Endolymphatic Potential Research Articles

Development of cochlear spiral ligament fibrocytes of the common marmoset, a nonhuman model animal

Spiral ligament fibrocytes generate potassium gradients, which hair cells require to convert mechanical sound waves into electrical palsy. Together with the stria vascularis, they regulate endolymph electrolyte homeostasis. Developing spiral ligament fibrocytes and generating endocochlear potential with an appropriate endolymph ion composition are essential for hearing. Understanding spiral ligament fibrocyte development is useful for studying age-related and genetic hearing loss, as well as for regenerative therapy and cochlear immunology. Despite interspecies differences, most studies of cochlear development have been conducted in rodent models due to the difficulty of using human fetal samples. This study investigated the cochlear development of spiral ligament fibrocytes in a small New World monkey species, the common marmoset (Callithrix jacchus). We examined the developmental expression of specific genes in spiral ligament fibrocytes, including those essential for the generation of endolymphatic potential. Our results showed that this animal model of spiral ligament fibrocyte development is similar to that of humans and is a suitable alternative for the analysis of human cochlear development. The time course established in this study will be useful for studying the primate-specific developmental biology of the inner ear, which may lead to novel treatment strategies for human hearing loss.

Nuclear Translocation Triggered at the Onset of Hearing in Cochlear Inner Hair Cells of Rats and Mice

PurposeNuclear position is precisely orchestrated during cell division, migration, and maturation of cells and tissues. Here we report a previously unrecognized, programmed movement of the nucleus in rat and mouse cochlear inner hair cells (IHCs) coinciding with the functional maturation of inner hair cells around the onset of hearing.MethodsWe measured hair cell length and nuclear position from confocal scans of immunofluorescence-labeled hair cells from whole-mount cochlear preparations throughout post-natal development.ResultsIn early post-natal days, the IHC experiences a period of sustained growth, during which the nucleus sits at the very basal pole of the cell, far from the apically located mechano-transducing stereocilia, but close to where synapses with primary afferent and efferent neurons are forming. After IHCs reach their final length, the nucleus moves to occupy a new position half-way along the length of the cell. Nuclear translocation begins in the middle turn, completes throughout the cochlea within 2–3 days, and coincides with the emergence of endolymphatic potential, the acquisition of big-conductance potassium channels (BK), and the onset of acoustic hearing. IHCs cultured in-vitro without endolymphatic potential (EP) do not grow, do not express BK, and do not experience nuclear movement. IHCs cultured in high K+ solutions (to simulate EP) grow but do not experience nuclear movement or acquire BK channels.ConclusionNuclear migration at the onset of hearing is a key step in the morphological maturation of IHCs. Whether this plays a role in functional maturation remains to be explored.

Analysis of NFKB1 and NFKB2 gene expression in the blood of patients with sudden sensorineural hearing loss.

Sudden Sensorineural Hearing Loss (SSNHL) is an increasingly common health problem today. Although the direct mortality rate of this disorder is relatively low, its impact on quality of life is enormous; this is why accurate identification of pathogenesis and influencing factors in the disease process can play an essential role in preventing and treating the disease. Acute inflammation, which leads to chronic inflammation due to aberrant expression of inflammation-mediating genes, may play a significant role in the pathogenesis of the disease. The essential Nuclear factor kappa B (NF-kB) pathway genes, NFKB1 and NFKB2, serve as prothrombotic agents when expressed abnormally, compromising the cochlea by disrupting the endolymphatic potential and causing SSNHL. This study investigates the expression levels of NFKB1 and NFKB2 in peripheral blood (PB) through a quantitative polymerase chain reaction in 50 Iranian patients with SSNHL, and 50 healthy volunteers were of the same age and sex as controls. As a result, NFKB2 expression levels in patients were higher than in controls, regardless of sex or age (posterior beta=0.619, adjusted P-value=0.016), and NFKB1 expression levels did not show significant differences between patients and controls. The expression levels of NFKB1 and NFKB2 had significantly strong positive correlations in both SSNHL patients and healthy individuals (r=0.620, P=0.001 and r=0.657, P 0.001, respectively), suggesting the presence of an interconnected network. NFKB2 has been identified as a significant inflammatory factor in the pathophysiology of SSNHL disease. Inflammation can play an essential role in developing SSNHL, and our findings could be used as a guide for future research.

