Developmentally-regulated coexpression of vimentin and cytokeratins in the rat inner ear

Evidence for a partial epithelial-mesenchymal transition in postnatal stages of rat auditory organ morphogenesis

The epithelial-mesenchymal transition (EMT) plays a crucial role in the differentiation of many tissues and organs. So far, an EMT was not detected in the development of the auditory organ. To determine whether an EMT may play a role in the morphogenesis of the auditory organ, we studied the spatial localization of several EMT markers, the cell-cell adhesion molecules and intermediate filament cytoskeletal proteins, in epithelium of the dorsal cochlea during development of the rat Corti organ from E18 (18th embryonic day) until P25 (25th postnatal day). We examined by confocal microscopy immunolabelings on cryosections of whole cochleae with antibodies anti-cytokeratins as well as with antibodies anti-vimentin, anti-E-cadherin and anti-β-catenin. Our results showed a partial loss of E-cadherin and β-catenin and a temporary appearance of vimentin in pillar cells and Deiters between P8 and P10. These observations suggest that a partial EMT might be involved in the remodelling of the Corti organ during the postnatal stages of development in rat.

Analysis of the mouse mutant Cloth-ears shows a role for the voltage-gated sodium channel Scn8a in peripheral neural hearing loss

Deafness is the most common sensory disorder in humans and the aetiology of genetic deafness is complex. Mouse mutants have been crucial in identifying genes involved in hearing. However, many deafness genes remain unidentified. Using N-ethyl N−nitrosourea (ENU) mutagenesis to generate new mouse models of deafness, we identified a novel semi-dominant mouse mutant, Cloth-ears (Clth). Cloth-ears mice show reduced acoustic startle response and mild hearing loss from ∼30 days old. Auditory-evoked brainstem response (ABR) and distortion product otoacoustic emission (DPOAE) analyses indicate that the peripheral neural auditory pathway is impaired in Cloth-ears mice, but that cochlear function is normal. In addition, both Clth/Clth and Clth/+ mice display paroxysmal tremor episodes with behavioural arrest. Clth/Clth mice also show a milder continuous tremor during movement and rest. Longitudinal phenotypic analysis showed that Clth/+ and Clth/Clth mice also have complex defects in behaviour, growth, neurological and motor function. Positional cloning of Cloth-ears identified a point mutation in the neuronal voltage-gated sodium channel α-subunit gene, Scn8a, causing an aspartic acid to valine (D981V) change six amino acids downstream of the sixth transmembrane segment of the second domain (D2S6). Complementation testing with a known Scn8a mouse mutant confirmed that this mutation is responsible for the Cloth-ears phenotype. Our findings suggest a novel role for Scn8a in peripheral neural hearing loss and paroxysmal motor dysfunction.

Macrophage contribution to the response of the rat organ of Corti to amikacin

Transdifferentiation of nonsensory supporting cells into sensory hair cells occurs naturally in the damaged avian inner ear. Such transdifferentiation was achieved experimentally in the cochlea of deaf guinea pigs through Atoh 1 gene transfection. Supporting cells may therefore serve as targets for transdifferentiation therapy. Supporting cells rapidly degenerate after hair cell disappearance, however, limiting the therapeutic window for gene transfer. We studied the time course of ultrastructural and phenotypical changes occurring in Deiters cells (hair cell supporting cells) after ototoxic treatment in the rat. The presence of macrophages in the cochlea was also investigated, to study any deleterious effects they may have on pathologic tissues. One week after treatment most hair cells had disappeared. Deiters cells no longer expressed the glial marker vimentin but instead displayed typical hair cell markers, the calcium binding proteins calbindin and parvalbumin. This suggests that a process of transdifferentiation of Deiters cells into hair cells was activated. By 3 weeks post-treatment, however, the Deiters cells began to degenerate and by 10 weeks post-treatment the organ of Corti was degraded fully. Interestingly, a marked increase in macrophage density was seen after the end of amikacin treatment to 10 weeks post-treatment. This suggests chronic inflammation is involved in epithelium degeneration. Consequently, early treatments with anti-inflammatory factors might promote supporting cell survival, thus improving the efficacy of more specific strategies aimed to regenerate hair cells from nonsensory cells.

