Effect of locally applied drugs on the endolymphatic sac potential

“Reversed polarization” of Na/K-ATPase—a sign of inverted transport in the human endolymphatic sac: a super-resolution structured illumination microscopy (SR-SIM) study

The human endolymphatic sac (ES) is believed to regulate inner ear fluid homeostasis and to be associated with Meniere’s disease (MD). We analyzed the ion transport protein sodium/potassium-ATPase (Na/K-ATPase) and its isoforms in the human ES using super-resolution structured illumination microscopy (SR-SIM). Human vestibular aqueducts were collected during trans-labyrinthine vestibular schwannoma surgery after obtaining ethical permission. Antibodies against various isoforms of Na/K-ATPase and additional solute-transporting proteins, believed to be essential for ion and fluid transport, were used for immunohistochemistry. A population of epithelial cells of the human ES strongly expressed Na/K-ATPase α1, β1, and β3 subunit isoforms in either the lateral/basolateral or apical plasma membrane domains. The β1 isoform was expressed in the lateral/basolateral plasma membranes in mostly large cylindrical cells, while β3 and α1 both were expressed with “reversed polarity” in the apical cell membrane in lower epithelial cells. The heterogeneous expression of Na/K-ATPase subunits substantiates earlier notions that the ES is a dynamic structure where epithelial cells show inverted epithelial transport. Dual absorption and secretion processes may regulate and maintain inner ear fluid homeostasis. These findings may shed new light on the etiology of endolymphatic hydrops and MD.

Molecular architecture underlying fluid absorption by the developing inner ear

Mutations of SLC26A4 are a common cause of hearing loss associated with enlargement of the endolymphatic sac (EES). Slc26a4 expression in the developing mouse endolymphatic sac is required for acquisition of normal inner ear structure and function. Here, we show that the mouse endolymphatic sac absorbs fluid in an SLC26A4-dependent fashion. Fluid absorption was sensitive to ouabain and gadolinium but insensitive to benzamil, bafilomycin and S3226. Single-cell RNA-seq analysis of pre- and postnatal endolymphatic sacs demonstrates two types of differentiated cells. Early ribosome-rich cells (RRCs) have a transcriptomic signature suggesting expression and secretion of extracellular proteins, while mature RRCs express genes implicated in innate immunity. The transcriptomic signature of mitochondria-rich cells (MRCs) indicates that they mediate vectorial ion transport. We propose a molecular mechanism for resorption of NaCl by MRCs during development, and conclude that disruption of this mechanism is the root cause of hearing loss associated with EES.

Gastric-type H+,K+-ATPase in mouse vestibular end organs

CONCLUSION

Gastric type H⁺,K⁺-ATPase in the vestibular end organs may be of importance for K⁺ circulation and may also be related to pH regulation in vestibular end organs and endolymphatic sac.

OBJECTIVE

To analyze the expression of gastric-type H⁺,K⁺-ATPase in normal mouse vestibular end organs.

METHODS

8 weeks old CBA/J mice were used in this study. The presence of gastric-type H⁺,K⁺-ATPase α and β in the vestibular end organs, viz. utricle, saccule, ampulla, vestibular ganglion, and endolymphatic sac, was investigated using immunohistochemistry.

RESULTS

In the vestibular end organs, H⁺,K⁺-ATPase α and β were almost identical. H⁺,K⁺-ATPase was expressed in sensory cells, the basolateral surface of dark cells, fibrocytes, in vestibular ganglion cells, and in the upper region of the endolymphatic sac epithelial cells.

Ion transport its regulation in the endolymphatic sac: suggestions for clinical aspects of Meniere's disease

