Developmental change of Na+-absorptive function in Reissner's membrane epithelia

Slc26a7 chloride channel activity and localization in mouse Reissner's membrane epithelium

Several members of the SLC26 gene family have highly-restricted expression patterns in the auditory and vestibular periphery and mutations in mice of at least two of these (SLC26A4 and SLC26A5) lead to deficits in hearing and/or balance. A previous report pointed to SLC26A7 as a candidate gene important for cochlear function. In the present study, inner ears were assayed by immunostaining for Slc26a7 in neonatal and adult mice. Slc26a7 was detected in the basolateral membrane of Reissner’s membrane epithelial cells but not neighboring cells, with an onset of expression at P5; gene knockout resulted in the absence of protein expression in Reissner’s membrane. Whole-cell patch clamp recordings revealed anion currents and conductances that were elevated for NO₃⁻ over Cl⁻ and inhibited by I⁻ and NPPB. Elevated NO₃⁻ currents were absent in Slc26a7 knockout mice. There were, however, no major changes to hearing (auditory brainstem response) of knockout mice during early adult life under constitutive and noise exposure conditions. The lack of Slc26a7 protein expression found in the wild-type vestibular labyrinth was consistent with the observation of normal balance. We conclude that SLC26A7 participates in Cl⁻ transport in Reissner’s membrane epithelial cells, but that either other anion pathways, such as ClC-2, possibly substitute satisfactorily under the conditions tested or that Cl⁻ conductance in these cells is not critical to cochlear function. The involvement of SLC26A7 in cellular pH regulation in other epithelial cells leaves open the possibility that SLC26A7 is needed in Reissner’s membrane cells during local perturbations of pH.

Developmental changes of ENaC expression and function in the inner ear of pendrin knock-out mice as a perspective on the development of endolymphatic hydrops

Pendrin mutations cause enlarged vestibular aqueducts and various degrees of sensorineural hearing loss. The selective abolition of pendrin causes dilation of the membranous labyrinth known as endolymphatic hydrops, loss of the endocochlear potential, and consequently loss of hearing function. Because Na+ transport is one of the most important driving forces for fluid transport, the epithelial Na+ channel (ENaC) is believed to play an important role in fluid volume regulation in the inner ear. Therefore, the dysfunction of Na+ transport through ENaC by the acidification of endolymph in Pendred syndrome is one of the potential causes of endolymphatic hydrops. We investigated the changes of ENaC expression and function during the development of the pendrin knock-out mouse. In the cochlea, the expression of β and γENaC was significantly increased at P56 in Pds-/- mice compared with Pds+/+ mice. In the vestibule, the expression of βENaC was significantly increased at P56, and γENaC expression significantly increased from P6 to P56 in Pds-/- mice. The ENaC-dependent trans-epithelial current was not significantly different between Pds+/+ and Pds-/- mice in Reissner's membrane or the saccular extramacular roof epithelium at P0, but the current was significantly increased in Pds-/- mice at P56 compared with Pds+/+ mice. These findings indicate that the expression and function of ENaC were enhanced in Pds-/- mice after the development of endolymphatic hydrops as a compensatory mechanism. This result provides insight into the role of Na+ transport in the development and regulation of endolymphatic hydrops due to pendrin mutations.

Endolymphatic Na⁺ and K⁺ concentrations during cochlear growth and enlargement in mice lacking Slc26a4/pendrin

Slc26a4 (Δ/Δ) mice are deaf, develop an enlarged membranous labyrinth, and thereby largely resemble the human phenotype where mutations of SLC26A4 cause an enlarged vestibular aqueduct and sensorineural hearing loss. The enlargement is likely caused by abnormal ion and fluid transport during the time of embryonic development, however, neither the mechanisms of ion transport nor the ionic composition of the luminal fluid during this time of development are known. Here we determine the ionic composition of inner ear fluids at the time at which the enlargement develops and the onset of expression of selected ion transporters. Concentrations of Na(+) and K(+) were measured with double-barreled ion-selective electrodes in the cochlea and the endolymphatic sac of Slc26a4 (Δ/+), which develop normal hearing, and of Slc26a4 (Δ/Δ) mice, which fail to develop hearing. The expression of specific ion transporters was examined by quantitative RT-PCR and immunohistochemistry. High Na(+) (∼141 mM) and low K(+) concentrations (∼11 mM) were found at embryonic day (E) 16.5 in cochlear endolymph of Slc26a4 (Δ/+) and Slc26a4 (Δ/Δ) mice. Shortly before birth the K(+) concentration began to rise. Immediately after birth (postnatal day 0), the Na(+) and K(+) concentrations in cochlear endolymph were each ∼80 mM. In Slc26a4 (Δ/Δ) mice, the rise in the K(+) concentration occurred with a ∼3 day delay. K(+) concentrations were also found to be low (∼15 mM) in the embryonic endolymphatic sac. The onset of expression of the K(+) channel KCNQ1 and the Na(+)/2Cl(-)/K(+) cotransporter SLC12A2 occurred in the cochlea at E19.5 in Slc26a4 (Δ/+) and Slc26a4 (Δ/Δ) mice. These data demonstrate that endolymph, at the time at which the enlargement develops, is a Na(+)-rich fluid, which transitions into a K(+)-rich fluid before birth. The data suggest that the endolymphatic enlargement caused by a loss of Slc26a4 is a consequence of disrupted Na(+) transport.