Ventral otic cell lines as developmental models of auditory epithelial and neural precursors

Conditionally immortal cell lines were established from the ventral otocyst of the Immortomouse at embryonic day 10.5 and selected to represent precursors of auditory sensory neural and epithelial cells. Selection was based upon dissection, tissue-specific markers, and expression of the transcription factor GATA3. Two cell lines expressed GATA3 but possessed intrinsically different genetic programs under differentiating conditions. US/VOT-E36 represented epithelial progenitors with potential to differentiate into sensory and nonsensory epithelial cells. US/VOT-N33 represented migrating neuroblasts. Under differentiating conditions in vitro the cell lines expressed very different gene expression profiles. Expression of several cell- and tissue-specific markers, including the transcription factors Pax2, GATA3, and NeuroD, differed between the cell lines in a pattern consistent with that observed between their counterparts in vivo. We suggest that these and other conditionally immortal cell lines can be used to study transient events in development against different backgrounds of cell competence.

Nestin expression in the developing rat cochlea sensory epithelia

An intermediate filament (IF), nestin, is used as an immature cell marker because nestin occurs in neural progenitors during early development. Recent cell culture studies have indicated that proliferating otic progenitor cells express nestin in vitro. However, localization of nestin in the developing inner ear has not yet been clarified. In this study, the ontogenetical expression of nestin epitopes in the rat cochlea was examined immunohistochemically. Sensory epithelial cells in the rat Corti organ (e.g. hair cells and support cells) transiently demonstrated immunoreactivity for nestin during the late embryonic period. After birth, nestin expression in the sensory epithelia disappeared gradually. The findings of this study indicate that the expression of nestin epitopes in the developing cochlea is linked with the plasticities of sensory epithelial cells, such as proliferation or differentiation.

Structure and innervation of the cochlea

The role of the cochlea is to transduce complex sound waves into electrical neural activity in the auditory nerve. Hair cells of the organ of Corti are the sensory cells of hearing. The inner hair cells perform the transduction and initiate the depolarization of the spiral ganglion neurons. The outer hair cells are accessory sensory cells that enhance the sensitivity and selectivity of the cochlea. Neural feedback loops that bring efferent signals to the outer hair cells assist in sharpening and amplifying the signals. The stria vascularis generates the endocochlear potential and maintains the ionic composition of the endolymph, the fluid in which the apical surface of the hair cells is bathed. The mechanical characteristics of the basilar membrane and its related structures further enhance the frequency selectivity of the auditory transduction mechanism. The tectorial membrane is an extracellular matrix, which provides mass loading on top of the organ of Corti, facilitating deflection of the stereocilia. This review deals with the structure of the normal mature mammalian cochlea and includes recent data on the molecular organization of the main cell types within the cochlea.

Postnatal maturation of the dog stria vascularis-- an immunohistochemical study

The lateral wall of the dog cochlear duct was investigated by classical staining and immunohistochemistry for NaK/ATPase beta2 isoform, cytokeratins (Cks), vimentin, nestin, and S100A6 during the postnatal cochlear maturation, i.e., from birth to postnatal day 110. The dog stria vascularis was immature at birth. Fine melanin granules were evident in the stria from the second week of life, and melanin concentration increased drastically beyond the first month. The marginal cells were NaK/ATPase- and Ck-positive; intermediate cells were either nestin- and S100A6-positive or vimentin-positive; the basal cells were vimentin-positive; the capillary endothelium showed vimentin and nestin labeling; the cell layer underlying the stria was nestin-positive. The fibrocytes of the spiral ligament and spiral prominence expressed nestin and vimentin. The epithelial cells overlaying the spiral prominence and the external sulcus were Ck-positive, and transiently nestin- and vimentin-positive. Double immunolabeling, for S100A6 and either nestin, vimentin, or NaK/ATPase, and for nestin and vimentin suggested the presence of two distinct intermediate cell types. The results enabled us to differentiate the cell types forming the lateral wall of the dog cochlear duct, and to follow their postnatal maturation. This study may form a basis for future investigations about spontaneous cochleosaccular degeneration in dogs. This species is an important companion animal, and a possible model for the study of comparable diseases in humans.

Cell lines in inner ear research

Cell lines have provided important experimental tools that have enhanced our understanding of neural and sensory function. They are particularly valuable in inner ear research because the auditory and vestibular systems are small, complex, and encased in several layers of bone. Organotypic cultures provide an invaluable experimental resource but require repeated microdissection and culture, and remain complex in terms of cell types and states of differentiation. A number of laboratories have established cell lines that offer a range of potential applications to hearing research. This review describes the advances that have already been made with these lines and the potential applications that they offer in the future. The majority of the cell lines are immortalized with a conditionally expressed, temperature sensitive variant of the SV40 tumor antigen. We discuss the value of these cells in developmental studies.

