The Suppression of Ubiquitin C-Terminal Hydrolase L1 Promotes the Transdifferentiation of Auditory Supporting Cells into Hair Cells by Regulating the mTOR Pathway

In mammals, hearing loss is irreversible due to the lack of the regenerative capacity of the auditory epithelium. However, stem/progenitor cells in mammalian cochleae may be a therapeutic target for hearing regeneration. The ubiquitin proteasome system plays an important role in cochlear development and maintenance. In this study, we investigated the role of ubiquitin C-terminal hydrolase L1 (UCHL1) in the process of the transdifferentiation of auditory supporting cells (SCs) into hair cells (HCs). The expression of UCHL1 gradually decreased as HCs developed and was restricted to inner pillar cells and third-row Deiters' cells between P2 and P7, suggesting that UCHL1-expressing cells are similar to the cells with Lgr5-positive progenitors. UCHL1 expression was decreased even under conditions in which supernumerary HCs were generated with a γ-secretase inhibitor and Wnt agonist. Moreover, the inhibition of UCHL1 by LDN-57444 led to an increase in HC numbers. Mechanistically, LDN-57444 increased mTOR complex 1 activity and allowed SCs to transdifferentiate into HCs. The suppression of UCHL1 induces the transdifferentiation of auditory SCs and progenitors into HCs by regulating the mTOR pathway.

Knockdown of Foxg1 in Sox9+ supporting cells increases the trans-differentiation of supporting cells into hair cells in the neonatal mouse utricle

Foxg1 plays important roles in regeneration of hair cell (HC) in the cochlea of neonatal mouse. Here, we used Sox9-CreER to knock down Foxg1 in supporting cells (SCs) in the utricle in order to investigate the role of Foxg1 in HC regeneration in the utricle. We found Sox9 an ideal marker of utricle SCs and bred Sox9^CreER/+Foxg1^loxp/loxp mice to conditionally knock down Foxg1 in utricular SCs. Conditional knockdown (cKD) of Foxg1 in SCs at postnatal day one (P01) led to increased number of HCs at P08. These regenerated HCs had normal characteristics, and could survive to at least P30. Lineage tracing showed that a significant portion of newly regenerated HCs originated from SCs in Foxg1 cKD mice compared to the mice subjected to the same treatment, which suggested SCs trans-differentiate into HCs in the Foxg1 cKD mouse utricle. After neomycin treatment in vitro, more HCs were observed in Foxg1 cKD mice utricle compared to the control group. Together, these results suggest that Foxg1 cKD in utricular SCs may promote HC regeneration by inducing trans-differentiation of SCs. This research therefore provides theoretical basis for the effects of Foxg1 in trans-differentiation of SCs and regeneration of HCs in the mouse utricle.

Paraquat initially damages cochlear support cells leading to anoikis-like hair cell death

Paraquat (PQ), one of the most widely used herbicides, is extremely dangerous because it generates the highly toxic superoxide radical. When paraquat was applied to cochlear organotypic cultures, it not only damaged the outer hair cells (OHCs) and inner hair cells (IHCs), but also caused dislocation of the hair cell rows. We hypothesized that the dislocation arose from damage to the support cells (SCs) that anchors hair cells within the epithelium. To test this hypothesis, rat postnatal cochlear cultures were treated with PQ. Shortly after PQ treatment, the rows of OHCs separated from one another and migrated radially away from IHCs suggesting loss of cell-cell adhesion that hold the hair cells in proper alignment. Hair cells dislocation was associated with extensive loss of SCs in the organ of Corti, loss of tympanic border cells (TBCs) beneath the basilar membrane, the early appearance of superoxide staining and caspase-8 labeling in SCs below the OHCs and disintegration of E-cadherin and β-catenin in the organ of Corti. Damage to the TBCs and SCs occurred prior to loss of OHC or IHC loss suggesting a form of detachment-induced apoptosis referred to as anoikis.

Identification of mouse cochlear progenitors that develop hair and supporting cells in the organ of Corti

The adult mammalian cochlear sensory epithelium houses two major types of cells, mechanosensory hair cells and underlying supporting cells, and lacks regenerative capacity. Recent evidence indicates that a subset of supporting cells can spontaneously regenerate hair cells after ablation only within the first week postparturition. Here in vivo clonal analysis of mouse inner ear cells during development demonstrates clonal relationship between hair and supporting cells in sensory organs. We report the identification in mouse of a previously unknown population of multipotent stem/progenitor cells that are capable of not only contributing to the hair and supporting cells but also to other cell types, including glia, in cochlea undergoing development, maturation and repair in response to damage. These multipotent progenitors originate from Eya1-expressing otic progenitors. Our findings also provide evidence for detectable regenerative potential in the postnatal cochlea beyond 1 week of age.

Styrene enhances the noise induced oxidative stress in the cochlea and affects differently mechanosensory and supporting cells

Experimental and human investigations have raised the level of concern about the potential ototoxicity of organic solvents and their interaction with noise. The main objective of this study was to characterize the effects of the combined noise and styrene exposure on hearing focusing on the mechanism of damage on the sensorineural cells and supporting cells of the organ of Corti and neurons of the ganglion of Corti. The impact of single and combined exposures on hearing was evaluated by auditory functional testing and histological analyses of cochlear specimens. The mechanism of damage was studied by analyzing superoxide anion and lipid peroxidation expression and by computational analyses of immunofluorescence data to evaluate and compare the oxidative stress pattern in outer hair cells versus the supporting epithelial cells of the organ of Corti. The oxidative stress hypothesis was further analyzed by evaluating the protective effect of a Coenzyme Q₁₀ analogue, the water soluble Q_ter, molecule known to have protective antioxidant properties against noise induced hearing loss and by the analysis of the expression of the endogenous defense enzymes. This study provides evidence of a reciprocal noise-styrene synergism based on a redox imbalance mechanism affecting, although with a different intensity of damage, the outer hair cell (OHC) sensory epithelium. Moreover, these two damaging agents address preferentially different cochlear targets: noise mainly the sensory epithelium, styrene the supporting epithelial cells. Namely, the increase pattern of lipid peroxidation in the organ of Corti matched the cell damage distribution, involving predominantly OHC layer in noise exposed cochleae and both OHC and Deiters' cell layers in the styrene or combined exposed cochleae. The antioxidant treatment reduced the lipid peroxidation increase, potentiated the endogenous antioxidant defense system at OHC level in both exposures but it failed to ameliorate the oxidative imbalance and cell death of Deiters' cells in the styrene and combined exposures. Current antioxidant therapeutic approaches to preventing sensory loss focus on hair cells alone. It remains to be seen whether targeting supporting cells, in addition to hair cells, might be an effective approach to protecting exposed subjects.

Spontaneous Activity of Cochlear Hair Cells Triggered by Fluid Secretion Mechanism in Adjacent Support Cells

Spontaneous electrical activity of neurons in developing sensory systems promotes their maturation and proper connectivity. In the auditory system, spontaneous activity of cochlear inner hair cells (IHCs) is initiated by the release of ATP from glia-like inner supporting cells (ISCs), facilitating maturation of central pathways before hearing onset. Here, we find that ATP stimulates purinergic autoreceptors in ISCs, triggering Cl(-) efflux and osmotic cell shrinkage by opening TMEM16A Ca(2+)-activated Cl(-) channels. Release of Cl(-) from ISCs also forces K(+) efflux, causing transient depolarization of IHCs near ATP release sites. Genetic deletion of TMEM16A markedly reduces the spontaneous activity of IHCs and spiral ganglion neurons in the developing cochlea and prevents ATP-dependent shrinkage of supporting cells. These results indicate that supporting cells in the developing cochlea have adapted a pathway used for fluid secretion in other organs to induce periodic excitation of hair cells.