Role of Kir4.1 Channels in Aminoglycoside-Induced Ototoxicity of Hair Cells.

The Kir4.1 channel, an inwardly rectifying potassium ion (K+) channel, is located in the hair cells of the organ of Corti as well as the intermediate cells of the stria vascularis. The Kir4.1 channel has a crucial role in the generation of endolymphatic potential and maintenance of the resting membrane potential. However, the role and functions of the Kir4.1 channel in the progenitor remain undescribed. To observe the role of Kir4.1 in the progenitor treated with the one-shot ototoxic drugs (kanamycin and furosemide), we set the proper condition in culturing Immortomouse-derived HEI-OC1 cells to express the potassium-related channels well. And also, that was reproduced in mice experiments to show the important role of Kir4.1 in the survival of hair cells after treating the ototoxicity drugs. In our results, when kanamycin and furosemide drugs were cotreated with HEI-OC1 cells, the Kir4.1 channel did not change, but the expression levels of the NKCC1 cotransporter and KCNQ4 channel are decreased. This shows that inward and outward channels were blocked by the two drugs (kanamycin and furosemide). However, noteworthy here is that the expression level of Kir4.1 channel increased when kanamycin was treated alone. This shows that Kir4.1, an inwardly rectifying potassium channel, acts as an outward channel in place of the corresponding channel when the KCNQ4 channel, an outward channel, is blocked. These results suggest that the Kir4.1 channel has a role in maintaining K+ homeostasis in supporting cells, with K+ concentration compensator when the NKCC1 cotransporter and Kv7.4 (KCNQ4) channels are deficient.

Electrochemical properties of the non-excitable tissue stria vascularis of the mammalian cochlea are sensitive to sounds.

Excitable cochlear hair cells convert the mechanical energy of sounds into the electrical signals necessary for neurotransmission. The key process is cellular depolarization via K+ entry from K+ -enriched endolymph through hair cells' mechanosensitive channels. Positive 80mV potential in endolymph accelerates the K+ entry, thereby sensitizing hearing. This potential represents positive extracellular potential within the epithelial-like stria vascularis; the latter potential stems from K+ equilibrium potential (EK ) across the strial membrane. Extra- and intracellular [K+ ] determining EK are likely maintained by continuous unidirectional circulation of K+ through a putative K+ transport pathway containing hair cells and stria. Whether and how the non-excitable tissue stria vascularis responds to acoustic stimuli remains unclear. Therefore, we analysed a cochlear portion for the best frequency, 1kHz, by theoretical and experimental approaches. We have previously developed a computational model that integrates ion channels and transporters in the stria and hair cells into a circuit and described a circulation current composed of K+ . Here, in this model, mimicking of hair cells' K+ flow induced by a 1kHz sound modulated the circulation current and affected the strial ion transport mechanisms; the latter effect resulted in monotonically decreasing potential and increasing [K+ ] in the extracellular strial compartment. Similar results were obtained when the stria in acoustically stimulated animals was examined using microelectrodes detecting the potential and [K+ ]. Measured potential dynamics mirrored the EK change. Collectively, because stria vascularis is electrically coupled to hair cells by the circulation current in vivo too, the strial electrochemical properties respond to sounds. KEY POINTS: A highly positive potential of +80mV in K+ -enriched endolymph in the mammalian cochlea accelerates sound-induced K+ entry into excitable sensory hair cells, a process that triggers hearing. This unique endolymphatic potential represents an EK -based battery for a non-excitable epithelial-like tissue, the stria vascularis. To examine whether and how the stria vascularis responds to sounds, we used our computational model, in which strial channels and transporters are serially connected to those hair cells in a closed-loop circuit, and found that mimicking hair cell excitation by acoustic stimuli resulted in increased extracellular [K+ ] and decreased the battery's potential within the stria. This observation was overall verified by electrophysiological experiments using live guinea pigs. The sensitivity of electrochemical properties of the stria to sounds indicates that this tissue is electrically coupled to hair cells by a radial ionic flow called a circulation current.