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.

Differentiation of mammalian vestibular hair cells from conditionally immortal, postnatal supporting cells

We provide evidence from a newly established, conditionally immortal cell line (UB/UE-1) that vestibular supporting cells from the mammalian inner ear can differentiate postnatally into more than one variant of hair cell. A clonal supporting cell line was established from pure utricular sensory epithelia of H2k(b)tsA58 transgenic mice 2 d after birth. Cell proliferation was dependent on conditional expression of the immortalizing gene, the "T" antigen from the SV40 virus. Proliferating cells expressed cytokeratins, and patch-clamp recordings revealed that they all expressed small membrane currents with little time-dependence. They stopped dividing within 2 d of being transferred to differentiating conditions, and within a week they formed three defined populations expressing membrane currents characteristic of supporting cells and two kinds of neonatal hair cell. The cells expressed several characteristic features of normal hair cells, including the transcription factor Brn3.1, a functional acetylcholine receptor composed of alpha9 subunits, and the cytoskeletal proteins myosin VI, myosin VIIa, and fimbrin. Immunofluorescence labeling and electron microscopy showed that the cells formed complex cytoskeletal arrays on their upper surfaces with structural features resembling those at the apices of normal hair cells. The cell line UB/UE-1 provides a valuable in vitro preparation in which the expression of numerous structural and physiological components can be initiated or upregulated during early stages of mammalian hair cell commitment and differentiation.

Cyclic GMP and outer hair cell electromotility

The aim of this study is to examine the effect of phosphorylation pathways on the electrically evoked fast motile response of isolated outer hair cells (OHCs). Transcellular electrical stimulation was applied in the microchamber to guinea pig OHCs and motility was measured before and after drug application. Forskolin (adenylate cyclase activator), phorbol 12-myristate 13-acetate (PMA, protein kinase C activator) and dibutyryl 3',5'-cyclic guanosine monophosphate (cGMP agonist) were studied. As controls, L15 medium and dimethyl-sulfoxide (DMSO) were used. In each group, 12 cells were measured. Forskolin and PMA were dissolved in 0.1% DMSO to render them membrane permeable. DMSO by itself caused a statistically significant electromotility magnitude decrease. Forskolin and PMA could not reverse the motility decrease due to DMSO, the effects seen in their presence were the same as observed with DMSO alone. Thus, neither 3',5'-cyclic AMP-dependent protein kinase nor calcium/phospholipid-dependent protein kinase appear to have modulatory effects on electromotility. Dibutyryl cGMP (DBcGMP), in concentrations of 200 microM, elicited a significant electromotility magnitude increase. The DBcGMP effect could be inhibited by co-application of 200 microM DBcGMP and 100 microM 8-Rp-pCPT-cGMPS (8-4-chlorophenylthio-guanosine 3',5'-cyclic monophosphothioate, Rp isomer, a cGMP antagonist). Our results suggest that OHC electromotility is modulated by a cGMP-dependent pathway.

Antigen retrieval immunohistochemistry used for routinely processed celloidin-embedded human temporal bone sections: standardization and development

The use of immunohistochemistry (IHC) in routinely processed celloidin-embedded human temporal bone section has created a fruitful field in understanding the pathogenesis and pathophysiology of the human inner ear at a molecular level since the early 1990s when the antigen retrieval (AR) technique was developed. This review article focuses on several critical technical issues of AR technique based predominantly on our experiences and suggestions concerning further development and standardization of AR-IHC for IHC study of human temporal bone section, as well as other tissues embedded in celloidin. Examples of using AR-IHC in human temporal bone sections collected include our unpublished data in order to indicate the potential utility of this novel method. Suggestions of further development of AR techniques are proposed for references and comments.

Auditory hair cell precursors immortalized from the mammalian inner ear

Mammalian auditory hair cells are few in number, experimentally inaccessible, and do not proliferate postnatally or in vitro. Immortal cell lines with the potential to differentiate into auditory hair cells would substantially facilitate auditory research, drug development, and the isolation of critical molecules involved in hair cell biology. We have established two conditionally immortal cell lines that express at least five characteristic hair cell markers. These markers are the transcription factor Brn3.1, the alpha 9 subunit of the acetylcholine receptor, the stereociliary protein fimbrin and the myosins VI and VIIA. These hair cell precursors permit functional studies of cochlear genes and in the longer term they will provide the means to explore therapeutic methods of stimulating auditory hair cell regeneration.