The RNA-binding protein LIN28B regulates developmental timing in the mammalian cochlea

Proper tissue development requires strict coordination of proliferation, growth, and differentiation. Strict coordination is particularly important for the auditory sensory epithelium, where deviations from the normal spatial and temporal pattern of auditory progenitor cell (prosensory cell) proliferation and differentiation result in abnormal cellular organization and, thus, auditory dysfunction. The molecular mechanisms involved in the timing and coordination of auditory prosensory proliferation and differentiation are poorly understood. Here we identify the RNA-binding protein LIN28B as a critical regulator of developmental timing in the murine cochlea. We show that Lin28b and its opposing let-7 miRNAs are differentially expressed in the auditory sensory lineage, with Lin28b being highly expressed in undifferentiated prosensory cells and let-7 miRNAs being highly expressed in their progeny-hair cells (HCs) and supporting cells (SCs). Using recently developed transgenic mouse models for LIN28B and let-7g, we demonstrate that prolonged LIN28B expression delays prosensory cell cycle withdrawal and differentiation, resulting in HC and SC patterning and maturation defects. Surprisingly, let-7g overexpression, although capable of inducing premature prosensory cell cycle exit, failed to induce premature HC differentiation, suggesting that LIN28B's functional role in the timing of differentiation uses let-7 independent mechanisms. Finally, we demonstrate that overexpression of LIN28B or let-7g can significantly alter the postnatal production of HCs in response to Notch inhibition; LIN28B has a positive effect on HC production, whereas let-7 antagonizes this process. Together, these results implicate a key role for the LIN28B/let-7 axis in regulating postnatal SC plasticity.

Mechanisms of programmed cell death signaling in hair cells and support cells post-electrode insertion trauma

CONCLUSION

Programmed cell death (PCD) initially starts in the support cells (SCs) after electrode insertion trauma (EIT), followed by PCD in hair cells (HCs). Activation of caspase-3 was observed only in SCs. Protecting both SCs and HCs with selective otoprotective drugs at an early stage post implantation may help to preserve residual hearing.

OBJECTIVES

Cochlear implant EIT can initiate sensory cell losses via necrosis and PCD within the organ of Corti, which can lead to a loss of residual hearing. PCD appears to be a major factor in HC loss post-EIT. The current study aimed to: (1) determine the onset of PCD in both SCs and HCs within the traumatized organ of Corti; and (2) identify the molecular mechanisms active within the HCs and SCs that are undergoing PCD.

METHODS

Adult guinea pigs were assigned to one of two groups: (1) EIT and (2) unoperated contralateral ears as controls. Immunostaining of dissected organ of Corti surface preparations for phosphorylated-Jun, cleaved caspase-3, and 4-hydroxy-2,3-nonenal (HNE) were performed at 6, 12, and 24 h post-EIT and for contralateral control ears.

RESULTS

At 6 h post-EIT the SCs immunolabeled for the presence of phosphorylated-Jun and activated caspase-3. Phosphorylated p-Jun labeling was observed at 12 h in both the HCs and SCs of middle and basal cochlear turns. Cleaved caspase-3 was not observed in HCs of any cochlear turn at up to 24 h post-EIT. Lipid peroxidation (HNE immunostaining) was first observed at 12 h post-EIT in both the HCs and SCs of the basal turn, and reached the apical turn by 24 h post-EIT.

Notch signaling limits supporting cell plasticity in the hair cell-damaged early postnatal murine cochlea

In mammals, auditory hair cells are generated only during embryonic development and loss or damage to hair cells is permanent. However, in non-mammalian vertebrate species, such as birds, neighboring glia-like supporting cells regenerate auditory hair cells by both mitotic and non-mitotic mechanisms. Based on work in intact cochlear tissue, it is thought that Notch signaling might restrict supporting cell plasticity in the mammalian cochlea. However, it is unresolved how Notch signaling functions in the hair cell-damaged cochlea and the molecular and cellular changes induced in supporting cells in response to hair cell trauma are poorly understood. Here we show that gentamicin-induced hair cell loss in early postnatal mouse cochlear tissue induces rapid morphological changes in supporting cells, which facilitate the sealing of gaps left by dying hair cells. Moreover, we provide evidence that Notch signaling is active in the hair cell damaged cochlea and identify Hes1, Hey1, Hey2, HeyL, and Sox2 as targets and potential Notch effectors of this hair cell-independent mechanism of Notch signaling. Using Cre/loxP based labeling system we demonstrate that inhibition of Notch signaling with a γ- secretase inhibitor (GSI) results in the trans-differentiation of supporting cells into hair cell-like cells. Moreover, we show that these hair cell-like cells, generated by supporting cells have molecular, cellular, and basic electrophysiological properties similar to immature hair cells rather than supporting cells. Lastly, we show that the vast majority of these newly generated hair cell-like cells express the outer hair cell specific motor protein prestin.

In vivo visualization of Notch1 proteolysis reveals the heterogeneity of Notch1 signaling activity in the mouse cochlea

Mechanosensory hair cells (HCs) and surrounding supporting cells (SCs) in the mouse cochlea are important for hearing and are derived from the same prosensory progenitors. Notch1 signaling plays dual but contrasting and age-dependent roles in mouse cochlear development: early lateral induction and subsequent lateral inhibition. However, it has been difficult to directly visualize mouse cochlear cells experiencing various levels of Notch1 activity at single cell resolution. Here, we characterized two knock-in mouse lines, Notch1^{Cre (Low)/+} and Notch1^{Cre (High)/+}, with different Cre recombinase activities, that can detect Notch1 receptor proteolysis or Notch1 activity at high and low thresholds, respectively. Using both lines together with a highly sensitive Cre reporter line, we showed that Notch1 activity is nearly undetectable during lateral induction but increases to medium and high levels during lateral inhibition. Furthermore, we found that within the neonatal organ of Corti, the vast majority of cells that experience Notch1 activity were SCs not HCs, suggesting that HCs kept undetectable Notch1 activity during the entire lineage development. Furthermore, among SC subtypes, ∼85–99% of Deiters’ and outer pillar cells but only ∼19–38% of inner pillar cells experience medium and high levels of Notch1 activity. Our results demonstrate that Notch1 activity is highly heterogeneous: 1) between lateral induction and inhibition; 2) between HC and SC lineages; 3) among different SC subtypes; 4) among different cells within each SC subtype. Such heterogeneity should elucidate how the development of the cochclear sensory epithelium is precisely controlled and how HC regeneration can be best achieved in postnatal cochleae.

Na+ absorption by Claudius' cells is regulated by purinergic signaling in the cochlea

CONCLUSION

Claudius' cells absorb Na(+) through the amiloride-sensitive epithelial sodium channel (ENaC). Transepithelial ion transport through ENaC and possibly a Cl(-) secretory pathway is regulated by P2Y purinergic signaling.

OBJECTIVES

The purpose of this study was to investigate ion transport in Claudius' cells and its purinergic regulation.

