Endolymphatic Na+ and K+ Concentrations during Cochlear Growth and Enlargement in Mice Lacking Slc26a4/pendrin

Abstract

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.

Epithelial Sodium Transport and Its Control by Aldosterone: The Story of Our Internal Environment Revisited

“…The CSF is thus a "secondary" internal milieu allowing the development of all remarkable cognitive performances associated with the human brain justifying Homer Smith's remarks. It is interesting to note that an even more restricted extracellular compartment, i.e., the endolymph of the cochlea (a few hundred microliters) with a unique ionic composition (high potassium, very low sodium) that is of critical importance for the mechanoelectric transduction that takes place in hair cells (195,196). The development of audition required the evolution of a specific extracellular ionic composition just opposite to what is required in the brain.…”

“…The development of audition required the evolution of a specific extracellular ionic composition just opposite to what is required in the brain. In the mouse, the switch between a high sodium/low potassium to a low sodium/high potassium takes place during embryonic development (E16,5 in mouse) and is linked to the expression of pendrin (196). Interestingly, three components of the aldosterone signaling cascade (MR, ENaC, and Na ϩ -K ϩ -ATPase) are expressed in both the choroid plexus and in the cochlea, but their physiological roles are not yet fully understood.…”

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

“…This may imply that fluid accumulation occurs from E14.5 and that there is a failure in the regulation of the endolymphatic Na + concentration in the endolymphatic sac during that period. It was reported that the expression levels of βENaC rose between P4 and P8 in the cochlea of Pds +/− mice and γENaC expression rose between E17.5 and P0 in the cochlea of Pds +/− mice, which is earlier than Pds −/− mice [11]. And the Na + concentration in the cochlea was similar at E16.5 between Pds +/− and Pds −/− mice but was higher in the Pds −/− than Pds +/− mice at P0 [11].…”

“…It was reported that the expression levels of βENaC rose between P4 and P8 in the cochlea of Pds +/− mice and γENaC expression rose between E17.5 and P0 in the cochlea of Pds +/− mice, which is earlier than Pds −/− mice [11]. And the Na + concentration in the cochlea was similar at E16.5 between Pds +/− and Pds −/− mice but was higher in the Pds −/− than Pds +/− mice at P0 [11]. This result suggests that a Na + absorption failure in the endolymphatic space occurs that may be caused by the decreased activity of ENaC in the endolymphatic sac by the acidification of endolymph.…”

Synopsis of the 48thAnnual Meeting of the Lake Cumberland Biological Transport Group and the Second Biannual Meeting of the Pendrin Consortium

“…In the last ten years, the work of Wangemann and colleagues illuminated the role of pendrin in the physiology of development of the inner ear [24,25,26,27,28,29,30,31,32,33,34]. Mouse models allowed tremendous advances in this field [35].…”