SLC26A4 mutations can cause a distinctive hearing loss phenotype with sudden drops and fluctuation in patients. Existing Slc26a4 mutant mouse lines have a profound loss of hearing and vestibular function, with severe inner ear malformations that do not model this human phenotype. In this study, we generated Slc26a4-insufficient mice by manipulation of doxycycline administration to a transgenic mouse line in which all Slc26a4 expression was under the control of doxycycline. Doxycycline was administered from conception to embryonic day 17.5, and then discontinued. Auditory brainstem response thresholds showed significant fluctuation of hearing loss from 1 through 3 months of age. The endocochlear potential, which is required for inner ear sensory cell function, correlated with auditory brainstem response thresholds. We observed degeneration of stria vascularis intermediate cells, the cells that generate the endocochlear potential, but no other abnormalities within the cochlea. We conclude that fluctuations of hearing result from fluctuations of the endocochlear potential and stria vascularis dysfunction in Slc26a4-insufficient mouse ears. This model can now be used to test potential interventions to reduce or prevent sudden hearing loss or fluctuation in human patients. Our strategy to generate a hypomorphic mouse model utilizing the tet-on system will be applicable to other diseases in which a hypomorphic allele is needed to model the human phenotype.
Conditional; doxycycline; endocochlear potential; fluctuation; hearing loss; hypomorph; inducible; SLC26A4; stria vascularis; transgenic; vestibular aqueduct
Pendred syndrome (PS) is characterized by autosomal recessive inheritance of goiter associated with a defect of iodide organification, hearing loss, enlargement of the vestibular aqueduct (EVA), and mutations of the SLC26A4 gene. However, not all EVA patients have PS or SLC26A4 mutations. Two mutant alleles of SLC26A4 are detected in ¼ of North American or European EVA populations, one mutant allele is detected in another ¼ of patient populations, and no mutations are detected in the other ½. The presence of two mutant alleles of SLC26A4 is associated with abnormal iodide organification, increased thyroid gland volume, increased severity of hearing loss, and bilateral EVA. The presence of a single mutant allele of SLC26A4 is associated with normal iodide organification, normal thyroid gland volume, less severe hearing loss and either bilateral or unilateral EVA. When other underlying correlations are accounted for, the presence of a cochlear malformation or the size of EVA does not have an effect on hearing thresholds. This is consistent with observations of an Slc26a4 mutant mouse model of EVA in which hearing loss is independent of endolymphatic hydrops or inner ear malformations. Segregation analyses of EVA in families suggest that the patients carrying one mutant allele of SLC26A4 have a second, undetected mutant allele of SLC26A4, and the probability of a sibling having EVA is consistent with its segregation as an autosomal recessive trait. Patients without any mutations are an etiologically heterogeneous group in which siblings have a lower probability of having EVA. SLC26A4 mutation testing can provide prognostic information to guide clinical surveillance and management, as well as the probability of EVA affecting a sibling.
SLC26A4; Pendred syndrome; Genetic testing; Goiter; Hearing loss; Vestibular aqueduct; Genotype-phenotype correlation
The human gene SLC26A4 and the mouse ortholog Slc26a4 code for the protein pendrin, which is an anion exchanger expressed in apical membranes of selected epithelia. In the inner ear, pendrin is expressed in the cochlea, the vestibular labyrinth and the endolymphatic sac. Loss-of-function and hypo-functional mutations cause an enlargement of the vestibular aqueduct (EVA) and sensorineural hearing loss. The relatively high prevalence of SLC26A4 mutations provides a strong imperative to develop rational interventions that delay, ameliorate or prevent pendrin-associated loss of cochlear and vestibular function. This review summarizes recent studies in mouse models that have been developed to delineate the role of pendrin in the physiology of hearing and balance and that have brought forward the concept that a temporally and spatially limited therapy may be sufficient to secure a life-time of normal hearing in children bearing mutations of SLC26A4.
