Development of monovalent ions in the endolymph in mouse cochlea

An L1 retrotransposon insertion-induced deafness mouse model for studying the development and function of the cochlear stria vascularis

Dysregulation of ion and potential homeostasis in the scala media is the most prevalent cause of hearing loss in mammals. However, it is not well understood how the development and function of the stria vascularis regulates this fluid homeostasis in the scala media. From a mouse genetic screen, we characterize a mouse line, named 299, that displays profound hearing impairment. Histology suggests that 299 mutant mice carry a severe, congenital structural defect of the stria vascularis. The in vivo recording of 299 mice using double-barreled electrodes shows that endocochlear potential is abolished and potassium concentration is reduced to ∼20 mM in the scala media, a stark contrast to the +80 mV endocochlear potential and the 150 mM potassium concentration present in healthy control mice. Genomic analysis revealed a roughly 7-kb-long, interspersed nuclear element (LINE-1 or L1) retrotransposon insertion on chromosome 11. Strikingly, the deletion of this L1 retrotransposon insertion from chromosome 11 restored the hearing of 299 mutant mice. In summary, we characterize a mouse model that enables the study of stria vascularis development and fluid homeostasis in the scala media.

Potential Mechanisms Underlying Inflammation-Enhanced Aminoglycoside-Induced Cochleotoxicity

Aminoglycoside antibiotics remain widely used for urgent clinical treatment of life-threatening infections, despite the well-recognized risk of permanent hearing loss, i.e., cochleotoxicity. Recent studies show that aminoglycoside-induced cochleotoxicity is exacerbated by bacteriogenic-induced inflammation. This implies that those with severe bacterial infections (that induce systemic inflammation), and are treated with bactericidal aminoglycosides are at greater risk of drug-induced hearing loss than previously recognized. Incorporating this novel comorbid factor into cochleotoxicity risk prediction models will better predict which individuals are more predisposed to drug-induced hearing loss. Here, we review the cellular and/or signaling mechanisms by which host-mediated inflammatory responses to infection could enhance the trafficking of systemically administered aminoglycosides into the cochlea to enhance the degree of cochleotoxicity over that in healthy preclinical models. Once verified, these mechanisms will be potential targets for novel pharmacotherapeutics that reduce the risk of drug-induced hearing loss (and acute kidney damage) without compromising the life-saving bactericidal efficacy of aminoglycosides.

In vivo genetic manipulation of inner ear connexin expression by bovine adeno-associated viral vectors

We have previously shown that in vitro transduction with bovine adeno–associated viral (BAAV) vectors restores connexin expression and rescues gap junction coupling in cochlear organotypic cultures from connexin–deficient mice that are models DFNB1 nonsyndromic hearing loss and deafness. The aims of this study were to manipulate inner ear connexin expression in vivo using BAAV vectors, and to identify the optimal route of vector delivery. Injection of a BAAV vector encoding a bacterial Cre recombinase via canalostomy in adult mice with floxed connexin 26 (Cx26) alleles promoted Cre/LoxP recombination, resulting in decreased Cx26 expression, decreased endocochlear potential, increased hearing thresholds, and extensive loss of outer hair cells. Injection of a BAAV vector encoding GFP-tagged Cx30 via canalostomy in P4 mice lacking connexin 30 (Cx30) promoted formation of Cx30 gap junctions at points of contacts between adjacent non-sensory cells of the cochlear sensory epithelium. Levels of exogenous Cx30 decayed over time, but were still detectable four weeks after canalostomy. Our results suggest that persistence of BAAV-mediated gene replacement in the cochlea is limited by the extensive remodeling of the organ of Corti throughout postnatal development and associated loss of non-sensory cells.