Ion transport and its regulation in the endolymphatic sac (ES) are reviewed on the basis of recent lines of evidence. The morphological and physiological findings demonstrate that epithelial cells in the intermediate portion of the ES are more functional in ion transport than those in the other portions. Several ion channels, ion transporters, ion exchangers, and so on have been reported to be present in epithelial cells of ES intermediate portion. An imaging study has shown that mitochondria-rich cells in the ES intermediate portion have a higher activity of Na⁺, K⁺-ATPase and a higher Na⁺ permeability than other type of cells, implying that molecules related to Na⁺ transport, such as epithelial sodium channel (ENaC), Na⁺–K⁺–2Cl⁻ cotransporter 2 (NKCC2) and thiazide-sensitive Na⁺–Cl⁻ cotransporter (NCC), may be present in mitochondria-rich cells. Accumulated lines of evidence suggests that Na⁺ transport is most important in the ES, and that mitochondria-rich cells play crucial roles in Na⁺ transport in the ES. Several lines of evidence support the hypothesis that aldosterone may regulate Na⁺ transport in ES, resulting in endolymph volume regulation. The presence of molecules related to acid/base transport, such as H⁺-ATPase, Na⁺–H⁺ exchanger (NHE), pendrin (SLC26A4), Cl⁻–HCO₃⁻ exchanger (SLC4A2), and carbonic anhydrase in ES epithelial cells, suggests that acid/base transport is another important one in the ES. Recent basic and clinical studies suggest that aldosterone may be involved in the effect of salt-reduced diet treatment in Meniere’s disease.

Electrogenic transport and K(+) ion channel expression by the human endolymphatic sac epithelium

The endolymphatic sac (ES) is a cystic organ that is a part of the inner ear and is connected to the cochlea and vestibule. The ES is thought to be involved in inner ear ion homeostasis and fluid volume regulation for the maintenance of hearing and balance function. Many ion channels, transporters, and exchangers have been identified in the ES luminal epithelium, mainly in animal studies, but there has been no functional study investigating ion transport using human ES tissue. We designed the first functional experiments on electrogenic transport in human ES and investigated the contribution of K(+) channels in the electrogenic transport, which has been rarely identified, even in animal studies, using electrophysiological/pharmacological and molecular biological methods. As a result, we identified functional and molecular evidence for the essential participation of K(+) channels in the electrogenic transport of human ES epithelium. The identified K(+) channels involved in the electrogenic transport were KCNN2, KCNJ14, KCNK2, and KCNK6, and the K(+) transports via those channels are thought to play an important role in the maintenance of the unique ionic milieu of the inner ear fluid.

Contemporary perspectives on the pathophysiology of Meniere's disease: implications for treatment

PURPOSE OF REVIEW

Meniere's disease is characterized by episodic vertigo, fluctuating hearing loss, aural fullness and tinnitus. Endolymphatic hydrops, found on post-mortem examination, is the histologic hallmark. Recent research suggests that endolymphatic hydrops results from cytochemical perturbations of unknown etiology that lead to disturbance of the normal endolymphatic fluid homeostasis. This consequent hydropic state or the associated cytochemical perturbations appears to create a neurotoxic environment that ultimately leads to spiral ganglion cell death likely via the apoptotic mechanism. This review highlights some of the recent advances in the understanding of the pathophysiology of endolymphatic hydrops and progressive cochleovestibular deterioration, with emphasis placed on its potential therapeutic implications.

RECENT FINDINGS

Recent evidence supports that endolymphatic hydrops is possibly an epiphenomenon, and is preceded by perturbation of the normal ionic transport regulatory mechanisms. Furthermore, chronic cochleovestibular deterioration appears to be the result of an excitotoxic response to chronic hydrops. A recently described animal model, the Phex mouse, carrying a mutation in the Phex Hyp-Duk gene, provides a novel insight to genetically regulated postnatal endolymphatic hydrops and a useful tool to expand our understanding.

SUMMARY

Despite encouraging recent advances, there are considerable challenges that remain in the development of targeted therapeutic interventions that may offer new avenues of neuroprotection in known cases of Meniere's disease. These advances will hopefully provide pharmacotherapeutic interventions aimed at preventing progressive cochleovestibular dysfunction.

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.

Meniere's disease: update of etiopathogenetic theories and proposal of a possible model of explanation

Meniere's Disease (MD) is an affection consisting of an association of sensorineural hearing loss, tinnitus and vertigo initially presenting by crises. A review of the most considered possible causative factors and pathophysiologic interpretations allows us to underline the uncertainties which still exist about the genesis of this illness. We propose a mechanistic model based on the effect of a haemodynamic imbalance leading to transient ischaemia which could have an effect on the pH of the inner ear as well as on the work of the inner ear proton pumps. It is hypothesized that under ischaemic conditions and consequent metabolic acidity a preserved proton pump activity can generate an overload of anions in the endolymphatic partition, which is a closed system, thus resulting in an enhancement of osmolarity and consequently in the formation of a hydrops resulting in the development of fluctuating hearing loss, tinnitus and vertigo which characterize Meniere's Disease.