METHODS

Young adult Sprague-Dawley rats and gerbils were studied. The Claudius' cell layer on the basilar membrane was dissected from the basal turn of the cochlea. A voltage-sensitive vibrating probe was used to measure transepithelial short circuit current (I(sc) ). The baseline I(sc) of Claudius' cells was measured in the perilymph-like control solution and the change of I(sc) after application of amiloride (10 μM) or uridine triphosphate (UTP, 100 μM).

RESULTS

A negative baseline I(sc) was observed in the control solution (-12.50 ± 3.95 μA/cm(2), n = 8) and the addition of amiloride resulted in a decrease of I(sc) by 75.8%. The application of UTP, an agonist for P2Y purinergic receptors, led to a partial inhibition of I(sc) (by 38.2 ± 3.2%, n = 5), and subsequent addition of amiloride abolished the remaining I(sc).

Fgf signaling regulates development and transdifferentiation of hair cells and supporting cells in the basilar papilla

The avian basilar papilla (BP) is a likely homolog of the auditory sensory epithelium of the mammalian cochlea, the organ of Corti. During mammalian development Fibroblast growth factor receptor-3 (Fgfr3) is known to regulate the differentiation of auditory mechanosensory hair cells (HCs) and supporting cells (SCs), both of which are required for sound detection. Fgfr3 is expressed in developing progenitor cells (PCs) and SCs of both the BP and the organ of Corti; however its role in BP development is unknown. Here we utilized an in vitro whole organ embryonic culture system to examine the role of Fgf signaling in the developing avian cochlea. SU5402 (an antagonist of Fgf signaling) was applied to developing BP cultures at different stages to assay the role of Fgf signaling during HC formation. Similar to the observed effects of inhibition of Fgfr3 in the mammalian cochlea, Fgfr inhibition in the developing BP increased the number of HCs that formed. This increase was not associated with increased proliferation, suggesting that inhibition of the Fgf pathway leads to the direct conversion of PCs or supporting cells into HCs, a process known as transdifferentiation. This also implies that Fgf signaling is required to prevent the conversion of PCs and SCs into HCs. The ability of Fgf signaling to inhibit transdifferentiation suggests that its down-regulation may be essential for the initial steps of HC formation, as well as for the maintenance of SC phenotypes.

Viral vector tropism for supporting cells in the developing murine cochlea

Gene-based therapeutics are being developed as novel treatments for genetic hearing loss. One roadblock to effective gene therapy is the identification of vectors which will safely deliver therapeutics to targeted cells. The cellular heterogeneity that exists within the cochlea makes viral tropism a vital consideration for effective inner ear gene therapy. There are compelling reasons to identify a viral vector with tropism for organ of Corti supporting cells. Supporting cells are the primary expression site of connexin 26 gap junction proteins that are mutated in the most common form of congenital genetic deafness (DFNB1). Supporting cells are also primary targets for inducing hair cell regeneration. Since many genetic forms of deafness are congenital it is necessary to administer gene transfer-based therapeutics prior to the onset of significant hearing loss. We have used transuterine microinjection of the fetal murine otocyst to investigate viral tropism in the developing inner ear. For the first time we have characterized viral tropism for supporting cells following in utero delivery to their progenitors. We report the inner ear tropism and potential ototoxicity of three previously untested vectors: early-generation adenovirus (Ad5.CMV.GFP), advanced-generation adenovirus (Adf.11D) and bovine adeno-associated virus (BAAV.CMV.GFP). Adenovirus showed robust tropism for organ of Corti supporting cells throughout the cochlea but induced increased ABR thresholds indicating ototoxicity. BAAV also showed tropism for organ of Corti supporting cells, with preferential transduction toward the cochlear apex. Additionally, BAAV readily transduced spiral ganglion neurons. Importantly, the BAAV-injected ears exhibited normal hearing at 5 weeks of age when compared to non-injected ears. Our results support the use of BAAV for safe and efficient targeting of supporting cell progenitors in the developing murine inner ear.

Supporting cell characteristics in long-deafened aged mouse ears

Significant sensory hair cell loss leads to irreversible hearing and balance deficits in humans and other mammals. Future therapeutic strategies to repair damaged mammalian auditory epithelium may involve inserting stem cells into the damaged epithelium, inducing non-sensory cells remaining in the epithelium to transdifferentiate into replacement hair cells via gene therapy, or applying growth factors. Little is currently known regarding the status and characteristics of the non-sensory cells that remain in the deafened auditory epithelium, yet this information is integral to the development of therapeutic treatments. A single high-dose injection of the aminoglycoside kanamycin coupled with a single injection of the loop diuretic furosemide was used to kill hair cells in adult mice, and the mice were examined 1 year after the drug insult. Outer hair cells are lost throughout the entire length of the cochlea and less than a third of the inner hair cells remain in the apical turn. Over 20% and 55% of apical organ of Corti support cells and spiral ganglion cells are lost, respectively. We examined the expression of several known support cell markers to investigate for possible support cell dedifferentiation in the damaged ears. The support cell markers investigated included the microtubule protein acetylated tubulin, the transcription factor Sox2, and the Notch signaling ligand Jagged1. Non-sensory epithelial cells remaining in the organ of Corti retain acetylated tubulin, Sox2 and Jagged1 expression, even when the epithelium has a monolayer-like appearance. These results suggest a lack of marked SC dedifferentiation in these aged and badly damaged ears.

Non-sensory cells in the deafened organ of Corti: approaches for repair

After moderate cochlear trauma, hair cells degenerate and their places are taken by phalangeal scars formed by differentiated supporting cells. A short time after trauma, these supporting cells can respond to an induced expression of genes which signal hair cell differentiation during normal development and transdifferentiate into new hair cells. However, these non-sensory cells often lose their differentiated features after severe insults or prolonged hearing loss and become a simple, flat epithelium. The flat organ of Corti can serve as a substrate for gene- and stem cell-based therapies. Despite its prevalence, the flat epithelium is not well characterized. Recent data show that cells of the flat epithelium can divide and maintain the structural confluence of the membranous labyrinth. The mitotic potential of these cells should facilitate production of cells for therapies based on recapitulation of development or insertion of stem cells.

Selective cochlear degeneration in mice lacking the F-box protein, Fbx2, a glycoprotein-specific ubiquitin ligase subunit

Little is known about the role of protein quality control in the inner ear. We now report selective cochlear degeneration in mice deficient in Fbx2, a ubiquitin ligase F-box protein with specificity for high-mannose glycoproteins (Yoshida et al., 2002). Originally described as a brain-enriched protein (Erhardt et al., 1998), Fbx2 is also highly expressed in the organ of Corti, in which it has been called organ of Corti protein 1 (Thalmann et al., 1997). Mice with targeted deletion of Fbxo2 develop age-related hearing loss beginning at 2 months. Cellular degeneration begins in the epithelial support cells of the organ of Corti and is accompanied by changes in cellular membrane integrity and early increases in connexin 26, a cochlear gap junction protein previously shown to interact with Fbx2 (Henzl et al., 2004). Progressive degeneration includes hair cells and the spiral ganglion, but the brain itself is spared despite widespread CNS expression of Fbx2. Cochlear Fbx2 binds Skp1, the common binding partner for F-box proteins, and is an unusually abundant inner ear protein. Whereas cochlear Skp1 levels fall in parallel with the loss of Fbx2, other components of the canonical SCF (Skp1, Cullin1, F-box, Rbx1) ubiquitin ligase complex remain unchanged and show little if any complex formation with Fbx2/Skp1, suggesting that cochlear Fbx2 and Skp1 form a novel, heterodimeric complex. Our findings demonstrate that components of protein quality control are essential for inner ear homeostasis and implicate Fbx2 and Skp1 as potential genetic modifiers in age-related hearing loss.