Critical role of ATP-induced ATP release for Ca2+ signaling in nonsensory cell networks of the developing cochlea

Spatially and temporally coordinated variations of the cytosolic free calcium concentration ([Ca²⁺]_c) play a crucial role in a variety of tissues. In the developing sensory epithelium of the mammalian cochlea, elevation of extracellular adenosine trisphosphate concentration ([ATP]_e) triggers [Ca²⁺]_c oscillations and propagation of intercellular inositol 1,4,5-trisphosphate (IP₃)-dependent Ca²⁺ waves. What remains uncertain is the relative contribution of gap junction channels and connexin hemichannels to these fundamental mechanisms, defects in which impair hearing acquisition. Another related open question is whether [Ca²⁺]_c oscillations require oscillations of the cytosolic IP₃ concentration ([IP₃]_c) in this system. To address these issues, we performed Ca²⁺ imaging experiments in the lesser epithelial ridge of the mouse cochlea around postnatal day 5 and constructed a computational model in quantitative adherence to experimental data. Our results indicate that [Ca²⁺]_c oscillations are governed by Hopf-type bifurcations within the experimental range of [ATP]_e and do not require [IP₃]_c oscillations. The model replicates accurately the spatial extent and propagation speed of intercellular Ca²⁺ waves and predicts that ATP-induced ATP release is the primary mechanism underlying intercellular propagation of Ca²⁺ signals. The model also uncovers a discontinuous transition from propagating regimes (intercellular Ca²⁺ wave speed > 11 μm⋅s^-1) to propagation failure (speed = 0), which occurs upon lowering the maximal ATP release rate below a minimal threshold value. The approach presented here overcomes major limitations due to lack of specific connexin channel inhibitors and can be extended to other coupled cellular systems.

Deletion of Brg1 causes abnormal hair cell planer polarity, hair cell anchorage, and scar formation in mouse cochlea

Hair cells (HCs) are mechanosensors that play crucial roles in perceiving sound, acceleration, and fluid motion. The precise architecture of the auditory epithelium and its repair after HC loss is indispensable to the function of organ of Corti (OC). In this study, we showed that Brg1 was highly expressed in auditory HCs. Specific deletion of Brg1 in postnatal HCs resulted in rapid HC degeneration and profound deafness in mice. Further experiments showed that cell-intrinsic polarity of HCs was abolished, docking of outer hair cells (OHCs) by Deiter’s cells (DCs) failed, and scar formation in the reticular lamina was deficient. We demonstrated that Brg1 ablation disrupted the Gαi/Insc/LGN and aPKC asymmetric distributions, without overt effects on the core planer cell polarity (PCP) pathway. We also demonstrated that Brg1-deficient HCs underwent apoptosis, and that leakage in the reticular lamina caused by deficient scar formation shifted the mode of OHC death from apoptosis to necrosis. Together, these data demonstrated a requirement for Brg1 activity in HC development and suggested a role for Brg1 in the proper cellular structure formation of HCs.

Deletion of Tricellulin Causes Progressive Hearing Loss Associated with Degeneration of Cochlear Hair Cells

Tricellulin (also known as MARVELD2) is considered as a central component of tricellular tight junctions and is distributed among various epithelial tissues. Although mutations in the gene encoding tricellulin are known to cause deafness in humans (DFNB49) and mice, the influence of its systemic deletion in vivo remains unknown. When we generated tricellulin-knockout mice (Tric^−/−), we found an early-onset rapidly progressive hearing loss associated with the degeneration of hair cells (HCs); however, their body size and overall appearance were normal. Tric^−/− mice did not show any morphological change pertaining to other organs such as the gastrointestinal tract, liver, kidney, thyroid gland and heart. The endocochlear potential (EP) was normal in Tric^−/− mice, suggesting that the tight junction barrier is maintained in the stria vascularis, where EP is generated. The degeneration of HCs, which occurred after the maturation of EP, was prevented in the culture medium with an ion concentration similar to that of the perilymph. These data demonstrate the specific requirement of tricellulin for maintaining ion homeostasis around cochlear HCs to ensure their survival. The Tric^−/− mouse provides a new model for understanding the distinct roles of tricellulin in different epithelial systems as well as in the pathogenesis of DFNB49.