Functional and molecular expression of epithelial sodium channels in cultured human endolymphatic sac epithelial cells

HYPOTHESIS

Epithelial sodium channels are expressed in cultured human endolymphatic sac (ES) epithelial (HESE) cells and epithelial sodium channel (ENaC) expression is suppressed by interleukin 1beta.

BACKGROUND

The ES is part of the membranous labyrinth in the inner ear that plays an important role in maintaining homeostasis of the endolymphatic fluid system. However, the exact mechanism of fluid volume regulation is not yet known.

METHODS

The ES specimens were harvested during acoustic neuroma surgery (n = 13) using the translabyrinthine approach and were subcultured with high-epidermal growth factor (25 ng/mL) media.

RESULTS

The serially passaged HESE cells differentiated into a monolayer of confluent cells and some of the cultured cells had features of mitochondria-rich cells. Reverse transcription-polymerase chain reaction revealed that ENaC subunits are expressed in the cultured HESE cells. We also confirmed the presence of an ENaC-dependent short-circuit current in the cultured HESE cells. Interestingly, ENaC mRNA expression and ENaC-dependent current decreased after treatment with interleukin 1beta (10 nmol/L for 24 h).

CONCLUSION

These findings suggest that ENaC plays an important role in fluid absorption in the human ES and that its function may be altered during inflammatory conditions.

Measurement of the endolymphatic sac potential in human

In this study, we measured human endolymphatic sac potential (ESP) in 8 patients with vestibular schwannoma and in five patients with Ménière's disease during surgery. ESP was measured with a glass electrode filled with 154 mM NaCl and with an outside tip diameter ranging from 2 to 3 microm. The mean value of human ESP in patients with vestibular schwannoma was +13.3+/-1.9 mV. Since electron microscopy showed that the endolymphatic sacs of the eight patients with vestibular schwannoma were normal in the ultrastructures the value can be close to normal human ESP. While in Ménière's disease, three cases showed low potentials and two cases showed almost the same values observed as in the eight patients with vestibular schwannoma. In the two cases with Ménière's disease, the epithelial cells of the endolymphatic sac were preserved. Our study can be considered as the first successful measurement of human ESP and revealed the existence of Ménière's disease having normal endolymphatic sac in function as well as morphology.

Le sac endolymphatique : ses fonctions au sein de l’oreille interne

The endolymphatic sac is a non-sensory organ of the inner ear. It is connected to the endolymphatic compartment that is filled with endolymph, a potassium-rich fluid that bathes the apical side of inner ear sensory cells. The main functions ascribed to the endolymphatic sac are the regulation of the volume and pressure of endolymph, the immune response of the inner ear, and the elimination of endolymphatic waste products by phagocytosis. Functional alteration of these functions, leading to deficient endolymph homeostasis and/or altered inner ear immune response, may participate to the pathophysiology of Ménière's disease, an inner ear pathology that causes episodes of vertigo, sensorineural hearing loss and tinnitus, and is characterized by an increase in volume of the cochleo-vestibular endolymph (endolymphatic hydrops).

Quantitative anatomy of the guinea pig endolymphatic sac

The endolymphatic sac is believed to represent one of the primary loci for endolymph volume regulation in the inner ear. Quantitative analysis of physiologic measurements from the endolymphatic sac requires knowledge of the anatomy of the structure, specifically the luminal volume and the variation of cross-sectional area with distance along the sac. Recently techniques have become available to make these measurements. In the present study, fixed, isolated specimens of the guinea pig endolymphatic sac were imaged by high-resolution magnetic resonance microscopy (MRM) or by histological serial sections. Structures were reconstructed and quantified using image analysis software. In specimens imaged by MRM the endolymphatic sac volume, including tissue and lumen, was 359 nl for the intraosseous region and 106 nl for the extraosseous region, totaling 465 nl for the entire structure. The luminal volumes were 131 nl for the intraosseous region and 13 nl for the extraosseous region, totaling 144 nl. In histological specimens the volume, including tissue and lumen, was 414 nl for the intraosseous region and 121 nl for the extraosseous region, totaling 535 nl for the entire structure. The luminal volumes were 152 nl for the intraosseous region and 26 nl for the extraosseous region, totaling 179 nl. Differences in volume estimates obtained by the two methods were not statistically significant and variation was dominated by inter-specimen variation. Pooling the data, the total volume of the endolymphatic sac in the guinea pig including tissue and lumen was 506 nl (S.D. 100, n=17) and the volume of the lumen was 169 nl (S.D. 48, n=14).

