (E-cadherin, alpha- and beta-catenin) and the cytoskeleton (actin, cytokeratin and tubulin) in whole-mounts of the normal and ototoxically damaged organ of Corti. In normal ears the proteins of adherens junctions were found in all cell types of the reticular lamina. We now demonstrate that all cells forming the reticular lamina partially overlap each other organizing extensive cell contacts with a complex three-dimensional shape. During scar formation, the tight junctions as well as adherens junctions between hair and supporting cells appeared in two distinct focal planes, which could help to preserve the ionic barrier and tissue integrity during hair cell degeneration. During scar formation all cytoskeletal structures in the reticular lamina were reorganized in a specific spatio-temporal pattern. We present a three-dimensional model of cell contact organization in the reticular lamina of normal ears and during scar formation.

Ultrastructural localization of cadherin in the adult guinea-pig organ of Corti

The apices of the majority of cells of the organ of Corti are connected together by junctional complexes to form the reticular lamina, a barrier that prevents the mixing of endolymph and perilymph. These complexes include tight junctions, adherens junctions and desmosomes. Further information is required about the identity and distribution of the molecules involved in these connections if the function and organization of the reticular lamina are to be well understood. One major category of molecules occurring in adherens junctions and desmosomes, and involved in the maintenance of tissue integrity, is the cadherins. However, although cadherin has been identified in junctions between supporting cells in the adult mammalian organ of Corti at the light microscopic level, its ultrastructural distribution has not so far been described. A post-embedding immunogold labelling technique has therefore been used in conjunction with a monoclonal antibody to cadherin to investigate its ultrastructural distribution in the adult guinea-pig reticular lamina. Immunolabelling is observed in hair cell-supporting cell junctions and in supporting cell-supporting cell junctions. In addition, there is more labelling associated with inner hair cell-supporting cell junctions than with outer hair cell-supporting cell junctions. This may indicate that the junctions associated with the two types of hair cell have different functional properties.

Morphological evidence for supporting cell to hair cell conversion in the mammalian utricular macula

The possible origin of the immature hair cells that appear in the utricular maculae of guinea pigs following gentamicin-induced hair cell death was investigated. Guinea pigs were continuously infused with bromodeoxyuridine, to label proliferating cells and their progeny, for 2 weeks after inducing damage to the inner ear on one side with gentamicin. The opposite ear in each animal served as control. Serial sections were cut through the entire utricular maculae of both ears of each animal and the number of labelled cells in the epithelium and underlying connective tissue was counted. Label was present in cells in the sensory epithelium in the utricles from the drug exposed ears but not in the controls. The nuclei of cells in the underlying connective tissue were also labelled in both ears. Some of the labelled nuclei in the epithelium were at the level normally occupied by hair cells, but most were at the level of supporting cell nuclei. However, the total number of labelled nuclei in the sensory epithelium was small; the maximum was 12 in one animal. The number of labelled nuclei in the connective tissue of the treated ears was significantly greater than the number in the untreated ear. This confirms that cell proliferation is stimulated in the mature mammalian utricular macula after hair cell loss, but the extent to which it occurs appears to be insufficient to explain the recovery in hair cell numbers which is observed. Detailed thin section studies of the utricular maculae of gentamicin-treated animals over a prolonged post-treatment period were also performed. In utricles which had suffered damage, there were cells which, like supporting cells but unlike hair cells, were resting on basement membrane, but which possessed at their apical ends organized bundles of microvilli similar to immature hair cell stereocilia. Other cells with more obvious stereocilia remained in contact with the basement membrane via and a small feet process. In still other cells, where a stereociliary bundle was obvious and almost mature in appearance, there was a foot process extending towards the basement membrane but not quite in contact, suggesting it had just detached. All these cells were contacted by nerve endings and specialization of the membranes were apparent at the site of cell-neurone contact. The morphological characteristics of these cells are consistent with phenotypic conversion of supporting cells into hair cells and this may account for some of the hair cell production in the mature mammalian vestibular sensory epithelia after hair cell death.

Relationship between the development of outer hair cell electromotility and efferent innervation: a study in cultured organ of corti of neonatal gerbils

Outer hair cell (OHC) electromotility, which powers the cochlear amplifier, develops at a later stage of hearing ontogeny. There has been speculation whether efferents play a necessary role in directing or achieving OHC maturation in mammals. In this study, we examine whether the development of OHC motility depends on the establishment of efferent innervation of the cells' synaptic pole by measuring electromotility of OHCs grown in cultures, deprived of efferent innervation. Tissue cultures of the organ of Corti were prepared from the cochleas of newborn gerbils. Solitary OHCs were obtained from 4- to 15-d-old cultures by enzymatic digestion and mechanical trituration. Length changes evoked by transcellular electrical stimulation were detected and measured with a photodiode sensor. Results show that OHCs develop electromotility between 6 and 13 d in culture without the presence of efferent innervation. The timetable for the onset of OHC electromotility is comparable with that in vivo. This demonstrates that the ontogeny of OHC electromotility is an intrinsic process that does not require the influence of efferent innervation.