Deafness in occludin-deficient mice with dislocation of tricellulin and progressive apoptosis of the hair cells

Occludin is the first identified protein in the tight junction (TJ), but its function has remained for the most part obscure. TJs have been demonstrated to play important roles in the inner ear function, and occludin is expressed in all the epithelial TJs in the inner ear. Thus, we examined the inner ears of occludin-deficient (Occ^−/−) mice. Although inner ears initially developed normally in Occ^−/− mice, apoptosis occurs in hair cells in the organ of Corti around day 12 after birth, and deafness develops. Since hair cell degeneration was not observed in cochlear explant cultures of Occ^−/− mice, environmental changes were considered to be the trigger of cell death. As for the vestibular system, both the morphologies and functions are normal in Occ^−/− mice. These phenotypes of Occ^−/− mice are very similar with those of claudin-14 or claudin-9 deficient mice, leading us to speculate on the existence of imbalance induced by TJ abnormalities, such as localized ionic components. Moreover, the occludin deficiency led to dislocalization of tricellulin, a gene responsible for human deafness DFNB49. The deafness in Occ^−/− mice may be due to this dislocalization of tricellulin.

Mouse models for pendrin-associated loss of cochlear and vestibular function

The human gene SLC26A4 and the mouse ortholog Slc26a4 code for the protein pendrin, which is an anion exchanger expressed in apical membranes of selected epithelia. In the inner ear, pendrin is expressed in the cochlea, the vestibular labyrinth and the endolymphatic sac. Loss-of-function and hypo-functional mutations cause an enlargement of the vestibular aqueduct (EVA) and sensorineural hearing loss. The relatively high prevalence of SLC26A4 mutations provides a strong imperative to develop rational interventions that delay, ameliorate or prevent pendrin-associated loss of cochlear and vestibular function. This review summarizes recent studies in mouse models that have been developed to delineate the role of pendrin in the physiology of hearing and balance and that have brought forward the concept that a temporally and spatially limited therapy may be sufficient to secure a life-time of normal hearing in children bearing mutations of SLC26A4.

ATP-dependent intercellular Ca2+ signaling in the developing cochlea: facts, fantasies and perspectives

Hearing relies on a sensitive mechanoelectrical transduction process in the cochlea of the inner ear. The cochlea contains sensory, secretory, neural, supporting and epithelial cells which are all essential to the sound transduction process. It is well known that a complex extracellular purinergic signaling system contributes to cochlear homeostasis, altering cochlear sensitivity and neural output via ATP-gated ion channels (P2X receptors) and G protein-coupled P2Y receptors. This review focuses on the emerging roles of ATP that are currently under investigation in the developing sensory epithelium, with particular emphasis on the link between ATP release, Ca(2+) signaling, the expression and function of gap junction proteins connexin26 and connexin30, and the acquisition of hearing.

The role of pendrin in the development of the murine inner ear

Enlargement of the vestibular aqueduct (EVA) is a common inner ear malformation found in children with sensorineural hearing loss that is frequently associated with loss-of-function or hypo-function mutations of SLC26A4. SLC26A4 codes for pendrin, which is a protein that is expressed in apical membranes of selected epithelia and functions as an anion exchanger. The comparatively high prevalence of EVA provides a strong imperative to develop rational interventions that delay, ameliorate or prevent hearing loss associated with this phenotype. The development of rational interventions requires a fundamental understanding of the role that pendrin plays in the normal development of hearing, as well as a detailed understanding of the pathobiologic mechanisms that, in the absence of fully functional pendrin, lead to an unstable hearing phenotype, with fluctuating or progressive loss of hearing. This review summarizes studies in mouse models that have focused on delineating the role of pendrin in the physiology of the inner ear and the pathobiology that leads to hearing loss.

Development of gap junctional intercellular communication within the lateral wall of the rat cochlea