Neural stem cells injected into the sound-damaged cochlea migrate throughout the cochlea and express markers of hair cells, supporting cells, and spiral ganglion cells

Most cases of hearing loss are caused by the death or dysfunction of one of the many cochlear cell types. We examined whether cells from a neural stem cell line could replace cochlear cell types lost after exposure to intense noise. For this purpose, we transplanted a clonal stem cell line into the scala tympani of sound damaged mice and guinea pigs. Utilizing morphological, protein expression and genetic criteria, stem cells were found with characteristics of both neural tissues (satellite, spiral ganglion, and Schwann cells) and cells of the organ of Corti (hair cells, supporting cells). Additionally, noise-exposed, stem cell-injected animals exhibited a small but significant increase in the number of satellite cells and Type I spiral ganglion neurons compared to non-injected noise-exposed animals. These results indicate that cells of this neural stem cell line migrate from the scala tympani to Rosenthal's canal and the organ of Corti. Moreover, they suggest that cells of this neural stem cell line may derive some information needed from the microenvironment of the cochlea to differentiate into replacement cells in the cochlea.

Manipulating cell cycle regulation in the mature cochlea

Sensorineural hearing loss, which is often caused by degeneration of hair cells in the auditory epithelium, is permanent because lost hair cells are not replaced. Several conceptual approaches can be used to place new hair cells in the auditory epithelium. One possibility is to enhance proliferation of non-sensory cells that remain in the deaf ear and induce transdifferentiation of some of these cells into the hair cell phenotype. Several genes, including p27(Kip1), have been shown to regulate proliferation and differentiation in the developing auditory epithelium. The role of p27(Kip1) in the mature ear is not well characterized. We now show that p27(Kip1) is present in the nuclei of non-sensory cells of the mature auditory epithelium. We determined that forced expression of Skp2 using a recombinant adenovirus vector, resulted in presence of BrdU-positive cells in the auditory epithelium. When SKP2 over-expression was combined with forced expression of Atoh1, ectopic hair cells were found in the auditory epithelium in greater numbers than were seen with Atoh1 alone. Skp2 over-expression alone did not result in ectopic hair cells. These findings suggest that the p27(Kip1) protein remains in the mature auditory epithelium and therefore p27(Kip1) can serve as a target for gene manipulation. The data also suggest that induced proliferation, by itself, does not generate new hair cells in the cochlea.

[Inhibition of nifedipine on inward current of Hensen cell induced by ATP]

OBJECTIVE

To investigate the effect of nifedipine on the non-selective inward current of cochlear Hensen cell induced by ATP in high concentrations.

METHODS

The organ of Corti was treated using enzyme, and then dissociated mechanically to isolate Hensen cells. The whole cell patch-clamp technique was used to record ion currents in Hensen cells which had integrated border, round shape and translucent intracellular cytoplasm. Drugs were delivered to the cell by a micro-manifold consisting made by three 100 microm diameter microtubules, including 0.1 mmol/L ATP, 1 mmol/L ATP, 10 mmol/L ATP, 0.1 mmol/L ATP + 0. 1 mmol/L suramin (purinergic antagonist), stimulation of extracellular fluid alone, 140 mmol/L CsCl (replace KCL in intracellular fluid) + 1 mmol/L ATP, 40 mmol/L TEA (blocker of potassium channel) + 1 mmol/L ATP, and 1 mmol/L ATP + 10 micromol/L nifedipine, respectively.

RESULTS

When isolated Hensen cell was given 0.1 mmol/L (n = 10), 1 mmol/L (n = 10), 10 mmol/L( n = 6) ATP separately, an inward ion current could be recorded, which enhanced with increased ATP concentration and showed dose-dependence. Further study indicated that the inward ion current could be inhibited by 0.1 mmol/L suramin (n = 5), 140 mmol/L CsCl (n = 5) and 40 mmol/L TEA (n = 5). There was no ion current be recorded when the cell was stimulated with the extracellular fluid alone, neither inward nor outward. However, the inward ion current vanished and an outward ion current appeared instead, when 1 mmol/L ATP and 10 micromol/L nifedipine were given together (n = 5).

CONCLUSIONS

An inward current was evoked in isolated Hensen cell by ATP in high concentrations. This inward current seems to be associated closely with potassium channels without the participation of mechanical channels. Nifedipine can inhibit this inward current and induce an outward current, which is similar to the normal potassium current in isolated Hensen cell. It suggests that nifedipine have partly protective effect on the function of cochlea by inducing modulate of the potassium circle of cochlea in Hensen cell's tache.

Regulation of cell fate in the sensory epithelia of the inner ear

The sensory epithelia of the inner ear contain mechanosensory hair cells and non-sensory supporting cells. Both classes of cell are heterogeneous, with phenotypes varying both between and within epithelia. The specification of individual cells as distinct types of hair cell or supporting cell is regulated through intra- and extracellular signalling pathways that have been poorly understood. However, new methodologies have resulted in significant steps forward in our understanding of the molecular pathways that direct cells towards these cell fates.

Establishment of a proneural field in the inner ear

Hair-cells, supporting cells and sensory neurons are the main specialized cell-types responsible for mechanotransduction in the inner ear. They derive from precursors expressing proneural genes and recent data has underlined the importance of SoxB1 genes as upstream activators of proneural genes during cranial placode development. Here we review the steps of establishing a proneural field and propose several models for how early otic regionalization into a proneural territory is achieved.

Distinct and gradient distributions of connexin26 and connexin30 in the cochlear sensory epithelium of guinea pigs

Connexin26 (Cx26) and Cx30 are predominant isoforms of gap junction channels in the cochlea and play a critical role in hearing. In this study, the cellular distributions of Cx26 and Cx30 in the cochlear sensory epithelium of guinea pigs were examined by immunofluorescent staining and confocal microscopy in whole mounts of the cochlear sensory epithelium and dissociated cell preparations. The expression of Cx26 and Cx30 demonstrated a longitudinal gradient distribution in the epithelium and was reduced threefold from the cochlear apex to base. The reduction was more pronounced in the Deiters cells and pillar cells than in the Hensen cells. Cx26 was expressed in all types of supporting cells, but little Cx30 labeling was seen in the Hensen cells. Cx26 expression in the Hensen cells was concentrated mainly in the second and third rows, forming a distinct band along the sensory epithelium at its outer region. In the dissociated Deiters cells and pillar cells, Cx30 showed dense labeling at the cell bodies and processes in the reticular lamina. Cx26 labeling largely overlapped that of Cx30 in these regions. Cx26 and Cx30 were also coexpressed in the gap junctional plaques between Claudius cells. Neither Cx26 nor Cx30 labeling was seen in the hair cells and spiral ganglion neurons. These observations demonstrate that Cx26 and Cx30 have a longitudinal gradient distribution and distinct cellular expression in the auditory sensory epithelium. This further supports our previous reports that Cx26 and Cx30 can solely and concertedly perform different functions in the cochlea.

