Deafness and imbalance associated with inactivation of the secretory Na-K-2Cl co-transporter

Abstract

Deafness can result from a variety of gene defects. Some genes involved in the physiology of hearing encode membrane transporters that regulate the ionic composition of the fluid bathing the inner ear. The endolymph is an extracellular fluid with an atypical composition that resembles the intracellular milieu, high in K+ and low in Na+. Recent studies have emphasized the prominent role of K+ channels in endolymph secretion and mechanical transduction. Coupled electroneutral transport of Na+, K+ and Cl- is mediated by two isoforms of the Na-K-2Cl co-transporter: the absorptive isoform BSC1 (also called NKCC2, encoded by Slc12a1 in mouse) that is exclusively expressed in kidney; and BSC2/NKCC1 (encoded by Slc12a2 in mouse), the secretory isoform which has a wider pattern of expression including epithelia, muscle cells, neurons and red blood cells. These co-transporters share 57% homology at the amino acid level and are pharmacologically inhibited by loop diuretics. There is functional and histochemical evidence for the presence of the secretory isoform of the Na-K-2Cl co-transporter in gerbil, rat and rabbit inner ear. We disrupted mouse Slc12a2 and report here that Slc12a2-/- mice are deaf and exhibit classic shaker/waltzer behaviour, indicative of inner-ear defects. We localized the co-transporter to key secreting epithelia of the mouse inner ear and show that absence of functional co-transporter leads to structural damages in the inner ear consistent with a decrease in endolymph secretion.

Differential Expression of Genes within the Cochlea as Defined by a Custom Mouse Inner Ear Microarray

“…Both carbonic anhydrase genes (Car2 and Car14) and Na + , K + ATPase (Atp1b3) are all expressed in the lateral wall spiral ligament fibrocytes and are thought to play a role in cochlear fluid and ion homeostasis (Ichimiya et al 1994;Spicer et al 1997). K + homeostasis function of the marginal cells is mediated through the action of membrane proteins, such as Na + , K + ATPases, protein phosphatase inhibitors, and Na + , K + , Cl co-transporters (Zhao et al 1994;Agrup et al 1997;Spicer et al 1997;Delpire et al 1999;Dixon et al 1999;Wangemann 2002). In the LW, we found a differentially expressed subunit of the protein phosphatase 1 regulatory (inhibitor) protein, Ppp1r1b.…”

Immunocytochemical Traits of Type IV Fibrocytes and Their Possible Relations to Cochlear Function and Pathology

“…2C, D). Type II fibrocytes being positive for NKCC1 and Na + , K + -ATPase have previously been reported (Schulte and Adams 1989;Crouch et al 1997;Delpire et al 1999). The NKCC1 protein is well recognized for its role in cell volume regulation in response to osmotic stress (e.g., Hoffmann and Dunham 1995).…”

Molecular Genetics of Vestibular Organ Development

“…All three mutant lines are deaf, with waltzer/shaker behavior indicative of vestibular deficits. In addition, their membranous labyrinths are collapsed, indicating a problem with endolymph secretion (Delpire et al 1999;Dixon et al 1999).…”

“…This protein is expressed in the basolateral membrane of the marginal cells in the stria vascularis, fibrocytes in the spiral ligament, and dark cells of the vestibule (Crouch et al 1997;Goto et al 1997;Mizuta et al 1997). Three mouse models are available for Slc12a2: a targeted deletion mutant; a radiation-induced mutant (Shaker-with-syndactylism (sy)) with a deletion that includes the Slc12a2 locus; and an allele of sy, sy ns (Shaker with no syndactylism), that has a frame-shift mutation in Slc12a2 (Delpire et al 1999;Dixon et al 1999). All three mutant lines are deaf, with waltzer/shaker behavior indicative of vestibular deficits.…”