Auditory function depends on gap junctional intercellular communication (GJIC) between fibrocytes within the cochlear spiral ligament, and basal cells and intermediate cells within stria vascularis. This communication within the lateral wall is hypothesized to support recirculation of K+ from perilymph to the intra-strial space, and thus is essential for the high [K+] measured within endolymph, and the generation of the endocochlear potential. In rats, the [K+] within endolymph reaches adult levels by postnatal day 7 (P7), several days before hearing onset, suggesting that GJIC matures before auditory responses are detectable. In this study we have mapped the postnatal development of GJIC within the cochlear lateral wall, to determine the stage at which direct communication first exists between the spiral ligament and stria vascularis. Connexin 30 immunofluorescence revealed a progressive increase of gap junction plaque numbers from P0 onwards, initially in the condensing mesenchyme behind strial marginal cells, and spreading throughout the lateral wall by P7-P8. Whole-cell patch clamp experiments revealed compartmentalized intercellular dye-coupling in the lateral wall between P2 and P5. There was extensive dye-coupling throughout the fibrocyte syncytium by P7. Also, by P7 dye introduced to fibrocytes could also be detected within strial basal cells and intermediate cells. These data suggest that lateral wall function matures several days in advance of hearing onset, and provide anatomical evidence of the existence of a putative K+ recirculation pathway within the cochlear lateral wall.

Failure of fluid absorption in the endolymphatic sac initiates cochlear enlargement that leads to deafness in mice lacking pendrin expression

Mutations of SLC26A4 are among the most prevalent causes of hereditary deafness. Deafness in the corresponding mouse model, Slc26a4^−/−, results from an abnormally enlarged cochlear lumen. The goal of this study was to determine whether the cochlear enlargement originates with defective cochlear fluid transport or with a malfunction of fluid transport in the connected compartments, which are the vestibular labyrinth and the endolymphatic sac. Embryonic inner ears from Slc26a4^+/− and Slc26a4^−/− mice were examined by confocal microscopy ex vivo or after 2 days of organ culture. Culture allowed observations of intact, ligated or partially resected inner ears. Cochlear lumen formation was found to begin at the base of the cochlea between embryonic day (E) 13.5 and 14.5. Enlargement was immediately evident in Slc26a4^−/− compared to Slc26a4^+/− mice. In Slc26a4^+/− and Slc26a4^−/− mice, separation of the cochlea from the vestibular labyrinth by ligation at E14.5 resulted in a reduced cochlear lumen. Resection of the endolymphatic sacs at E14.5 led to an enlarged cochlear lumen in Slc26a4^+/− mice but caused no further enlargement of the already enlarged cochlear lumen in Slc26a4^−/− mice. Ligation or resection performed later, at E17.5, did not alter the cochlea lumen. In conclusion, the data suggest that cochlear lumen formation is initiated by fluid secretion in the vestibular labyrinth and temporarily controlled by fluid absorption in the endolymphatic sac. Failure of fluid absorption in the endolymphatic sac due to lack of Slc26a4 expression appears to initiate cochlear enlargement in mice, and possibly humans, lacking functional Slc26a4 expression.

Opening the TRPML gates

The intracellular TRPML channels have multiple biological roles, but the physiological stimuli that open them remained unknown. In a previous issue of Chemistry & Biology, Grimm et al. report a high-throughput chemical screen that identified a plethora of selective activators of TRPML3 that should open the way to fully characterize these channels and their physiological roles.

Small molecule activators of TRPML3

We conducted a high-throughput screen for small molecule activators of the TRPML3 ion channel, which, when mutated, causes deafness and pigmentation defects. Cheminformatics analyses of the 53 identified and confirmed compounds revealed nine different chemical scaffolds and 20 singletons. We found that agonists strongly potentiated TRPML3 activation with low extracytosolic [Na(+)]. This synergism revealed the existence of distinct and cooperative activation mechanisms and a wide dynamic range of TRPML3 activity. Testing compounds on TRPML3-expressing sensory hair cells revealed the absence of activator-responsive channels. Epidermal melanocytes showed only weak or no responses to the compounds. These results suggest that TRPML3 in native cells might be absent from the plasma membrane or that the protein is a subunit of heteromeric channels that are nonresponsive to the activators identified in this screen.

The role of potassium recirculation in cochlear amplification

PURPOSE OF REVIEW

Normal cochlear function depends on maintaining the correct ionic environment for the sensory hair cells. Here we review recent literature on the cellular distribution of potassium transport-related molecules in the cochlea.

