Membrane potential in isolated epithelial cells of the endolymphatic sac in the guinea-pig

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.

Presence of FXYD6 in the endolymphatic sac epithelia

A homeostasis of the electrochemical properties and volume of the endolymph in the inner ear is essential for hearing and equilibrium sensing and is maintained by ion-transport across an epithelial tissue, the endolymphatic sac. One of the key proteins in the maintenance is Na(+), K(+)-ATPase. Although we previously found that the Na(+), K(+)-ATPase in the sac plays a pivotal role in the control of the endolymphatic volume, the mechanism remains unclear. Therefore, in this study, we examined the expression of FXYD6, a functional modulator of the Na(+), K(+)-ATPase, in the epithelial cells of the endolymphatic sac using various approaches. Laser capture microdissection RT-PCR was used to identify FXYD6 mRNA in the endolymphatic sac. Immunolabeling with the specific antibody showed that FXYD6 was predominantly expressed in the intermediate portion of the endolymphatic sac, and it was colocalized with the Na(+), K(+)-ATPase. Because the Na(+), K(+)-ATPase in this region is known to exhibit a high level of activity, an interaction of FXYD6 with this transporter may be critically involved in the regulation of the characteristics of the endolymph.

The mRNA of claudins is expressed in the endolymphatic sac epithelia

OBJECTIVE

Claudins are a family of membrane proteins which localize to tight junctions (TJs). Recent studies have shown that claudins can form pores for ions in the TJs and regulate the permeability of epithelial paracellular ion transport. The endolymphatic sac (ES) is a part of the inner ear, absorbing the endolymphatic fluid. ES dysfunction may result in endolymphatic hydrops. In this study, we focused on the paracellular transport and examined claudin mRNA expression in the ES epithelia.

MATERIALS AND METHODS

Total RNA was isolated from whole ES epithelia of rats by laser capture microdissection. RT-PCR was used to evaluate the expression of claudins. The expression of each claudin mRNA in the epithelial cells of rat ES was confirmed by in situ hybridization.

RESULTS

RT-PCR indicated the expression of cldn2, cldn4, cldn6, cldn7, cldn9, cldn11, cldn12, and cldn14. The expression of these claudin mRNAs in the epithelial cells of rat ES was confirmed by in situ hybridization.

CONCLUSION

We demonstrated mRNA expression of multiple claudins in the rat ES epithelia. These results in the ES epithelia were consistent with a role of claudins in paracellular ion transport.