Deletion of Kcnj16 in Mice Does Not Alter Auditory Function.

Endolymphatic potential (EP) is the main driving force behind the sensory transduction of hearing, and K+ is the main charge carrier. Kir5.1 is a K+ transporter that plays a significant role in maintaining EP homeostasis, but the expression pattern and role of Kir5.1 (which is encoded by the Kcnj16 gene) in the mouse auditory system has remained unclear. In this study, we found that Kir5.1 was expressed in the mouse cochlea. We checked the inner ear morphology and measured auditory function in Kcnj16–/– mice and found that loss of Kcnj16 did not appear to affect the development of hair cells. There was no significant difference in auditory function between Kcnj16–/– mice and wild-type littermates, although the expression of Kcnma1, Kcnq4, and Kcne1 were significantly decreased in the Kcnj16–/– mice. Additionally, no significant differences were found in the number or distribution of ribbon synapses between the Kcnj16–/– and wild-type mice. In summary, our results suggest that the Kcnj16 gene is not essential for auditory function in mice.

Endolymphatic Potential Measured From Developing and Adult Mouse Inner Ear.

The mammalian inner ear has two major parts, the cochlea is responsible for hearing and the vestibular organ is responsible for balance. The cochlea and vestibular organs are connected by a series of canals in the temporal bone and two distinct extracellular fluids, endolymph and perilymph, fill different compartments of the inner ear. Stereocilia of mechanosensitive hair cells in the cochlea and vestibular end organs are bathed in the endolymph, which contains high K+ ions and possesses a positive potential termed endolymphatic potential (ELP). Compartmentalization of the fluids provides an electrochemical gradient for hair cell mechanotransduction. In this study, we measured ELP from adult and neonatal C57BL/6J mice to determine how ELP varies and develops in the cochlear and vestibular endolymph. We measured ELP and vestibular microphonic response from saccules of neonatal mice to determine when vestibular function is mature. We show that ELP varies considerably in the cochlear and vestibular endolymph of adult mice, ranging from +95 mV in the basal turn to +87 mV in the apical turn of the cochlea, +9 mV in the saccule and utricle, and +3 mV in the semicircular canal. This suggests that ELP is indeed a local potential, despite the fact that endolymph composition is similar. We further show that vestibular ELP reaches adult-like magnitude around post-natal day 6, ~12 days earlier than maturation of cochlear ELP (i.e., endocochlear potential). Maturation of vestibular ELP coincides with the maturation of vestibular microphonic response recorded from the saccular macula, suggesting that maturation of vestibular function occurs much earlier than maturation of hearing in mice.

Forgotten Fibrocytes: A Neglected, Supporting Cell Type of the Cochlea With the Potential to be an Alternative Therapeutic Target in Hearing Loss.

Cochlear fibrocytes are a homeostatic supporting cell type embedded in the vascularized extracellular matrix of the spiral ligament, within the lateral wall. Here, they participate in the connective tissue syncytium that enables potassium recirculation into the scala media to take place and ensures development of the endolymphatic potential that helps drive current into hair cells during acoustic stimulation. They have also been implicated in inflammatory responses in the cochlea. Some fibrocytes interact closely with the capillaries of the vasculature in a way which suggests potential involvement, together with the stria vascularis, also in the blood-labyrinth barrier. Several lines of evidence suggests that pathology of the fibrocytes, along with other degenerative changes in this region, contribute to metabolic hearing loss (MHL) during aging that is becoming recognized as distinct from, and potentially a precursor for, sensorineural hearing loss (SNHL). This pathology may underlie a significant proportion of cases of presbycusis. Some evidence points also to an association between fibrocyte degeneration and Ménière’s disease (MD). Fibrocytes are mesenchymal; this characteristic, and their location, make them amenable to potential cell therapy in the form of cell replacement or genetic modification to arrest the process of degeneration that leads to MHL. This review explores the properties and roles of this neglected cell type and suggests potential therapeutic approaches, such as cell transplantation or genetic engineering of fibrocytes, which could be used to prevent this form of presbycusis or provide a therapeutic avenue for MD.