RECENT FINDINGS

Transgenic animal models have identified novel molecules essential for normal hearing and support the idea that potassium is recycled in the cochlea. The findings indicate that extracellular potassium released by outer hair cells into the space of Nuel is taken up by supporting cells, that the gap junction system in the organ of Corti is involved in potassium handling in the cochlea, that the gap junction system in stria vascularis is essential for the generation of the endocochlear potential, and that computational models can assist in the interpretation of the systems biology of hearing and integrate the molecular, electrical, and mechanical networks of the cochlear partition. Such models suggest that outer hair cell electromotility can amplify over a much broader frequency range than expected from isolated cell studies.

SUMMARY

These new findings clarify the role of endolymphatic potassium in normal cochlear function. They also help current understanding of the mechanisms of certain forms of hereditary hearing loss.

Endocochlear potential depends on Cl- channels: mechanism underlying deafness in Bartter syndrome IV

Human Bartter syndrome IV is an autosomal recessive disorder characterized by congenital deafness and severe renal salt and fluid loss. It is caused by mutations in BSND, which encodes barttin, a beta-subunit of ClC-Ka and ClC-Kb chloride channels. Inner-ear-specific disruption of Bsnd in mice now reveals that the positive potential, but not the high potassium concentration, of the scala media depends on the presence of these channels in the epithelium of the stria vascularis. The reduced driving force for K(+)-entry through mechanosensitive channels into sensory hair cells entails a profound congenital hearing loss and subtle vestibular symptoms. Although retaining all cell types and intact tight junctions, the thickness of the stria is reduced early on. Cochlear outer hair cells degenerate over several months. A collapse of endolymphatic space was seen when mice had additionally renal salt and fluid loss due to partial barttin deletion in the kidney. Bsnd(-/-) mice thus demonstrate a novel function of Cl(-) channels in generating the endocochlear potential and reveal the mechanism leading to deafness in human Bartter syndrome IV.

Deafness in mice lacking the T-box transcription factor Tbx18 in otic fibrocytes

In the cochlea, fibrocytes play important physiological roles, including the maintenance of the ionic composition of the endolymph. Human deafness upon fibrocyte alterations witnesses their crucial role for hearing. We demonstrate that differentiation of otic fibrocytes requires the T-box transcription factor gene Tbx18. Tbx18 expression during inner ear development is restricted to the sub-region of otic mesenchyme that is fated to differentiate into fibrocytes. We rescued the somitic defect that underlies the perinatal lethality of Tbx18-mutant mice by a transgenic approach, and measured auditory brainstem responses. Adult Tbx18-deficient mice showed profound deafness and a complete disruption of the endocochlear potential that is essential for the transduction of sound by sensory hair cells. The differentiation of otic fibrocytes of the spiral ligament was severely compromised. Tissue architecture of the stria vascularis of the lateral wall was disrupted, exhibiting an almost complete absence of the basal cell layer, and a reduction and changes of intermediate and marginal cells, respectively. Stria vascularis defects resulted from the failure of Tbx18-mutant otic fibrocytes to generate the basal cell layer by a mesenchymal-epithelial transition. Defects in otic fibrocyte differentiation may be subordinate to a primary role of Tbx18 in early compartmentalization of the otic mesenchyme, as lineage restriction and boundary formation between otic fibrocytes and the surrounding otic capsule were severely affected in the mutant. Our study sheds light on the genetic control of patterning and differentiation of the otic mesenchyme, uncovers distinct steps of stria vascularis formation and illuminates the importance of non-epithelially-derived otic cell types for normal hearing and the etiology of deafness.

Free radical stress-mediated loss of Kcnj10 protein expression in stria vascularis contributes to deafness in Pendred syndrome mouse model

