Expression of IsK protein mRNA in cultured rat strial marginal cells

Endothelin-1 mediated induction of extracellular matrix genes in strial marginal cells underlies strial pathology in Alport mice

Alport syndrome, a type IV collagen disorder, manifests as glomerular disease associated with hearing loss with thickening of the glomerular and strial capillary basement membranes (SCBMs). We have identified a role for endothelin-1 (ET-1) activation of endothelin A receptors (ETARs) in glomerular pathogenesis. Here we explore whether ET-1 plays a role in strial pathology. Wild type (WT) and Alport mice were treated with the ETAR antagonist, sitaxentan. The stria vascularis was analyzed for SCBM thickness and for extracellular matrix (ECM) proteins. Additional WT and Alport mice were exposed to noise or hypoxia and the stria analyzed for hypoxia-related and ECM genes. A strial marginal cell line cultured under hypoxic conditions, or stimulated with ET-1 was analyzed for expression of hypoxia-related and ECM transcripts. Noise exposure resulted in significantly elevated ABR thresholds in Alport mice relative to wild type littermates. Alport stria showed elevated expression of collagen α1(IV), laminin α2, and laminin α5 proteins relative to WT. SCBM thickening and elevated ECM protein expression was ameliorated by ETAR blockade. Stria from normoxic Alport mice and hypoxic WT mice showed upregulation of hypoxia-related, ECM, and ET-1 transcripts. Both ET-1 stimulation and hypoxia up-regulated ECM transcripts in cultured marginal cells. We conclude that ET-1 mediated activation of ETARs on strial marginal cells results in elevated expression of ECM genes and thickening of the SCBMs in Alport mice. SCBM thickening results in hypoxic stress further elevating ECM and ET-1 gene expression, exacerbating strial pathology.

Immunohistochemical localization of G protein betagamma subunits in the lateral wall of the rat cochlea

The role of G protein-mediated signal transduction in the production of endolymph, an extracellular fluid of unusual ionic composition, is beginning to be understood. The identity of Galpha subunits in the stria vascularis and the spiral ligament of the lateral wall of the cochlear duct is well established. However, little is known about the presence of betagamma subunits. This study used immunohistochemistry to investigate the distribution of G protein betagamma subunits in the lateral wall of the cochlea. Temporal bones of 6- to 8-week-old rats were fixed in 4% paraformaldehyde and 0.1% glutaraldehyde and processed for embedding in paraffin wax. The dewaxed, midmodiolar sections of the cochlea were incubated with subunit-specific polyclonal antibodies. The results show that the pattern of immunoreactivity varies for the G protein beta1-4 and gamma1-3, 5 and 7 subunits in the stria vascularis and spiral ligament. In the stria vascularis, immunoreactivity was detected for beta2, beta3, beta4, gamma1, gamma2 and gamma7 subunits. All five types of fibrocytes in the spiral ligament exhibited positive staining for gamma2 and gamma7. However, immunoreactivity for beta1-4 subunits was variable. Immunoreactivity for gamma3 and gamma5 subunits was not detected in the lateral cochlear wall. The expression pattern of G protein betagamma subunits in lateral wall provides a basis for interpreting the functions of G protein-coupled receptors in cochlear fluid homeostasis.

Influence of gain of function epithelial chloride channel ClC-Kb mutation on hearing thresholds

Hearing depends on functional ClC-K-type chloride channels composed of barttin with ClC-Ka or ClC-Kb. Loss-of-function mutations of the barttin gene BSND or of both, the ClC-Ka gene CLNKA and the ClC-Kb gene CLNKB lead to congenital deafness and renal salt wasting. Recently, we identified the gain-of-function mutation ClC-Kb(T481S) which is associated with increased blood pressure. To explore the impact of ClC-Kb(T481S) on hearing, healthy volunteers (n=329) and individuals suffering from tinnitus (n=246) volunteered for hearing tests (n=348) and genetic analysis (n=575). 19.1% of the individuals were heterozygote (ClC-Kb(T481S)/ClC-Kb) and 1.7% homozygote carriers. Pure tone average hearing threshold (PTAt) for air conduction was significantly (p<0.033) lower in ClC-Kb(T481S) carriers (13.2+/-1.2dB) than in wild-type individuals (17.1+/-0.9dB). The prevalence of ClC-Kb(T481S) carriers was significantly increased (29.7%) in individuals with PTAt<15dB (p<0.05) and significantly decreased (13.2%) in individuals with PTAt>30 dB (p<0.017). The difference was largely due to the female population. Bone conduction was less affected pointing to an effect of the mutation on middle ear function. Tinnitus tended to be more frequent in ClC-Kb(T481S) carriers, a difference, however, not statistically significant. In conclusion, hearing thresholds are slightly lower in carriers of ClC-Kb(T481S), i.e., the gain-of-function polymorphism ClC-Kb(T481S) exerts a subtle but significant protective effect against hearing loss.

Functional IsK/KvLQT1 potassium channel in a new corticosteroid-sensitive cell line derived from the inner ear

Endolymph, a high K(+)/low Na(+) fluid, participates in mechanoelectrical transduction in inner ear. Molecular mechanisms controlling endolymph ion homeostasis remain elusive, hampered by the lack of appropriate cellular models. We established an inner ear cell line by targeted oncogenesis. The expression of SV40 T antigen was driven by the proximal promoter of the human mineralocorticoid receptor (MR) gene, a receptor expressed in the inner ear. The EC5v cell line, microdissected from the semicircular canal, grew as a monolayer of immortalized epithelial cells forming domes. EC5v cells exhibited on filters of high transepithelial resistance and promoted K(+) secretion and Na(+) absorption. Functional MR and the 11beta-hydroxysteroid dehydrogenase type 2, a key enzyme responsible for MR selectivity were identified. Expression of the epithelial sodium channel and serum glucocorticoid-regulated kinase 1 was shown to be up-regulated by aldosterone, indicating that EC5v represents a novel corticosteroid-sensitive cell line. Ionic measurements and (86)Rb transport assays revealed an apical secretion of K(+) at least in part through the I(sK)/KvLQT1 potassium channel under standard culture conditions. However, when cells were exposed to high apically K(+)/low Na(+) fluid, mimicking endolymph exposure, I(sK)/KvLQT1 actually functioned as a strict apical to basolateral K(+) channel inhibited by clofilium. Quantitative reverse transcriptase-PCR further demonstrated that expression of KvLQT1 but not of I(sK) was down-regulated by high K(+) concentration. This first vestibular cellular model thus constitutes a valuable system to further investigate the molecular mechanisms controlling ionic transports in the inner ear and the pathophysiological consequences of their dysfunctions in vertigo and hearing loss.