Sensorineural Hearing Loss After Adoptive Cell Immunotherapy for Melanoma Using MART-1 Specific T Cells: A Case Report and Its Pathophysiology

To illustrate a case of sensorineural hearing loss (SNHL) after immunotherapy based on T cell receptor (TCR) gene therapy using modified T cells recognizing melanoma antigen recognized by T cells 1 for disseminated melanoma. We present a 59-year-old woman with profound subacute bilateral SNHL including unilateral deafness after immunotherapy based on TCR gene therapy using modified T cells recognizing melanoma antigen recognized by T cells 1 for disseminated melanoma. Ten days after treatment, the patient developed hearing loss of 57 dB hearing loss air conduction at pure-tone average 0.5-1-2-4 kHz in the right ear, and >100 dB hearing loss air conduction at pure-tone average 0.5-1-2-4 in the left ear. The right ear recovered partially, while the left ear remained deaf, despite oral prednisolone (1.0 mg/kg) and salvage treatment with three transtympanic injections of 0.5 ml dexamethasone (4.0 mg/ml). Based on our presented case and a vast amount of literature there is circumstantial evidence that TCR gene therapy for melanoma targets the perivascular macrophage-like melanocytes in the stria vascularis, resulting in SNHL. We suggest that SNHL after TCR gene therapy may be caused by a disruption of the blood-labyrinth-barrier and the endolymphatic potential and/or a sterile inflammation of the stria vascularis. In severe cases like our subject, we posit that endolymphatic hydrops or hair cell loss may cause irreversible and asymmetrical deafness. Steroid prophylaxis via transtympanic application is debatable.

Hearing Loss Controlled by Optogenetic Stimulation of Nonexcitable Nonglial Cells in the Cochlea of the Inner Ear.

Light-gated ion channels and transporters have been applied to a broad array of excitable cells including neurons, cardiac myocytes, skeletal muscle cells and pancreatic β-cells in an organism to clarify their physiological and pathological roles. Nonetheless, among nonexcitable cells, only glial cells have been studied in vivo by this approach. Here, by optogenetic stimulation of a different nonexcitable cell type in the cochlea of the inner ear, we induce and control hearing loss. To our knowledge, deafness animal models using optogenetics have not yet been established. Analysis of transgenic mice expressing channelrhodopsin-2 (ChR2) induced by an oligodendrocyte-specific promoter identified this channel in nonglial cells—melanocytes—of an epithelial-like tissue in the cochlea. The membrane potential of these cells underlies a highly positive potential in a K+-rich extracellular solution, endolymph; this electrical property is essential for hearing. Illumination of the cochlea to activate ChR2 and depolarize the melanocytes significantly impaired hearing within a few minutes, accompanied by a reduction in the endolymphatic potential. After cessation of the illumination, the hearing thresholds and potential returned to baseline during several minutes. These responses were replicable multiple times. ChR2 was also expressed in cochlear glial cells surrounding the neuronal components, but slight neural activation caused by the optical stimulation was unlikely to be involved in the hearing impairment. The acute-onset, reversible and repeatable phenotype, which is inaccessible to conventional gene-targeting and pharmacological approaches, seems to at least partially resemble the symptom in a population of patients with sensorineural hearing loss. Taken together, this mouse line may not only broaden applications of optogenetics but also contribute to the progress of translational research on deafness.