Pendred syndrome is due to loss-of-function mutations of Slc26a4, which codes for the HCO(3)(-) transporter pendrin. Loss of pendrin causes deafness via a loss of the K(+) channel Kcnj10 in stria vascularis and consequent loss of the endocochlear potential. Pendrin and Kcnj10 are expressed in different cell types. Here, we report that free radical stress provides a link between the loss of Kcnj10 and the loss of pendrin. Studies were performed using native and cultured stria vascularis from Slc26a4(+/-) and Slc26a4(-/-) mice as well as Chinese hamster ovary (CHO)-K1 cells. Kcnj10, oxidized proteins, and proteins involved in iron metabolism were quantified by Western blotting. Nitrated proteins were quantified by ELISA. Total iron was measured by ferrozine spectrophotometry and gene expression was quantified by qRT-PCR. At postnatal day 10 (P10), stria vascularis from Slc26a4(+/-) and Slc26a4(-/-) mice expressed similar amounts of Kcnj10. Slc26a4(-/-) mice lost Kcnj10 expression during the next 5 days of development. In contrast, stria vascularis, obtained from P10 Slc26a4(-/-) mice and kept in culture for 5 days, maintained Kcnj10 expression. Stria vascularis from Slc26a4(-/-) mice was found to suffer from free radical stress evident by elevated amounts of oxidized and nitrated proteins and other changes in protein and gene expression. Free radical stress induced by 3-morpholinosydnonimine-N-ethylcarbamide was found to be sufficient to reduce Kcnj10 expression in CHO-K1 cells. These data demonstrate that free radical stress provides a link between loss of pendrin and loss of Kcnj10 in Slc26a4(-/-) mice and possibly in human patients suffering from Pendred syndrome.

KCNQ1/KCNE1 K+ channel and P2Y4 receptor are co-expressed from the time of birth in the apical membrane of rat strial marginal cells

CONCLUSION

KCNQ1/KCNE1 K(+) channels and P2Y(4) receptors are expressed in the apical membrane of rat strial marginal cells from postnatal day 1 (P1) and maintained throughout development.

OBJECTIVES

The purpose of the present study was to investigate the developmental expression of KCNQ1/KCNE1 K(+) channel and of P2Y(4), which is an important metabotropic regulator of KCNQ1/KCNE1 K(+) channel in strial marginal cells.

MATERIALS AND METHODS

Sprague-Dawley rats at different stages of development (P1, P3, P5, P7, P14, and P21) were studied. The spiral ligament with the stria vascularis was detached from the cartilaginous or bony cochlea and prepared for a voltage-sensitive vibrating probe and immunohistochemistry.

RESULTS

Chromanol 293B, a blocker of KCNQ1/KCNE1 K(+) channel, inhibited short-circuit currents (I ( sc )) from P1 to P21. Similarly, I ( sc ) were found to be decreased by uridine 5'-triphosphate at all ages. The antagonist profiles indicated that the apical P2Y receptor is P2Y(4) subtype. KCNQ1, KCNE1, and P2Y(4) were immunolocalized in the apical region of stria vascularis at P1.

Changes in P2Y4 receptor expression in rat cochlear outer sulcus cells during development

Extracellular adenosine triphosphate (ATP) released from cellular sources plays an important role in variety of the cochlear physiologic processes. The primary purinergic receptor subtype in the cochlea is the P2X2 receptor, which is a subtype of P2X receptor. This receptor appears to mediate a protective decrease in the electrical driving force in response to acoustic overstimulation. Outer sulcus cells (OSCs) in the cochlear lateral wall appear to maintain an adequate K+ concentration in the cochlear endolymph in response to varying intensities of auditory stimulation. However, little is known about developing OSCs. The purpose of this study was to investigate subtypes of purinergic receptors in developing rat OSCs using a voltage-sensitive vibrating probe. Results showed that only two P2 receptors (P2Y4 and P2X2) contributed to the regulation of short circuit currents in neonatal OSCs. ATP increased cation absorption via apical nonselective cation channels after activating P2Y4 receptors in early neonatal OSCs. P2Y4 expression rapidly declined postnatally and reached near adult levels on postnatal day 14. P2X2 was co-expressed with P2Y4 in early neonatal OSCs. Temporal changes in P2Y4 during OSC development might be involved in the establishment of the endolymphatic ion composition needed for normal auditory transduction and/or specific cellular differentiation.