Dopamine increases Na+ absorption in the Reissner's membrane of the gerbil cochlea

OBJECTIVE

The purpose of the present study was to investigate the effect of dopamine as a possible regulator of epithelial Na(+) channel (ENaC) in the Reissner's membrane (RM).

METHODS

RM was freshly dissected from the gerbil cochlea, and short-circuit current (Isc) was measured using the voltage-sensitive vibrating probe technique. The dopamine receptor expression was examined using immunohistochemistry.

RESULTS

The results showed that dopamine induced activation of the amiloride-sensitive Isc, but not after pre-treatment with amiloride. The D1-like receptor antagonist SCH-23390, but not the D2-like receptor antagonist sulpiride, decreased the stimulatory effect of dopamine on RM. The effect of dopamine on Na(+) transport via ENaC was still observed after blockade of the Na(+)-K(+)-ATPase by ouabain. D1 receptor immunoreactivity was observed in RM, stria vascularis and spiral ganglion.

CONCLUSION

Na(+) transport in RM is activated by dopamine possibly via D1-like receptors, and intracellular mechanisms other than cAMP-mediated pathway may be involved.

Expression of osmotic stress protein 94 in murine endolymphatic hydrops model

CONCLUSION

The up-regulation of osmotic stress protein 94 (OSP94) in the murine endolymphatic hydrops (EH) models suggests that OSP94 might be involved in cellular adaptation in response to ionic and osmotic stress in the murine inner ear.

OBJECTIVES

The purpose of the present study was to investigate the expression of OSP94 in cochlear tissues of the murine EH models and control animals.

METHODS

Nine adult BALB/c mice were treated with both intratympanic injection of lipopolysaccharide and intraperitoneal administration of aldosterone to induce EH. Nine mice were used as control animals. The expression level of OSP94 in the EH and control groups was compared using immunohistochemistry and real-time RT-PCR.

RESULTS

Immunohistochemical staining of tissues in the EH group showed an up-regulation of OSP94 expression in the cochlea, especially in the stria vascularis and Reissner's membrane. Quantitative real-time PCR analysis also showed that transcription of the OSP94 gene in the cochlea was significantly up-regulated in the EH group.

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

CONCLUSION

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

OBJECTIVES

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

METHODS

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

RESULTS

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

Ion homeostasis in the ear: mechanisms, maladies, and management

PURPOSE OF REVIEW

To describe ion and water homeostatic mechanisms in the inner ear, how they are compromised in hearing disorders, and what treatments are employed to restore auditory function.

RECENT FINDINGS

The ion and water transport functions in the inner ear help maintain the proper endolymph K concentration required for hair cell function. Gene defects and idiopathic alterations in these transport functions cause hearing loss, but often the underlying cause is unknown. Current therapies largely involve glucocorticoid treatment, although the mechanisms of restoration are often undeterminable. Recent studies of these ion homeostatic functions in the ear are characterizing their cellular and molecular control. It is anticipated that future management of these hearing disorders will be more targeted to the cellular processes involved and improve the likelihood of hearing recovery.

SUMMARY

A better understanding of the ion homeostatic processes in the ear will permit more effective management of their associated hearing disorders. Sufficient insight into many homeostatic hearing disorders has now been attained to usher in a new era of better therapies and improved clinical outcomes.

P2Y4-mediated regulation of Na+ absorption in the Reissner's membrane of the cochlea

The epithelial cells of Reissner's membrane (RM) are capable of transporting Na(+) out of endolymph via epithelial Na(+) channel (ENaC). However, much remains to be known as to mechanism of regulation of Na(+) absorption in RM. We investigated P2Y signaling as a possible regulatory mechanism of ENaC in gerbil RM using voltage-sensitive vibrating probe technique and immunohistochemistry. Results showed that UTP induced partial inhibition of the amiloride-sensitive short-circuit current but did not change short-circuit current when applied in the presence of amiloride. The inhibitory effect of UTP was not completely reversible in minutes. The response to UTP was inhibited by reactive blue-2 and 2',3'-O-(4-benzoylbenzoyl)adenosine 5'-triphosphate but not by suramin or pyridoxalphosphate-6-azophenyl-2', 4'-disulfonic acid, which indicates this P2Y receptor as the P2Y(4) subtype. The phospholipase C (PLC) inhibitors 1-[6[[(17beta)-3-methoxyestra-1,3,5(10)-trien-17-yl]amino]hexyl]-1H-pyrrole-2,5-dione and 1-O-octadecyl-2-O-methyl-rac-glycero-3-phosphocholine markedly inhibited the effect of UTP on ENaC. In contrast, neither modulation of protein kinase C nor application of 2-aminoehoxydiphenyl borate affected P2Y(4)-mediated inhibition of ENaC. Immunoreactive staining for P2Y(4) was observed in the RM, apical membrane of stria vascularis, spiral ligament, and organ of Corti, including outer hair cell, inner hair cell, outer pillar cell, Deiters' cell, and Hensen cell. These results suggest that the physiological role of P2Y(4) receptor in RM is likely to regulate Na(+) homeostasis in the endolymph. The acute inhibition of ENaC activity by activation of P2Y(4) receptor is possibly mediated by decrease of phosphatidylinositol 4,5-biphosphate in the plasma membrane through PLC activation.