Molecular bases of K+ secretory cells in the inner ear: shared and distinct features between birds and mammals.

In the cochlea, mammals maintain a uniquely high endolymphatic potential (EP), which is not observed in other vertebrate groups. However, a high [K+] is always present in the inner ear endolymph. Here, we show that Kir4.1, which is required in the mammalian stria vascularis to generate the highly positive EP, is absent in the functionally equivalent avian tegmentum vasculosum. In contrast, the molecular repertoire required for K+ secretion, specifically NKCC1, KCNQ1, KCNE1, BSND and CLC-K, is shared between the tegmentum vasculosum, the vestibular dark cells and the marginal cells of the stria vascularis. We further show that in barn owls, the tegmentum vasculosum is enlarged and a higher EP (~+34 mV) maintained, compared to other birds. Our data suggest that both the tegmentum vasculosum and the stratified stria vascularis evolved from an ancestral vestibular epithelium that already featured the major cell types of the auditory epithelia. Genetic recruitment of Kir4.1 specifically to strial melanocytes was then a crucial step in mammalian evolution enabling an increase in the cochlear EP. An increased EP may be related to high-frequency hearing, as this is a hallmark of barn owls among birds and mammals among amniotes.

The difference in endolymphatic hydrostatic pressure elevation induced by isoproterenol between the ampulla and the cochlea.

The purpose of the study was to investigate the difference in the responses of endolymphatic hydrostatic pressure to isoproterenol, β-adrenergic receptor agonist, between pars superior and pars inferior. The hydrostatic pressure of endolymph and perilymph and endolymphatic potential in the ampulla and the cochlea during the intravenous administration of isoproterenol were recorded using a servo-null system in guinea pigs. The hydrostatic pressure of endolymph and perilymph in the ampulla and cochlea was similar in magnitude. Isoproterenol significantly increased hydrostatic pressure of ampullar and cochlear endolymph and perilymph with no change in the ampullar endolymphatic potential and endocochlear potential, respectively. The isoproterenol-induced maximum change of endolymphatic hydrostatic pressure in ampulla was significantly (p<0.01) smaller than that in the cochlea. In ears with an obstructed endolymphatic sac, the action of isoproterenol on endolymphatic hydrostatic pressure in the ampulla disappeared like that in the cochlea. Isoproterenol elevates endolymphatic hydrostatic pressure in different manner between the vestibule and the cochlea.

Measurement of endolymphatic pressure

Endolymphatic pressure measurement is of interest both to researchers in the physiology and pathophysiology of hearing and ENT physicians dealing with Menière's disease or similar conditions. It is generally agreed that endolymphatic hydrops is associated with Menière's disease and is accompanied by increased hydrostatic pressure. Endolymphatic pressure, however, cannot be measured precisely without endangering hearing, making the association between hydrops and increased endolymphatic pressure difficult to demonstrate. Several integrated in vivo models have been developed since the 1960s, but only a few allow measurement of endolymphatic hydrostatic pressure. Models associating measurement of hydrostatic pressure and endolymphatic potential and assessment of cochlear function are of value to elucidate the pathophysiology of endolymphatic hydrops. The present article presents the main types of models and discusses their respective interest.

Relationship between endolymphatic hydrops and vestibular-evoked myogenic potential

Conclusion: Vestibular-evoked myogenic potential (VEMP) can be used to examine endolymphatic hydrops, especially in the vestibule. Objective: To investigate the relationship between the degree of endolymphatic hydrops revealed by magnetic resonance imaging (MRI) and VEMP. Methods: Gadolinium diluted with saline was injected intratympanically in 49 ears (40 patients). One day after the injection, the endolymphatic space in the vestibule and the cochlea was visualized by 3 Tesla MRI. A VEMP test was done, and VEMP was judged as absent when the VEMP was within the noise level. Results: VEMP was present in 21 ears and absent in 28 ears. Endolymphatic hydrops was significantly associated with the disappearance of VEMP. Endolymphatic hydrops in the vestibule had a stronger effect than endolymphatic hydrops in the cochlea. Five patients with extremely large vestibular hydrops showed no response of VEMP.