Inactivation of NADPH oxidase organizer 1 results in severe imbalance

Otoconia are biominerals of the vestibular system that are indispensable for the perception of gravity. Despite their importance, the process of otoconia genesis is largely unknown. Reactive oxygen species (ROS) have been recognized for their toxic effects in antimicrobial host defense as well as in aging and carcinogenesis. Enzymes evolved for ROS production belong to the recently discovered NADPH oxidase (Nox) enzyme family . Here we show that the inactivation of a regulatory subunit, NADPH oxidase organizer 1 (Noxo1), resulted in the severe balance deficit seen in the spontaneous mutant "head slant" (hslt) mice whose phenotype was rescued by Noxo1 transgenes. Wild-type Noxo1 was expressed in the vestibular and cochlear epithelia and was required for ROS production by an oxidase complex. In contrast, the hslt mutation of Noxo1 was biochemically inactive and led to an arrest of otoconia genesis, characterized by a complete lack of calcium carbonate mineralization and an accumulation of otoconial protein, otoconin-90/95 (OC-90/95). These results suggest that ROS generated by a Noxo1-dependent vestibular oxidase are critical for otoconia formation and may be required for interactions among otoconial components. Noxo1 mutants implicate a constructive developmental role for ROS, in contrast to their previously described toxic effects.

Localization and functional studies of pendrin in the mouse inner ear provide insight about the etiology of deafness in pendred syndrome

Immunolocalization studies of mouse cochlea and vestibular end-organ were performed to study the expression pattern of pendrin, the protein encoded by the Pendred syndrome gene (PDS), in the inner ear. The protein was restricted to the areas composed of specialized epithelial cells thought to play a key role in regulating the composition and resorption of endolymph. In the cochlea, pendrin was abundant in the apical membrane of cells in the spiral prominence and outer sulcus cells (along with their root processes). In the vestibular end-organ, pendrin was found in the transitional cells of the cristae ampullaris, utriculi, and sacculi as well as in the apical membrane of cells in the endolymphatic sac. Pds-knockout (Pds-/-) mice were found to lack pendrin immunoreactivity in all of these locations. Histological studies revealed that the stria vascularis in Pds-/- mice was only two-thirds the thickness seen in wild-type mice, with the strial marginal cells showing irregular shapes and sizes. Functional studies were also performed to examine the role of pendrin in endolymph homeostasis. Using double-barreled electrodes placed in both the cochlea and the utricle, the endocochlear potential and endolymph potassium concentration were measured in wild-type and Pds-/- mice. Consistent with the altered strial morphology, the endocochlear potential in Pds-/- mice was near zero and did not change during anoxia. On the other hand, the endolymphatic potassium concentration in Pds-/- mice was near normal in the cochlea and utricle. Together, these results suggest that pendrin serves a key role in the functioning of the basal and/or intermediate cells of the stria vascularis to maintain the endocochlear potential, but not in the potassium secretory function of the marginal cells.

Age-related changes in cochlear endolymphatic potassium and potential in CD-1 and CBA/CaJ mice

The CD-1 mouse strain is known to have early onset of hearing loss that is progressive with aging. We sought to determine whether a disturbance of K+ homeostasis and pathological changes in the cochlear lateral wall were involved in the age-related hearing loss (AHL) of CD-1 as compared to the CBA/CaJ strain which has minimal AHL. In the present study, the endocochlear potential (EP) and endolymphatic K+ concentration ([K+]e) were measured in both strains of mice with double-barrel microelectrodes at "young" (1-2 mo) and "old" (5-9 mo) ages. CBA/CaJ mice displayed no changes with aging in EP and [K+]e of the basal turn. In the apical turn, there was a small positive shift of the EP (10 mV) with aging under both normoxic and acute anoxic conditions (-EP), without any change of [K+]e. Further, there were no obvious pathological changes in the lateral wall of CBA/CaJ mice. By contrast, old CD-1 mice displayed a significantly reduced [K+]e by 30% in both basal and apical turns with no significant changes in normoxic EP. The -EP in the apical turn was significantly reduced in magnitude by 6 mV. A severe loss of cells with aging was observed in the region of type IV fibrocytes of the apical and basal turns and of type II fibrocytes in the basal turn. A complete degeneration of organ of Corti was also observed at the basal turn of old CD-1 mice, as well as a basalward decline of spiral ganglion neuron density. The pathological changes in spiral ligament of CD-1 mice were similar to those of an inbred mouse strain C57BL/6J that expresses an AHL gene (ahl) and might be a primary etiology of AHL of CD-1 mice. These findings have ramifications for our understanding of AHL and for interpretation of genetic mutations in a CD-1 background.

