Regulation of ENaC-mediated sodium transport by glucocorticoids in Reissner's membrane epithelium

Abstract

Kim SH, Kim KX, Raveendran NN, Wu T, Pondugula SR, Marcus DC. Regulation of ENaC-mediated sodium transport by glucocorticoids in Reissner's membrane epithelium. 's membrane epithelium forms much of the barrier that produces and sustains the large ionic differences between cochlear endolymph and perilymph. We have reported that Reissner's membrane contributes to normal cochlear function by absorbing Na ϩ from endolymph via amiloridesensitive channels in gerbil inner ear. We used mouse Reissner's membrane to 1) identify candidate genes involved in the Na ϩ transport pathway, 2) determine whether their level of expression was regulated by the synthetic glucocorticoid dexamethasone, and 3) obtain functional evidence for the physiological importance of these genes. Transcripts were present for ␣-, ␤-, and ␥-subunits of epithelial Na ϩ channel (ENaC); corticosteroid receptors GR (glucocorticoid receptor) and MR (mineralocorticoid receptor); GR agonist regulator 11␤-hydroxysteroid dehydrogenase (HSD) type 1 (11␤-HSD1); Na ϩ transport control components SGK1, Nedd4-2, and WNKs; and K ϩ channels and Na ϩ -K ϩ -ATPase. Expression of the MR agonist regulator 11␤-HSD2 was not detected. Dexamethasone upregulated transcripts for ␣-and ␤-subunits of ENaC (ϳ6-and ϳ3-fold), KCNK1 (ϳ3-fold), 11␤-HSD1 (ϳ2-fold), SGK1 (ϳ2-fold), and WNK4 (ϳ3-fold). Transepithelial currents from the apical to the basolateral side of Reissner's membrane were sensitive to amiloride (IC50 ϳ0.7 M) and benzamil (IC50 ϳ0.1 M), but not EIPA (IC50 ϳ34 M); amilorideblocked transepithelial current was not immediately changed by forskolin/IBMX. Currents were reduced by ouabain, lowered bath Na ϩ concentration (from 150 to 120 mM), and K ϩ channel blockers (XE-991, Ba 2ϩ , and acidification from pH 7.4 to 6.5). Dexamethasone-stimulated current and gene expression were reduced by mifepristone, but not spironolactone. These molecular, pharmacological, and functional observations are consistent with Na ϩ absorption by mouse Reissner's membrane, which is mediated by apical ENaC and/or other amiloride-sensitive channels, basolateral Na ϩ -K ϩ -ATPase, and K ϩ -permeable channels and is under the control of glucocorticoids. These results provide an understanding and a molecular definition of an important transport function of Reissner's membrane epithelium in the homeostasis of cochlear endolymph.

Development of the stria vascularis and potassium regulation in the human fetal cochlea: Insights into hereditary sensorineural hearing loss

“…It did, however, show identical reactivity in the developing marginal cells at W16 and W18 (data not shown). Although we cannot explain this difference as both antibodies should recognize the same KCNQ1 transcript, expression of KCNQ1 in Reissner's membrane has been reported previously (Kim et al, 2009). …”

Distribution of glucocorticoid receptors and 11 beta-hydroxysteroid dehydrogenase isoforms in the rat inner ear

“…The supporting cells are known to be related to hair cell regeneration (Löwenheim H, et al, 1999) and ion regulation (Mistrik and Ashmore, 2009 (Kim et al, 2009). GRs possibly participate in the regulation of cell cycle, gene, ion, and fluid transport in the rat cochlea.…”

Endolymphatic Sodium Homeostasis by Extramacular Epithelium of the Saccule

“…Functional Na ϩ ,K ϩ -ATPase consists of an ␣ and a ␤ subunit, of which there are four ␣ and three ␤ subunits (Kim et al, 2009). Different combinations of isoforms display a range of affinities for their substrates Na ϩ and K ϩ and a wide range of affinities for the specific inhibitor ouabain (Segall et al, 2001;Pierre et al, 2008).…”

“…We sought functional evidence of electrogenic transepithelial ion transport by extramacular epithelium of adult mouse saccule and examined whether it is controlled by glucocorticoids via the glucocorticoid receptor (GR) and/or the mineralocorticoid receptor (MR) by electrophysiologic and pharmacologic means. The experimental strategy used in the current study of transepithelial transport by the saccule is based on that used for the study of Na ϩ transport across many epithelia, including others in the inner ear (Pondugula et al, 2004;Pondugula et al, 2006;Kim et al, 2009). …”