Vacuolization and alterations of lysosomal membrane proteins in cochlear marginal cells contribute to hearing loss in neuraminidase 1-deficient mice

The neuraminidase-1 ( Neu1) knockout mouse model is a phenocopy of the lysosomal storage disease (LSD) sialidosis, characterized by multisystemic and neuropathic symptoms, including hearing loss. We have characterized the auditory defects in Neu1 − /− mice and found that hearing loss involves both conductive and sensorineural components. Auditory brainstem response (ABR) thresholds were significantly elevated in Neu1 − /− mice at P21 (48 –55 dB), and hearing loss appeared progressive (53 –66 dB at P60). At these ages Neu1 − /− mice accumulated cerumen in the external ear canal and had a thickened mucosa and inflammation in the middle ear. In cochleae of adult wild-type mice, Neu1 was expressed in several cell types in the stria vascularis, the organ of Corti, and spiral ganglion. Progressive morphological abnormalities such as extensive vacuolization were detected in the Neu1 − /− cochleae as early as P9. These early morphologic changes in Neu1 − /− cochleae were associated with oversialylation of several lysosomal associated membrane proteins (Lamps) in the stria vascularis. A marked increase in the expression and apical localization of Lamp-1 in marginal cells of the stria vascularis predicts exacerbation of lysosomal exocytosis into the endolymph. Consequently, the endolymphatic potential in Neu1 − /− mice was reduced by approximately 20 mV at ages P31–P44, which would cause dysfunction of transduction in sensory hair cells. This study suggests a molecular mechanism that contributes to hearing loss in sialidosis and identifies potential therapeutic targets.

The endolymphatic surface of the stria vascularis in the guinea-pig and the effects of ethacrynic acid as shown by scanning electron microscopy

Examination of the endolymphatic surface of the stria vascularis showed, in agreement with previous studies, the marginal cells to be hexagonal in shape and that most possessed microvilli. However, the cells in the apical turns were more profusely covered with microvilli than those at the base and, particularly in the basal turns, there was some variation in the surface detail of the cells. These results may indicate differences in the physiological state of the cells. In the early period of ethacrynic acid intoxication (15-30 minutes post-injection), when rapid changes in endolymphatic potential and ion-fluxes occur, the surface of the marginal cells showed only slight distortion. The most dramatic changes were noted 1 hour post-injection, and involved swelling of the cells and loss of microvilli. This distortion persisted up to 2 hours post-injection. The possible explanation for these findings is discussed.

Orphan Glutamate Receptor δ1 Subunit Required for High-Frequency Hearing

The function of the orphan glutamate receptor delta subunits (GluRdelta1 and GluRdelta2) remains unclear. GluRdelta2 is expressed exclusively in the Purkinje cells of the cerebellum, and GluRdelta1 is prominently expressed in inner ear hair cells and neurons of the hippocampus. We found that mice lacking the GluRdelta1 protein displayed significant cochlear threshold shifts for frequencies of >16 kHz. These deficits correlated with a substantial loss of type IV spiral ligament fibrocytes and a significant reduction of endolymphatic potential in high-frequency cochlear regions. Vulnerability to acoustic injury was significantly enhanced; however, the efferent innervation of hair cells and the classic efferent inhibition of outer hair cells were unaffected. Hippocampal and vestibular morphology and function were normal. Our findings show that the orphan GluRdelta1 plays an essential role in high-frequency hearing and ionic homeostasis in the basal cochlea, and the locus encoding GluRdelta1 represents a candidate gene for congenital or acquired high-frequency hearing loss in humans.