Factors modulating supernumerary hair cell production in the postnatal rat cochlea in vitro

It has been shown in the past that extra hair cells or supernumerary cells can be produced when neonatal cochleae are maintained in vitro. In this report, we investigated the effects of the culture methods, molecules and growth factors that are thought to be involved in cell proliferation. Quantitative studies of supernumerary hair cells were made by measuring the cell density over the entire spiral lamina at two postnatal stages: birth and 3 days after birth. With a standard feeding solution without serum, a difference in cell density was observed between the two methods of culture. Cochlear explants in a standard feeding solution supplemented with serum showed an increase of cell density only when the explantation is made at birth. Retinoic acid added to the standard feeding solution did not increase the hair cell density, while insulin induced an increase, especially at 5 micrograms/ml. Several growth factors were tested. Epidermal growth factor (EGF) presented a dose dependent effect with an increase of up to 30% of hair cell density that was observed in the basal region when the explantation was made at birth. Transforming growth factor-alpha did not induce an increase of cell density, whereas transforming growth factor-beta presented an effect on hair cell density, with a dose dependent effect reaching 37.4% for the basal inner hair cells. Interpretation of these results is limited because of the lack of data concerning the presence of specific membrane receptors. One possibility is that insulin stimulates hair cell differentiation from existing undifferentiated cells. Another hypothesis may be related to the EGF and transforming growth factor-beta, where these molecules might induce transdifferentiation of cells by acting on the transmembrane molecules and the extracellular matrix.

The early postnatal development of F-actin patterns in the organ of Corti of the gerbil (Meriones unguiculatus) and the horseshoe bat (Rhinolophus rouxi)

The arrangements of F-actin in hair cells and non-sensory cells were studied in paraformaldehyde-fixed cochleae of horseshoe bats and gerbils in several postnatal stages and in the adult. Phallotoxin-labeled midmodiolar cryostat sections of the organ of Corti were analyzed with confocal fluorescence microscopy. In both species, the arrangement of F-actin in the adult organ of Corti was essentially similar to that described in other mammals; however, both species showed their own species-typical specializations in staining of the Deiters cells. In the gerbil, a distinct baso-apical gradient in morphology and staining properties was found in the upper compartment of the Deiters cells. In the bat, F-actin label within the Deiters cups was most pronounced in the basal cochlear turn and less abundant in the apical turns. During the first postnatal week, the sensory epithelium of the gerbil lacked the tunnel of Corti and the spaces of Nuel. Only the reticular lamina and the surface of the greater epithelial ridge were intensely labeled for F-actin. At 9 days after birth (DAB), when the tunnel of Corti and the inner spiral sulcus were formed, the footplates of Deiters and pillar cells and the apices of pillar cells began to show intense F-actin label. At 12 DAB, corresponding to onset of hearing, F-actin staining was found throughout the supporting cell bodies, but was less intense than in the adult. The specialized upper compartment of the Deiters cells differentiated around 15-20 DAB. In the neonate bat, gross-morphology of the organ of Corti was almost adult-like, but only the reticular lamina and the head- and footplates of pillar cells showed intense F-actin staining. The F-actin cytoskeleton of the Deiters cells bodies was poorly developed. At the onset of hearing (between 3rd and 5th DAB), supporting cells showed only a slight increase of F-actin mainly at mechanically important cell regions, namely the Deiters cups, the contact zone of pillar headplates and the footplates of supporting cells. The most intense increase of F-actin occurred between onset of hearing and 16 DAB. At 16 DAB, the F-actin distribution within the supporting cells was similar to the adult. In both species, there were no clear baso-apical gradients in development of F-actin patterns. It is proposed that F-actin insertion in supporting cells after the onset of hearing contributes to maturation of cochlear function.

Cytological changes related to maturation of the organ of Corti and opening of Corti's tunnel

