Expression of α and β subunit isoforms of Na,K-ATPase in the mouse inner ear and changes with mutations at the W v or Sl d loci

Abstract

Mice homozygous for mutations at the viable dominant spotting (W v) and Steel-dickie (Sl d) loci exhibit a similar phenotype which includes deafness. The auditory dysfunction derives from failure of the stria vascularis to develop normally and to generate a high positive endocochlear potential (EP). Because strial function is driven by Na,K-ATPase its expression was investigated in inner ears of W v W v and Sl d Sl d mice and their wild-type littermates by immunostaining with antisera against four of the enzyme's subunit isoforms. Wild-type mice from two different genetic backgrounds showed an identical distribution of subunit isoforms among inner ear transport cells. Several epithelial cell types coexpressed the α1 and β1 subunits. Vestibular dark cells showed no reactivity for β1 but expressed abundant β2, whereas, strial marginal cells stained strongly for both β isoforms. The only qualitative difference between mutant and wild-type mice was the absence of β1 subunit in marginal cells of the mutant's stria. However, it is unlikely that this difference accounts for failure of mutants to generate a high EP because the β1 subunit is not present in the stria vascularis of either rats or gerbils with normal EP values. Strong immunostaining for Na,K-ATPase in lateral wall fibrocytes of normal mice along with diminished immunoreactivity in the mutants supports the concept that these strategically located transport fibrocytes actively resorb K + leaked across Reissner's membrane into scala vestibuli or effluxed from hair cells and nerves into scala tympani. It is further speculated that the resorbed K + normally is siphoned down its concentration gradient into the intrastrial space through gap junctions between fibrocytes and strial basal and intermediate cells where it is recycled back to endolymph via marginal cells. Thus, failure of mutants to generate a positive EP could be explained by the absence of intermediate cells which may form the final link in the conduit for moving K + from perilymph to the intrastrial compartment.