MicroRNA gene expression in the mouse inner ear

MicroRNAs (miRNAs) are small non-coding RNAs that function through the RNA interference (RNAi) pathway and post-transcriptionally regulate gene expression in eukaryotic organisms. While miRNAs are known to affect cellular proliferation, differentiation, and morphological development, neither their expression nor roles in mammalian inner ear development have been characterized. We have investigated the extent of miRNA expression at various time points throughout maturation of the postnatal mouse inner ear by microarray analysis. Approximately one third of known miRNAs are detected in the inner ear, and their expression persists to adulthood. Expression of such miRNAs is validated by quantitative PCR and northern blot analysis. Further analysis by in situ hybridization demonstrates that certain miRNAs exhibit cell-specific expression patterns in the mouse inner ear. Notably, we demonstrate that miRNAs previously associated with mechanosensory cells in zebrafish are also expressed in hair cells of the auditory and vestibular endorgans. Our results demonstrate that miRNA expression is abundant in the mammalian inner ear and that certain miRNAs are evolutionarily associated with mechanosensory cell development and/or function. The data suggest that miRNAs contribute substantially to genetic programs intrinsic to development and function of the mammalian inner ear and that specific miRNAs might influence formation of sensory epithelia from the primitive otic neuroepithelium.

Distribution of pendrin in the organ of Corti of mice observed by electron immunomicroscopy

The distribution of pendrin, which is encoded by the Pendred syndrome gene, has been investigated immunohistochemically in the inner ear. In the cochlea, pendrin has been found in the spiral prominence, external sulcus cells, Hensen's cells and Claudius cells, but its expression in the organ of Corti remains unclear. We examined whether pendrin localizes in the organ of Corti by postembedding immunogold analysis. In the organ of Corti, gold particles were clearly observed in outer and inner hair cells, including the stereocilia. The density of the particles was especially high in the cuticular plates of the hair cells. Gold particles were also detected in the external sulcus, in part of the spiral ligament adjacent to the external sulcus, in supporting cells, and in the spiral ganglion of the cochlea. Our study revealed that pendrin occurs in the organ of Corti. The role of pendrin in the organ of Corti and its association with the Cl- or pH regulation of neurotransmission require further study.

Adeno-associated virus-mediated gene transfer to hair cells and support cells of the murine cochlea

More than 28 million Americans suffer from various forms of hearing loss. The lack of effective treatments for many forms of hearing disorders has prompted interest in the potential application of gene delivery techniques to treat both inherited and pathological hearing disorders. However, to develop a gene therapy strategy that will successfully treat hearing disorders, appropriate vectors that are capable of transducing cochlear hair cells and support cells must be identified. In the present study, we examined the efficiency with which AAV vectors (serotypes 1, 2, and 5) transduce hair cells and support cells in cochlear explants from P0 and E13 mice. We further examined the ability of the CBA and GFAP promoters to drive expression of a GFP marker gene in hair cells and support cells. Robust GFP expression was observed in hair cells and support cells following transduction of primary murine cochlear explants with AAV serotypes 1 and 2, but not serotype 5. The CBA promoter predominantly drove GFP expression in hair cells. In contrast, strong expression from the GFAP promoter was observed primarily in support cells. Thus, using AAV vectors and specific promoters, cell-type-specific expression of transgenes can be established within the cochlea.

Survival of adult spiral ganglion neurons requires erbB receptor signaling in the inner ear

Degeneration of cochlear sensory neurons is an important cause of hearing loss, but the mechanisms that maintain the survival of adult cochlear sensory neurons are not clearly defined. We now provide evidence implicating the neuregulin (NRG)-erbB receptor signaling pathway in this process. We found that NRG1 is expressed by spiral ganglion neurons (SGNs), whereas erbB2 and erbB3 are expressed by supporting cells of the organ of Corti, suggesting that these molecules mediate interactions between these cells. Transgenic mice in which erbB signaling in adult supporting cells is disrupted by expression of a dominant-negative erbB receptor show severe hearing loss and 80% postnatal loss of type-I SGNs without concomitant loss of the sensory cells that they contact. Quantitative RT-PCR analysis of neurotrophic factor expression shows a specific downregulation in expression of neurotrophin-3 (NT3) in the transgenic cochleas before the onset of neuronal death. Because NT3 is critical for survival of type I SGNs during development, these results suggest that it plays similar roles in the adult. Together, the data indicate that adult cochlear supporting cells provide critical trophic support to the neurons, that survival of postnatal cochlear sensory neurons depends on reciprocal interactions between neurons and supporting cells, and that these interactions are mediated by NRG and neurotrophins.

Impaired permeability to Ins(1,4,5)P3 in a mutant connexin underlies recessive hereditary deafness

Connexins are membrane proteins that assemble into gap-junction channels and are responsible for direct, electrical and metabolic coupling between connected cells. Here we describe an investigation of the properties of a recombinantly expressed recessive mutant of connexin 26 (Cx26), the V84L mutant, associated with deafness. Unlike other Cx26 mutations, V84L affects neither intracellular sorting nor electrical coupling, but specifically reduces permeability to the Ca(2+)-mobilizing messenger inositol 1,4,5-trisphosphate (Ins(1,4,5)P(3)). Both the permeability to Lucifer Yellow and the unitary channel conductance of V84L-mutant channels are indistinguishable from those of the wild-type Cx26. Injection of Ins(1,4,5)P(3) into supporting cells of the rat organ of Corti, which abundantly express Cx26, ensues in a regenerative wave of Ca(2+) throughout the tissue. Blocking the gap junction communication abolishes wave propagation. We propose that the V84L mutation reduces metabolic coupling mediated by Ins(1,4,5)P(3) to an extent sufficient to impair the propagation of Ca(2+) waves and the formation of a functional syncytium. Our data provide the first demonstration of a specific defect of metabolic coupling and offer a mechanistic explanation for the pathogenesis of an inherited human disease.

Increased noise sensitivity and altered inner ear MENA distribution in VASP-/- mice

Vasodilator-stimulated phosphoprotein (VASP) and mammalian-enabled protein (MENA) share similar cellular localisation and functions (signal transduction pathways, regulation of actin cytoskeleton dynamics). Functional substitution and compensation among Ena/VASP proteins have been proposed as the reason for the absence of major morphological and functional deficits in VASP-/- mice. The aim of this study was to investigate VASP expression in the mouse cochlea, to analyse cochlear function in VASP-/- mice compared with wildtype mice, and to analyse cochlear MENA distribution taking into account that MENA protein might compensate VASP loss in the cochlea of VASP-/- mice. We confirmed specific VASP expression in the pillar cells of the mice organ of Corti as previously reported for rat cochlea. By analysing the hearing function in VASP-/- mice, we found no differences in auditory brainstem responses and distortion product otoacoustic emissions from those of wildtype mice but evidence for an increased noise sensitivity at lower frequencies. When MENA protein levels in cochlea tissue were tested in mutant and wildtype mice by Western blot analysis, no significant differences were found, as was also seen with regard to MENA mRNA levels in laser-microdissected single pillar cells. Most surprisingly, however, MENA protein was absent in pillar cells of VASP-/- mice, whereas it was detected in other cochlear cells. The finding of a cell-specific, and not organ-specific, redundancy of MENA protein expression noted for the first time in VASP-/- mice is proposed as the reason for the observed distinct cochlear phenotype.