Maturation of the organ of Corti in the gerbil was analyzed between 2 and 16 days after birth (DAB) by electron microscopy and immunostaining for beta-tubulin. At 2 DAB, the organ of Corti consisted of stratified epithelium bearing immature sensory hair cells (HCs) and supporting cells. Maturation of OHCs and Deiters cells progressed in a medial-to-lateral direction and cytoskeletal development in inner pillar cells preceded that in outer pillar cells at the single location studied along the frequency-place map. Pillar cell differentiation progressed through a unique stage characterized by the appearance and stratification of structural features apparently concerned with opening of Corti's tunnel and subsequently showed other structural changes related to maturity toward the adult form. Development of the microtubule cytoskeleton occurred first in the cell's apex and proceeded basally. Ruffling of a middle region of the cell surface by microvilli appeared to promote separation between inner and outer pillar cells and initiate tunnel opening at 4 DAB. Proliferation of distended cisternae of granular reticulum evidenced proteinaceous secretion by these cells between 4 and 8 DAB. Subsequent tunnel expansion at about 14 DAB coincided with appearance in outer pillar cells of tubulocisternal endoplasmic reticulum and associated Golgi complexes that are thought to mediate fluid and ion secretion. Sixteen days postnatally after disappearance of granular and tubulocisternal reticula and Golgi complexes and at the time of clearing of tunnel fluid, lysosomes interpreted as mediating catabolism of endocytosed protein congregated beneath the apical and apicolateral plasmalemmae of inner pillar cells. As with pillar cells, development of the microtubule system in Deiters cells proceeded from the cell's apex to base. Following differentiation of their microtubule system by 8 DAB, Deiters cells showed expansion of Golgi cisternae between 10 and 15 DAB and development of tubulocisternal endoplasmic reticulum at 15 DAB. Hair cells possessed abundant, distinctively large mitochondria from 4 to 10 DAB. The subsurface cisternae matured earlier in medial as opposed to lateral outer hair cells. Vesicles budding from underlying cisternae appeared associated with development of subsurface cisternae and at 16 DAB were still observed in third row but not in more mature first row HCs.

F-actin, tubulin and spectrin in the organ of Corti: comparative distribution in different cell types and mammalian species

Laser scanning confocal microscopy was used to determine the distribution of actin, spectrin and tubulin in whole mounts of the organ of Corti of guinea pig, monkey, rat and chinchilla. Actin, spectrin and tubulin were localized in all cell types in the auditory epithelium. No specialized cytoskeletal organization of tubulin was detected in the cytoplasmic domain of hair cells. The only specialized organization of actin and spectrin in the cytoplasmic domain was the infra-cuticular network, found exclusively in apical guinea pig outer hair cells. In contrast, the lateral wall of inner and outer hair cells contained a homogeneous distribution of label specific for actin and spectrin. The label intensity was similar in the base and the apex of the cochlea. These results indicate that the distribution of spectrin and actin in the auditory epithelium is similar to that in other epithelial cells, suggesting that actin and spectrin participate in the formation of cellular shape and possibly in docking molecules to the membrane.

Distribution of cytokeratins in the vestibular epithelium of the guinea pig

Cytokeratin expression in the vestibular labyrinth of the guinea pig was investigated with immunofluorescence and immunoperoxidase staining on surface preparations of the vestibular epithelium. Phalloidin, an F-actin-specific probe, was used to distinguish between hair cells and supporting cells. Cytokeratin expression was not found in the cytoplasmic domain of hair cells of the crista ampullaris, utricle, or saccule. Cytokeratin expression was abundant in supporting cells of the vestibular sensory epithelium. Electron microscopy revealed the presence of desmosomes, which are associated with cytokeratins, within type 2 hair cells of the vestibular epithelium. It appears that cytokeratins are absent within the cytoplasmic domain of hair cells, but are present in association with intercellular junctions. The functional significance of this unique pattern of cytokeratin expression within the vestibular epithelium is discussed.

Changes in cytokeratin, vimentin and desmoplakin distribution during the repair of irradiation-induced lung injury in adult rats

The expression of cytokeratins, desmoplakin and vimentin has been studied immunohistochemically in the rat lung injured by x-irradiation using 14 well characterized monoclonal antibodies. A time-dependent relationship between the cytokeratin expression pattern and the morphological alterations observed was apparent. A cytokeratin 8 and 18 expression in normally cytokeratectable even at 3-6 h after irradiation. Between 14 days and 2 months, a remarkable heterogeneity in the epithelial cell cytokeratin pattern and an increasing immunoreaction for desmoplakin was found. In terminal bronchial epithelial cells, a heterogeneous CK8, 18 and 19 staining and a neoexpression of cytokeratins 4 and 7 was detected. Finally, peribronchiolar and vascular smooth muscle cells were cytokeratin-positive. At 6 months after irradiation, cytokeratin 13 and vimentin were focally present in bronchial epithelial cells and atypical type I and II pneumocytes as well as scattered epithelioid cell complexes were noted. During the course of injury, a loss of type III alveolar epithelial cells was found, which was characterized in the rat by a specific globular cytokeratin pattern and restricted immunoreactivity with cytokeratin-specific antibodies. These results show that the expression pattern of cytokeratins is a sensitive marker in monitoring epithelial alterations during lung injury.