Morphological and functional characteristics of cells cultured from the endolymphatic sac

The endolymphatic sac is a part of the homeostasis-regulating system of the membranous labyrinth of the inner ear. Disturbances in the function of the endolymphatic sac are believed to be involved in the genesis of different inner ear disorders, such as endolymphatic hydrops and Ménière's disease. To make studies of the ion- and fluid-regulating mechanisms of the sac possible, a method to culture the tissue in vitro was developed. Epithelial cells and fibroblasts were morphologically characterised in the cell cultures with light and electron microscopy as well as immunohistochemically using antibodies against cytokeratin and vimentin. Since mesenchymal cells have been shown to express vimentin and epithelial cells cytokeratin, the antibodies against these two intermediate filament proteins were used to further confirm the morphological identification. In addition, some functional characteristics of the cultured cells from the endolymphatic sac were studied. ATP and K(+) were added to the cell cultures and changes in cytoplasmic free Ca(2+) concentration ([Ca(2+)](i)) were determined with the fura-2 method. A rapid and transient increase in [Ca(2+)](i) could be seen in both epithelial cells and fibroblasts after applying ATP (200 microM) extracellularly. However, when K(+) was added in concentrations of 50 mM and 100 mM, no changes in [Ca(2+)](i) could be seen in either the epithelial cells or the fibroblasts. The results show that the cultured endolymphatic sac cells preserve their morphological characteristics and maintain a high viability. Accordingly, this method provides a tool for further studies of ion transport mechanisms and fluid homeostasis in the endolymphatic sac.

Effect of locally applied drugs on the pH of luminal fluid in the endolymphatic sac of guinea pig

The aim of the present work was to assess the effect of various drugs applied locally on the pH of the luminal fluid (pH(lum)) in guinea pig endolymphatic sac. pH(lum) and transepithelial potential, when measured in vivo by means of double-barrelled pH-sensitive microelectrodes, were 7.06 +/- 0.08 and +6.1 +/- 0.34 mV (mean +/- SE; n = 84), respectively, which is consistent with a net acid secretion in the luminal fluid of the endolymphatic sac. Bafilomycin and acetazolamide increased and decreased, respectively, pH(lum). Amiloride, ethylisopropylamiloride, ouabain, and Schering 28080 had no effect on pH(lum). Results obtained with inhibitors of anionic transport systems were inconclusive; e.g., DIDS reduced pH(lum), whereas neither SITS nor triflocin had any effect. We conclude that bafilomycin-sensitive H(+)-ATPase activity accounts for the transepithelial acid gradient measured in the endolymphatic sac and that intracellular and membrane-bound carbonic anhydrase probably participates in regulating endolymphatic sac pH(lum). The relationship between acid pH, endolymph volume, and Ménière's disease remains to be further investigated.

Ionic and potential changes of the endolymphatic sac induced by endolymph volume changes

The endolymphatic sac (ES) is believed to be the locus for endolymph volume regulation in the inner ear. It has recently been shown that induced endolymph volume changes in the cochlea result in anatomical changes in the ES, suggesting that function of the sac varies according to endolymph volume status. In the present study we have recorded luminal concentrations of K(+) and Na(+) from the ES and the endolymphatic sac potential (ESP) during cochlear endolymph volume changes. ES recordings were made by an extradural approach, thereby preserving normal cerebrospinal fluid resting pressure. Cochlear endolymph volume changes were generated by performing injections or withdrawals through a pipette inserted into endolymph by a round window approach. The pre-treatment concentrations of K(+) and Na(+) in the ES were found to be 8.4 mM (S.D. 3.3, n=8) and 128. 6 mM (S.D. 18.4, n=10) respectively, and the mean ESP was 14.4 mV (S. D. 5.2, n=18). Endolymphatic injections were found to produce a sustained increase in the K(+) content of the ES by an average of 19. 9 mM and to decrease Na(+) by 30.7 mM measured 50 min after the start of injection. The time for K(+) increase to occur was found to correlate with the injected volume, with larger injected volumes producing a more rapid increase. Endolymphatic withdrawals were found to induce a slow decline in endolymphatic K(+) by an average of 3.4 mM measured at 50 min after withdrawal, although no significant change of Na(+) was detected. Volume-induced ESP changes were highly variable. Injections produced a small increase in the mean ESP and withdrawals produced a small decrease but neither change was statistically significant and some animals showed potential changes in the opposite direction. These data show that a change in cochlear endolymph volume status results in a physiologic response of the ES which is sustained for a considerable period. If the ES plays a part in the restoration of normal endolymph volume, this process appears to proceed slowly, based on the prolonged time courses of ionic changes observed.