Expression of glutamate transporter GLAST in the developing mouse cochlea

The immunohistochemical localization of glutamate transporter GLAST in the developing mouse cochlea was studied at different ages between 0 and 30 days after birth (DAB). In the adult mouse cochlea, intense GLAST-like immunoreactivity was found in the supporting cells adjacent to the inner hair cells of the organ of Corti, the type II and suprastrial fibrocytes of the cochlear lateral wall, the fibrocytes of the spiral limbus and the satellite cells surrounding the spiral ganglion cells. At 0 DAB, weak GLAST-like immunoreactivity was found in the supporting cells around the immature inner hair cells. Immature fibrocytes in the cochlea were also positively immunostained. At 3 DAB, weak immunostaining of GLAST appeared in the immature satellite cells in the spiral ganglion. The GLAST-like immunoreactivity in the supporting cells around the inner hair cells, in the fiborocytes in the spiral ligament and the spiral limbus and in the satellite cells in the spiral ganglion increased progressively during the second postnatal week, and reached the adult level at 15 DAB. This time course correlates with the electrophysiological onset and maturation of the mouse auditory function, which is mediated by glutamatergic neurotransmission. These results suggest that the expression of GLAST may be needed for the efficient removal and metabolism of the released glutamate in the cochlea and may play important roles in the onset and maturation of the auditory system.

Identification and characterization of choline transporter-like protein 2, an inner ear glycoprotein of 68 and 72 kDa that is the target of antibody-induced hearing loss

The Kresge Hearing Research Institute-3 (KHRI-3) antibody binds to a guinea pig inner ear supporting cell antigen (IESCA) and causes hearing loss. To gain insight into the mechanism of antibody-induced hearing loss, we used antibody immunoaffinity purification to isolate the IESCA, which was then sequenced by mass spectroscopy, revealing 10 guinea pig peptides identical to sequences in human choline transporter-like protein 2 (CTL2). Full-length CTL2 cDNA sequenced from guinea pig inner ear has 85.9% identity with the human cDNA. Consistent with its expression on the surface of supporting cells in the inner ear, CTL2 contains 10 predicted membrane-spanning regions with multiple N-glycosylation sites. The 68 and 72 kDa molecular forms of inner ear CTL2 are distinguished by sialic acid modification of the carbohydrate. The KHRI-3 antibody binds to an N-linked carbohydrate on CTL2 and presumably damages the organ of Corti by blocking the transporter function of this molecule. CTL2 mRNA and protein are abundantly expressed in human inner ear. Sera from patients with autoimmune hearing loss bind to guinea pig inner ear with the same pattern as CTL2 antibodies. Thus, CTL2 is a possible target of autoimmune hearing loss in humans.

Cell type-specific reduction of beta tubulin isotypes synthesized in the developing gerbil organ of Corti

There are seven isotypic forms of the microtubule protein beta tubulin in mammals, but not all isotypes are synthesized in every cell type. In the adult organ of Corti, each of the five major cell types synthesizes a different subset of isotypes. Inner hair cells synthesize only betaI and betaII tubulin, while outer hair cells make betaI and betaIV tubulin. Only betaII and betaIV tubulin are found in inner and outer pillar cells, while betaI, betaII, and betaIV tubulin are present in Deiters cells, and betaI, betaII and betaIII tubulin are found in organ of Corti dendrites. During post-natal organ of Corti development in the gerbil, microtubules are elaborated in an orderly temporal sequence beginning with hair cells, followed by pillar cells and Deiters cells. Using beta tubulin isotype-specific antibodies, we show that, in the gerbil cochlea, the same three isotypes are present in each cell type at birth, and that a cell type-specific reduction in the isotypes synthesized occurs in hair cells and pillar cells at an unusually late stage in development. No beta tubulin isotypes were detected in mature afferent dendrites, but we show that this is because few microtubules are present in mature dendrites. In addition, we show that primary cilia in inner hair cells, a feature of early development, persist much later than previously reported. The findings represent the first description of developmental cell type-specific reductions in tubulin isotypes in any system.

[Distribution of calcitonin gene-related peptide in guinea pig cochlea]

OBJECTIVE

To observe the distribution of CGRP in guinea pig cochlea.

METHOD

CGRP was labeled by immunostaning technique. Distribution of CGRP immunoreactivity was observed with light microscopy.

RESULT

CGRP immunoreactivity was located in SGN, nerve fibers at osseous spiral lamina, stria vascularis, IHC, adjacent area of OHC and Deiters' cells.

CONCLUSION

CGRP distributes extensively in guinea pig cochlea. CGRP may play an important role as neurotransmitter and/or neuromodulator to hearing function by modulating HC, supporting cells, SGN and other neurons along the process of auditory nerve. It may have effect on on cochlear blood flow, too.

Ultrastructure indicative of ion transport in tectal, Deiters, and tunnel cells: differences between gerbil and chinchilla basal and apical cochlea

Ultrastructural examination revealed an epithelium of about five tectal cells (TCs) roofing the outer tunnel (OT) in the mid to upper, but not the basal, region of gerbil and chinchilla cochlea. Structures in TCs that are apparently specialized for retrieval of K(+) released into tunnel fluid from outer hair cells (OHCs) include surface fimbriae in the gerbil and canalicular reticulum in the chinchilla. A tunnel roof of organelle-rich TCs appeared to be better equipped for ion resorption than a roof composed of organelle-poor Hensen cells (HCs). Fimbriae, filopodia, and the cell body of TCs descended to contact the third Deiters cell (DC3) in the gerbil, and the hypertrophied DC3 phalanx rose to contact TCs in the chinchilla, which suggests a solute exchange between TCs and DCs. Previously unrecognized structures that are speculated to provide ATP ligand for cochlear purinoreceptors occurred in the chinchilla DC and gerbil TC. The observation of a microtubule stalk in DCs indicated that they also function in cochlear mechanics. A newly delineated lateral tunnel cell (LTC) intervened between the DC3 and HC in both species. The apicomedial plasmalemma of all DCs fitted closely to the base of OHCs and enveloped afferent nerves. The morphologic specializations reported here provide further support for the proposed transcellular lateral flow route for K(+) currents generated by sound exposure and neural activity. The previously demonstrated expansion of Boettcher cells, outer sulcus cell roots, type Il and IV fibrocytes, and apical microvilli on HCs and Claudius cells (CCs) in the base of the cochlea is postulated here to mediate a basal parallel current that could supply the increased K(+) transport required for the basally elevated electric potential (EP).

Expression of VASP and zyxin in cochlear pillar cells: indication for actin-based dynamics?

Vasodilator-stimulated phosphoprotein (VASP) is a member of the ENA/VASP-protein family. VASP is considered to be a crucial factor in the regulation of actin dynamics, which involves processes such as motility and cell adhesion, e.g. in filopodia or growth cones. In these processes zyxin acts as an important partner of VASP and is particularly concentrated at sites where VASP-dependent actin dynamics occur. Based on indirect evidence that actin-mediated dynamics may effect the mechanical properties of the cochlea, we have investigated expression of VASP and zyxin in the postnatal and adult rat cochlea using polymerase chain reaction and Western blot approaches, as well as immunohistochemistry and confocal microscopy. Besides an expected expression in vessels and fibroblasts, VASP and zyxin expression was also observed in pillar cells. Here, the staining was restricted to the head and foot plate of the pillar cells. Onset of VASP expression in pillar cells coincided with the beginning of hearing. In pillar cells, VASP and zyxin were co-localised with pan-actin, suggesting actin-based dynamics in these cochlear cells, which until now were rather presumed to form a highly rigid bridge between the inner and outer sensory cells. Thus, pillar cells may be more dynamically involved in controlling longer-lasting mechanical properties of the cochlea as hitherto presumed.