Long term effects of BAPTA in scala media on cochlear function

BAPTA was iontophoresed or allowed to diffuse into the scala media of the first turn of the guinea pig cochlea via pipettes inserted through the round window and basilar membrane. Cochlear action potential (CAP) thresholds for basal turn frequencies were elevated, scala media cochlear microphonic in response to a 207 Hz tone were drastically reduced and the distortion products 2 f1– f2 and f2– f2 in response to primaries set at 18 and 21.6 kHz were eliminated or severely reduced. The animals were recovered and the above measurements repeated between 24 and 240 h after the application of BAPTA. In all animals thresholds for basal turn frequencies remained elevated, and the distortion components were severely reduced. The endolymphatic potential (EP), measured through the basilar membrane on recovery, was not significantly different from the values measured before BAPTA was applied. If the effect of BAPTA, in lowering endolymphatic Ca 2+ concentration, is restricted to the destruction of tip links, as has been shown in many other preparations, then these results suggest that this effect has permanent consequences, either because the tip links failed to regenerate or because their destruction precipitated the degeneration of OHCs. These results may have a bearing on the mechanisms behind permanent threshold shift.

Does BAPTA leave outer hair cell transduction channels closed?

The calcium chelator BAPTA was iontophoresed into the scala media of the second turn of the guinea pig cochlea. This produced a reduction in low frequency cochlear microphonic (CM) measured in scala media and an elevation of the cochlear action potential (CAP) threshold that lasted for the duration of the experiment. Using two pipettes, one filled with KCl and the other KCl and BAPTA (50, 20 and 5 mM) it was possible to observe the effect of passing current through one electrode while measuring the endolymphatic potential (EP) with the other. The results demonstrated that current passed via the BAPTA pipette caused a sustained increase in EP of 8.2, 12.9 and 7.8 mV in the three animals used. This increase coincided with the decrease in low frequency CM that indicated a causal connection between the two. In a second series of experiments, pipettes with larger tips were inserted into scala media in the first cochlear turn and BAPTA was allowed to diffuse from the pipette. The results confirmed the relationship between EP increase and the fall of scala media CM. One interpretation of these results is that lowering the Ca 2+ concentration of endolymph with BAPTA inhibits mechano-electrical transduction in outer hair cells (OHCs) and leaves the hair cell transduction channels in a closed state, thus increasing the resistance across OHCs and increasing the EP. These findings are consistent with a model of hair cell transduction in which tension on stereo cilia opens the transduction channels.

Lack of pendrin HCO3−transport elevates vestibular endolymphatic [Ca2+] by inhibition of acid-sensitive TRPV5 and TRPV6 channels

The low Ca(2+) concentration ([Ca(2+)]) of mammalian endolymph in the inner ear is required for normal hearing and balance. We reported (Yamauchi et al., Biochem Biophys Res Commun 331: 1353-1357, 2005) that the epithelial Ca(2+) channels TRPV5 and TRPV6 (transient receptor potential types 5 and 6) are expressed in the vestibular system and that TRPV5 expression is stimulated by 1,25-dihydroxyvitamin D(3), as also reported in kidney. TRPV5/6 channels are known to be inhibited by extracellular acidic pH. Endolymphatic pH, [Ca(2+)], and transepithelial potential of the utricle were measured in Cl(-)/HCO(3)(-) exchanger pendrin (SLC26A4) knockout mice in vivo. Slc26a4(-/-) mice exhibit reduced pH and utricular endolymphatic potential and increased [Ca(2+)]. Monolayers of primary cultures of rat semicircular canal duct cells were grown on permeable supports, and cellular uptake of (45)Ca(2+) was measured individually from the apical and basolateral sides. Net uptake of (45)Ca(2+) was greater after incubation with 1,25-dihydroxyvitamin D(3). Net (45)Ca(2+) absorption was dramatically inhibited by low apical pH and was stimulated by apical alkaline pH. Gadolinium, lanthanum, and ruthenium red reduced apical uptake. These observations support the notion that one aspect of vestibular dysfunction in Pendred syndrome is a pathological elevation of endolymphatic [Ca(2+)] due to luminal acidification and consequent inhibition of TRPV5/6-mediated Ca(2+) absorption.