Anti-heat shock protein 70 antibodies in Meniere's disease

OBJECTIVES

To determine the prevalence of anti-heat shock protein 70 (anti-HSP70) antibodies in patients with Meniere's disease and healthy subjects and to probe the relationship between antibody status and clinical features of Meniere's disease.

STUDY DESIGN

Prospective cohort study of consecutive consenting patients with Meniere's disease.

METHODS

Serum samples were obtained prospectively from 134 patients with Meniere's disease and 124 blood donors. Serial samples were taken at 3-month intervals in 38 of 134 patients with Meniere's disease. Demographic data and clinical characterization of vestibular and auditory status were acquired with each sample. Serum was assayed for anti-HSP70 antibodies by Western blot using bovine renal extract, recombinant bovine HSP70, and recombinant human HSP70 antigens.

RESULTS

Immunoreactivity against bovine renal extract HSP70 was found in 38% of patients with Meniere's disease, compared with 25% of blood donors (P < .04). Reactivity with recombinant antigens was not significantly different between patients with Meniere's disease and healthy control subjects. Patients with Meniere's disease who reacted with all three antigens were more likely to have simultaneously active hearing and balance symptoms (P = .03). Neither univariate nor multivariate statistical analysis established any other association between serological findings and clinical features of Meniere's disease. Tests performed on serial samples of patients with Meniere's disease also showed no association of positive or negative test results with changes in clinical course.

CONCLUSIONS

Because of the high prevalence of antiHSP70 antibodies in healthy subjects and the very limited association of anti-HSP70 antibody status with clinical features or course of Meniere's disease, we conclude that, at present, the detection of anti-HSP70 antibodies by Western blotting offers little clinically useful information in Meniere's disease.

Morphological changes of the endolymphatic sac induced by microinjection of artificial endolymph into the cochlea

Morphological changes of the endolymphatic sac were analyzed in guinea pigs following microinjection of artificial endolymph into the cochlea or withdrawal of a quantity of native endolymph. Injections were performed into the second turn of scala media with a micro-pump at a rate of 60-100 nl/min, lasting for a period of 4, 7. 5, 15 or 18 min. In withdrawal experiments, endolymph was aspirated from the second cochlear turn over a period of 8 min. For each procedure the contralateral (non-treated) ear served as a histological control. Following artificial endolymph injections of 7. 5 min or more there was an almost total absence of the normal intraluminal homogeneous substance (HS) on the injected side. Our observations suggest that the disappearance of the HS occurs by both enzymatic and macrophagic activity. After endolymphatic withdrawals the ES was found to contain increased amounts of HS. The results could suggest that the volume of fluid in the ES, and hence the volume of the entire membranous labyrinth, may be regulated by a dynamic relationship between active secretion and enzymatic degradation of a lumen-expanding substance that is intimately related to the intraluminal macrophages. The exact mechanism governing these regulatory systems, and their relationship to ion and water movements across the epithelium of the sac, remain to be elucidated.

Evidence for apical K conductance and Na-K-2Cl cotransport in the endolymphatic sac of guinea pig

The transepithelial potential in the endolymphatic sac (ESP) was recorded up to 60 min after apical injection of ouabain, bumetanide, quinine, barium, tetraethylammonium, and 4-aminopyridine. After control injection, ESP decreased by 74% and completely recovered at 30 min. After ouabain, barium, or quinine injection, the ESP time course was similar to that in the control group. After bumetanide, tetraethylammonium, or 4-aminopyridine injection, complete recovery was only observed at 60 min. These results suggest that apical K+ conductance and Na-K-2Cl cotransporter could be involved in the genesis of ESP.