Hair cell regeneration: winging our way towards a sound future

The discovery of hair cell regeneration in the inner ear of birds provides new optimism that there may be a treatment for hearing and balance disorders. In this review we describe the process of hair cell regeneration in birds; including restoration of function, recovery of perception and what is currently known about molecular events, such as growth factors and signalling systems. We examine some of the key recent findings in both birds and mammals.

Alteration in expression of p27 in auditory epithelia and neurons of mice during degeneration

The aim of this study was to examine roles of p27, a cyclin-dependent kinase inhibitor, in cochleae of adult mice. Expression of p27 was found in cochlear supporting cells and spiral ganglion neurons of normal mice. Cisplatin treatment caused progressive degeneration of cochlear supporting cells and spiral ganglion neurons, and numbers of p27-positive cells in these cells decreased. This indicates a close relationship between p27 and cell death in cochleae. However, the relationships between decrease in number of p27-positive cells and that of survival cells differed according to type of cell. For Deiters' cells, there was apparent decrease in number of p27-positive cells, although no decrease in cell numbers. The present findings indicate that p27 plays roles in degeneration of cochleae according to cell type.

Transient expression of the t-isoform of plastins/fimbrin in the stereocilia of developing auditory hair cells

The transduction of auditory signals by cochlear hair cells depends upon the integrity of hair cell stereociliary bundles. Stereocilia contain a central core of actin filaments, cross-linked by actin bundling proteins. In the cochlea, the two proteins described to date as responsible for the spatial arrangement of actin filaments in sterocilia are fimbrin and the recently discovered espin. Fimbrin (the chick homolog of human I-plastin) belongs to the plastins/fimbrin family that includes two additional isoforms of plastins, T- and L-plastin. In the present study, we used isoform specific antibodies to investigate the presence of the T- and L-isoforms of plastin/fimbrin in the adult and developing rat cochlea. We found that T-plastin, but not L-plastin, is expressed in the rat cochlea. During postnatal development of the rat organ of Corti, T-plastin can be detected in the core of stereocilia from early stages of hair cell differentiation, and its expression gradually increases in stereocilia as hair cells mature. However, as opposed to other actin-binding proteins expressed in stereocilia, T-plastin is absent from the stereocilia of mature hair cells. Such temporally restricted expression strengthens the idea of functional differences between plastins isoforms, and suggests that T-plastin could have a specific role in stereocilia formation.

ATP-gated ion channels assembled from P2X2 receptor subunits in the mouse cochlea

Extracellular ATP has several neuro-humoral actions on cochlear physiology, many of which involve P2X receptor-mediated signal transduction. The present study extends the molecular physiology of P2X receptor gene expression in the cochlea to the principal platform for transgenic studies, the mouse model. P2X receptor subunits, which assemble to form ATP-gated ion channels, were localised in cryosections and whole-mount tissues from the adult mouse cochlea using a specific antiserum and immunoperoxidase histochemistry. Whole-cell voltage clamp recordings functionally correlated immunolocalisation of ATP-gated ion channels in isolated hair cells and supporting cells. P2X immunoreactivity was widespread throughout the epithelial lining of the cochlea (except vascular stria); spiral ganglion neurons, organ of Corti supporting cells, and outer hair cell (OHC) stereocilia exhibited strong P2X immunolabelling. Localisation of ATP-gated ion channels on the endolymphatic surface (cuticular plates and stereocilia) of outer hair cells was confirmed electrophysiologically. In contrast, Deiters' cells exhibited an even distribution of both immunolabelling over the whole cell membrane and inward currents could be evoked by localised ATP application anywhere on these cells. In both OHC and Deiters' cells, the slowly-desensitising inward currents were blocked by the P2X-selective antagonist pyridoxal-phosphate-6-azophenyl-2',4'-disulphonic acid (PPADS), compatible with P2X subunits contributing to the ATP-gated ion channels. Our immunohistochemical and functional localisation of P2X receptors in the mouse cochlea extends previous studies to verify and characterise extracellular ATP signalling in the cochlea and extends support for P2X receptor-mediated regulation of endolymphatic ionic homeostasis, sound transduction, auditory neurotransmission and cochlear mechanics.

REST mRNA expression in normal and regenerating avian auditory epithelium

Hair cells (HCs) and supporting cells (SCs) in the auditory epithelium initially arise from a sheet of undifferentiated cells. Although much has been learned about the initial steps leading to the fate determination of HCs and SCs, respectively, little is known about what molecular events 'finalize' cell fate determination. We investigated the role of repressor element-1 (RE-1) silencing transcription factor (REST), whose inactivation is known to be a requirement for a cell to assume a neuronal identity. Here we show by in situ hybridization (ISH) that SCs express REST messenger RNA (mRNA) but sensory HCs lack detectable expression. Using a more sensitive reverse transcription-polymerase chain reaction assay, however, we detected the presence of a neuron-specific splice variant in the epithelium, suggesting that HCs express REST mRNA at levels too low to be detectable by ISH. In regenerating auditory epithelium, we found that REST mRNA was expressed and upregulated in all remaining cells in the damaged region of the epithelium, consistent with its expression pattern during development prior to neurogenesis. Surprisingly, REST mRNA was also upregulated in SCs in the apical, undamaged region of the epithelium, and readily detectable by ISH in the HCs in this region. This finding suggests that the grossly undamaged region of the epithelium is in fact biochemically altered towards a 'less developed' state. Our results indicate that REST inactivation is an important step in finalizing HC fate in the chick inner ear.

Expression of members of Wnt and Frizzled gene families in the postnatal rat cochlea

The functioning of the mammalian cochlea is entirely based on its mechanical properties, which are supported by a highly complex tissue architecture resulting from the precise arrangement of sensory hair cells and non-sensory supporting cells. Growing evidence indicates that evolutionary conserved signaling pathways are involved in inner ear development and in the differentiation of its diverse cell types. We investigated whether members of the Wnt and Frizzled gene families, which play key roles in a wide variety of cellular and developmental processes, are expressed in the postnatal rat cochlea. A PCR screening of a rat cochlea cDNA library performed with degenerate primers allowed us to isolate five members of the Wnt gene family (RWnt-2B, -4, -5A, -5B, and -7A) and six members of the Frizzled gene family (Rfz1, Rfz2, Rfz3, Rfz4, Rfz6, Rfz9). In situ hybridization and immunocytochemistry experiments demonstrated that RWnt-4, -5B, -7A have distinct, although partly overlapping, expression patterns in the juvenile rat cochlea. These results suggest that the Wnt-Frizzled signaling pathway could be involved in several aspects of late cochlear differentiation and/or auditory function.

Innervation of supporting cells in the guinea pig cochlea detected in bloc-surface preparations

We immunohistochemically examined the distribution of nerve fibers among supporting cells of the cochlea by using the bloc-surface preparation. The existence of these nerve fibers was not very clear in the standard avidin-biotin complex (ABC) method. However, the standard ABC method complemented with silver intensification procedure provided very fine details of the nerve fibers. The nerves started to appear at low density about 55% of the distance from the apex, and their density gradually increased toward the upper turn. In each portion, the nerve fibers increased in thickness and length as well as the number of synapses made with the nuclei. Moreover, the distribution of these nerves in the fetal cochlea was similar to that in the adult. However, the functional significance and importance of these nerves remains to be determined. Our study also indicates that the silver intensification procedure combined with the standard ABC method is useful for the detailed observation of stereoscopic innervation in thick tissue preparations like such as the cochlea.

