Inward-rectifier chloride currents in Reissner’s membrane epithelial cells

Abstract

Sensory transduction in the cochlea depends on regulated ion secretion and absorption. Results of whole-organ experiments suggested that Reissner’s membrane may play a role in the control of luminal Cl−. We tested for the presence of Cl− transport pathways in isolated mouse Reissner’s membrane using whole-cell patch clamp recording and gene transcript analyses using RT-PCR. The current-voltage (I-V) relationship in the presence of symmetrical NMDG-Cl was strongly inward-rectifying at negative voltages, with a small outward current at positive voltages. The inward-rectifying component of the I-V curve had several properties similar to those of the ClC-2 Cl− channel. It was stimulated by extracellular acidity and inhibited by extracellular Cd2+, Zn2+, and intracellular ClC-2 antibody. Channel transcripts expressed include ClC-2, Slc26a7 and ClC-Ka, but not Cftr, ClC-1, ClCa1, ClCa2, ClCa3, ClCa4, Slc26a9, ClC-Kb, Best1, Best2, Best3 or the beta-subunit of ClC-K, barttin. ClC-2 is the only molecularly-identified channel present that is a strong inward rectifier. This study is the first report of conductive Cl− transport in epithelial cells of Reissner’s membrane and is consistent with an important role in endolymph anion homeostasis.

Regulation of sodium transport in the inner ear

“…The voltage across the apical membrane is likely greater than -100 mV (> 80 mV from the endocochlear potential and > −20 mV from the basolateral membrane voltage of the epithelial cells, both referenced to the potential of perilymph). The membrane voltage of the cells is likely controlled primarily by a large Cl − conductance (Kim and Marcus, 2010), which usually results in a membrane voltage less than that observed in cells that are dominated by K + conductance. The electrical equivalent of the Na + concentration is likely about 60 mV, outward directed (cytoplasmic [ Na + ] ~10 mM, endolymphatic [Na + ] ~1 mM).…”

Slc26a7 Chloride Channel Activity and Localization in Mouse Reissner’s Membrane Epithelium

“…Composite Cl − currents previously observed in Reissner’s membrane epithelial cells had characteristics consistent with mediation by Slc26a7 and ClC-2 anion channels [7]. We sought additional evidence here for functional Slc26a7 channels in Reissner’s membrane epithelial cells.…”

“…The basolateral membrane voltage in vivo has not been reported, but one can surmise that it is not highly polarized. Whole-cell patch currents are largest for Na + somewhat smaller for Cl − and comparatively very small for K + [7], [20]. The contribution of Na + conductance to the membrane voltage is likely very little under normal in vivo conditions since it is located on the apical membrane where it is in contact on both sides of the membrane with very low [Na + ] (1 mM in the lumen and presumably about 10 mM in the cytosol).…”

“…We recently reported that Cl − currents in Reissner’s membrane epithelial cells had composite characteristics consistent with the functional expression of both ClC-2 and Slc26a7 [7]. The highly-limited expression pattern of Slc26a7 and of two other Slc26 transporters in the ear led to the hypothesis that Slc26a7 may be a gene essential for hearing.…”

“…Analysis of gene array data [2] shows a 25-fold higher expression of Slc26a7 in Reissner’s membrane compared to the neighboring tissue, stria vascularis (Table S1 and Figure S1 in reference [7] ) [8] , or to kidney [9] . Expression in Reissner’s membrane is also greater than expression in thyroid, which is only 5-fold higher than in stria vascularis or kidney [10] .…”