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

Deaf ness 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(2-4) and mechanical transduction(5). Coupled electroneutral transport of Na+, K+ and Cl- is: mediated by two isoforms of the. Na-K-2Cl cotransporter:the absorptive isoform BSC1 (also called NKCC2, encoded by SIc12a1 in mouse) that is exclusively expressed in kidney;- and BSC2/NKCC1 (encoded by SIc12a2 in mouse).: the secretory isoform which has a wider pattern of expression including epithelia, muscle cells; neurons, and red blood cells(6,7). These Eo-transporters: share 57% homology at the amino acid level(8-11) and are pharmacologically inhibited by loop diuretics. There is functional(12-14) and histochemical(15-17) 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 SIc12a2 and report here that SIc12a2(-/-) 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.