In vivo and in vitro localization of brain-derived neurotrophic factor, fibroblast growth factor-2 and their receptors in the bullfrog vestibular end organs

The inner ear sensory epithelia of vertebrates are composed mainly of supporting cells and hair cells (HCs). Brain-derived neurotrophic factor (BDNF) and fibroblast growth factor-2 (FGF-2) are trophins that are believed to play an essential role in the development and innervation of inner ear epithelia. Both trophins also may play a crucial role in the maintenance and regeneration of hair cells in the adult vertebrate ear. In the bullfrog vestibular system, hair cells are produced throughout life, and the epithelia regenerates following ototoxicity. The expression of BDNF and FGF-2 in the vestibular organs of the adult bullfrog was investigated at a cellular level both in histological sections and in vitro in dissociated cell cultures. In histological sections of the crista ampullaris, in situ hybridization and immunocytochemical techniques demonstrated that HCs express both BDNF and its receptor trkB, while the supporting cells express the receptor trkB alone. Following dissociation and in vitro cell culture no changes in the pattern of BDNF and trkB receptor were observed. Immunocytochemical studies demonstrated that in vivo hair cells express FGF-2 and the receptors FGFR-1 and FGFR-2 while supporting cells do not express either molecule. Following dissociation, HCs continue to express FGF-2 and its two receptors, while supporting cells upregulate the expression of FGF-2 and its receptor FGFR-2. These data confirm the potential role of BDNF and FGF-2 trophic regulation of the sensory epithelia of the adult inner ear. The findings suggest that BDNF has a role in the maintenance of the vestibular epithelia while FGF-2 may regulate the proliferation of supporting cells.

ATP-induced movement of the stalks of isolated cochlear Deiters' cells

Deiters' cells, a type of supporting cell in the sensory epithelium of the cochlea, the organ of Corti, have been found to have P2X and P2Y adenosine triphosphate (ATP) receptors on their surfaces. Activation of these receptors may alter the mechanical properties of Deiters' cells and thus of the organ itself. ATP was applied to Deiters' cells isolated from guinea pig cochlea and the tip of the cells' stalk monitored for an ATP induced movement. Application of 100 microM ATP to the cell body of isolated Deiters' cells induced a small, reversible movement of the cells' stalk whereas application of bathing media did not. Results suggest that in vivo endogenous extracellular ATP released from unidentified locations could alter cochlear mechanics.

Directional rectification of gap junctional voltage gating between dieters cells in the inner ear of guinea pig

Deiters cells (DCs) are the cochlear supporting cells in inner ear and contain multiple gap junction connexin genes, which when mutated can induce hearing loss. In the present study, the gap junctions between DCs were investigated by a double voltage clamp technique. Besides asymmetric responses to the polarities of transjunctional voltage (V(j)) and transmembrane potential (V(m)), the channels were also sensitive to which cell side was stimulated in a cell pair, i.e. voltage gating had directional dependence. The direction-dependent voltage gating could result in asymmetric current flow between the cells and influenced K(+) passage. Multiple connexins may constitute non-homotypic channels with directional dependence of voltage gating to mediate functional gap junction pathways in the cochlea. This may explain how a single connexin mutation can produce hearing loss.

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.

Regulation of p27Kip1 during gentamicin mediated hair cell death

The INK4 and Kip/Cip families of Cyclin Dependent Kinase inhibitors (CKIs) are regulators of the cell cycle. In addition, CKIS including p27(Kip1) can protect cells from apoptosis in vitro. However, little is known about protective effect of p27(Kip1) in vivo. We used systemic treatment with aminoglycosides to induce hair-cell death in the basilar papilla (BP), the auditory organ of the avian inner ear, and characterised the expression of p27(Kip1) with confocal and immunofluorescence microscopy. In contrast to the adult mammalian cochlea where p27(Kip1) is expressed only in supporting cells, p27(Kip1) is found in the nuclei of both hair cells and supporting cells in the BP of the normal, mature bird. Forty-eight hours after gentamicin treatment, hair cells with TUNEL positive nuclei and hair cells with pyknotic nuclei were both detected, suggesting many hair cells die by apoptosis. When the BP was double labelled for p27(Kip1) and myosin VIIa, a hair-cell specific protein, all dying hair cells that had been ejected from the epithelium were found to be myosin VIIa positive but negative for p27(Kip1) even though nuclear remnants were still visible. In the transition zone where partial hair-cell loss occurs, freshly ejected hair cells lying immediately above the surface of the BP no longer expressed p27(Kip1). Damaged hair cells within the epithelium in the transition zone contained p27(Kip1) in their cytoplasm but not in their nuclei. These data support recent in vitro findings suggesting that p27(Kip1) protects cells from apoptosis and that its downregulation may be a general feature of programmed cell death.

Selective expression of mercurial-insensitive water channel (AQP-4) gene in Hensen and Claudius cells in the rat cochlea

We investigated the cellular localization of mercurial-insensitive water channel (MIWC) mRNA in the rat cochlea. MIWC gene expression was detected in the supporting cells in Corti's organ. The function of these supporting cells is not clear, but the results suggest that they reabsorb water and play a role in maintaining the ionic balance of inner ear fluids.

Morphologic evidence for innervation of Deiters' and Hensen's cells in the guinea pig

The presence of nerve fibers and terminals among Deiters' and Hensen's cells of the organ of Corti of the adult guinea pig is demonstrated using immunostaining for synaptophysin and neurofilaments, acetylcholinesterase histochemistry, and transmission electron microscopy. These nerve terminals appeared to form chemical synapses with Deiters' and Hensen's cells. Nerve fibers and synapses were more common in the apical as compared to the basal cochlea. The terminals were often present on basal appendages of Hensen's cells, which were rich in mitochondria and often contained a Golgi apparatus and dense core vesicles. Electron microscopy and immunostaining for neurofilaments showed that most Hensen's cells in the apical cochlea received innervation. Few of the nerve fibers and terminals were positive for acetylcholinesterase, which suggests that they were not collaterals of cholinergic olivocochlear fibers. The density of these fibers, as shown by immunohistochemistry for neurofilaments, was far greater than previous reports of GABA-ergic fibers, which suggests that they were not GABA-ergic olivocochlear fibers. The role of such fibers and synapses with supporting cells of the outer hair cell area is unknown. Determination of the origins and functions of these fibers will provide new insights into cochlear structure and function.

Immunofluorescence localizations of proteins in semithin 0.2--1 micron frozen sections of the ear. A report of improved techniques including gelatin encapsulation and cryoultramicrotomy

The present work describes a high resolution technique for locating proteins in frozen sections of the inner ear by immunofluorescence. Dissected organs are encapsulated in gelatin, and sections 0.1--1 micron thick are cut at --100 degrees C in a cryoultramicrotome. These are labelled with antibodies against two cytoskeletal proteins, actin and tubulin. Actin, which had previously only been described in the sensory cells, is found in the supporting cells as well. Tubulin is identified in the supporting cells and in outer spiral nerve fibres.