Expression of aquaporin 1 and 5 in the developing mouse inner ear and audiovestibular assessment of an Aqp5 null mutant

To examine the potential roles of aquaporins 1 and 5 (AQP1 and AQP5, respectively) in inner ear development and function, we defined their spatial and temporal expression patterns in the developing mouse inner ear and examined the morphologic and physiologic effects of loss of Aqp5 function. Standard in situ hybridization (ISH) and immunohistochemical (IHC) assays were used for expression studies with routine morphologic, behavioral, and physiologic assessments of hearing and balance in Aqp5 null mutant mice. AQP1 was first detected at embryonic day 10.5 (E10.5) in the otocyst but eventually localized to specific nonsensory portions of the inner ear and connective tissue cells surrounding the membranous labyrinth. AQP5 displayed specific cochlear expression, first detectable at E15.5 in the nonsensory epithelium and later restricted to the lateral wall of the cochlear duct near the spiral prominence. AQP5 expression continued through postnatal periods with a change of expression domain to the stria vascularis between postnatal day 7 (P7) and P14. By in situ hybridization and immunohistochemical techniques, subtle differences between transcript and protein expression patterns were noted for both AQP1 and 5. Although AQP5 is dynamically expressed in the developing mouse inner ear, adult Aqp5 knockout mice show normal hearing when tested and normal inner ear structural development. These results suggest redundant or alternative mechanisms that likely regulate water homeostasis in the developing and mature inner ear.

K+ cycling and the endocochlear potential

Sensory transduction in the cochlea and the vestibular labyrinth depends on the cycling of K+. In the cochlea, endolymphatic K+ flows into the sensory hair cells via the apical transduction channel and is released from the hair cells into perilymph via basolateral K+ channels including KCNQ4. K+ may be taken up by fibrocytes in the spiral ligament and transported from cell to cell via gap junctions into strial intermediate cells. Gap junctions may include GJB2, GJB3 and GJB6. K+ is released from the intermediate cells into the intrastrial space via the KCNJ10 K+ channel that generates the endocochlear potential. From the intrastrial space, K+ is taken up across the basolateral membrane of strial marginal cells via the Na+/2Cl-/K+ cotransporter SLC12A2 and the Na+/K+-ATPase ATP1A1/ATP1B2. Strial marginal cells secrete K+ across the apical membrane into endolymph via the K+ channel KCNQ1/KCNE1, which concludes the cochlear cycle. A similar K+ cycle exists in the vestibular labyrinth. Endolymphatic K+ flows into the sensory hair cells via the apical transduction channel and is released from the hair cells via basolateral K+ channels including KCNQ4. Fibrocytes connected by gap junctions including GJB2 may be involved in delivering K+ to vestibular dark cells. Extracellular K+ is taken up into vestibular dark cells via SLC12A2 and ATP1A1/ATP1B2 and released into endolymph via KCNQ1/KCNE1, which concludes the vestibular cycle. The importance of K+ cycling is underscored by the fact that mutations of KCNQ1, KCNE1, KCNQ4, GJB2, GJB3 and GJB6 lead to deafness in humans and that null mutations of KCNQ1, KCNE1, KCNJ10 and SLC12A2 lead to deafness in mouse models.

KCNJ10 (Kir4.1) potassium channel knockout abolishes endocochlear potential

Stria vascularis of the cochlea generates the endocochlear potential and secretes K(+). K(+) is the main charge carrier and the endocochlear potential the main driving force for the sensory transduction that leads to hearing. Stria vascularis consists of two barriers, marginal cells that secrete potassium and basal cells that are coupled via gap junctions to intermediate cells. Mice lacking the KCNJ10 (Kir4.1) K(+) channel in strial intermediate cells did not generate an endocochlear potential. Endolymph volume and K(+) concentration ([K(+)]) were reduced. These studies establish that the KCNJ10 K(+) channel provides the molecular mechanism for generation of the endocochlear potential in concert with other transport pathways that establish the [K(+)] difference across the channel. KCNJ10 is also a limiting pathway for K(+) secretion.