Slc26a4; Enlarged vestibular aqueduct; Hearing; Cochlea; Endolymphatic sac; Genetic disease model
Sensory transduction in the cochlea and vestibular labyrinth depends on fluid movements that deflect the hair bundles of mechanosensitive hair cells. Mechanosensitive transducer channels at the tip of the hair cell stereocilia allow K+ to flow into cells. This unusual process relies on ionic gradients unique to the inner ear. Linking genes to deafness in humans and mice has been instrumental in identifying the ion transport machinery important for hearing and balance. Morphological analysis is difficult in patients, but mouse models have helped to investigate phenotypes at different developmental time points. This review focuses on cellular ion transport mechanisms in the stria vascularis that generate the major electrochemical gradients for sensory transduction.
Pendred syndrome is due to loss-of-function mutations of Slc26a4, which codes for the HCO3− transporter pendrin. Loss of pendrin causes deafness via a loss of the K+ channel Kcnj10 in stria vascularis and consequent loss of the endocochlear potential. Pendrin and Kcnj10 are expressed in different cell types. Here, we report that free radical stress provides a link between the loss of Kcnj10 and the loss of pendrin. Studies were performed using native and cultured stria vascularis from Slc26a4+/− and Slc26a4−/− mice as well as Chinese hamster ovary (CHO)-K1 cells. Kcnj10, oxidized proteins, and proteins involved in iron metabolism were quantified by Western blotting. Nitrated proteins were quantified by ELISA. Total iron was measured by ferrozine spectrophotometry and gene expression was quantified by qRT-PCR. At postnatal day 10 (P10), stria vascularis from Slc26a4+/− and Slc26a4−/− mice expressed similar amounts of Kcnj10. Slc26a4−/− mice lost Kcnj10 expression during the next 5 days of development. In contrast, stria vascularis, obtained from P10 Slc26a4−/− mice and kept in culture for 5 days, maintained Kcnj10 expression. Stria vascularis from Slc26a4−/− mice was found to suffer from free radical stress evident by elevated amounts of oxidized and nitrated proteins and other changes in protein and gene expression. Free radical stress induced by 3-morpholinosydnonimine-N-ethylcarbamide was found to be sufficient to reduce Kcnj10 expression in CHO-K1 cells. These data demonstrate that free radical stress provides a link between loss of pendrin and loss of Kcnj10 in Slc26a4−/− mice and possibly in human patients suffering from Pendred syndrome.
cochlea; pendrin; oxidation; nitration
Mutations in the gene encoding the gap junction protein connexin26 (Cx26) are responsible for the autosomal recessive isolated deafness, DFNB1, which accounts for half of the cases of prelingual profound hereditary deafness in Caucasian populations [1–5]. To date, in vivo approaches to decipher the role of Cx26 in the inner ear have been hampered by the embryonic lethality of the Cx26 knockout mice . To overcome this difficulty, we performed targeted ablation of Cx26 specifically in one of the two cellular networks that it underlies in the inner ear, namely, the epithelial network. We show that homozygous mutant mice, Cx26OtogCre, have hearing impairment, but no vestibular dysfunction. The inner ear developed normally. However, on postnatal day 14 (P14), i.e., soon after the onset of hearing, cell death appeared and eventually extended to the cochlear epithelial network and sensory hair cells. Cell death initially affected only the supporting cells of the genuine sensory cell (inner hair cell, IHC), thus suggesting that it could be triggered by the IHC response to sound stimulation. Altogether, our results demonstrate that the Cx26-containing epithelial gap junction network is essential for cochlear function and cell survival. We conclude that prevention of cell death in the sensory epithelium is essential for any attempt to restore the auditory function in DFNB1 patients.
The spiral modiolar artery supplies blood and essential nutrients to the cochlea. Our previous functional study indicates the α1A–adrenergic receptor subtype mediates vasoconstriction of the gerbil spiral modiolar artery. Although the gerbil cochlea is often used as a model in hearing research, the molecular and pharmacological characteristics of the cloned gerbil α1a-adrenergic receptor have not been determined. Thus we cloned, expressed and characterized the gerbil α1a-adrenergic receptor and then compared its molecular and pharmacological properties to those of other mammalian α1a-adrenergic receptors. The cDNA clone contained 1404 nucleotides, which encoded a 467 amino acid peptide with a deduced sequence having 96.8, 96.4 and 91.6% identity to rat, mouse and human α1a-receptors, respectively. We transiently transfected the α1a-adrenergic receptor into COS-1 cells and determined its pharmacological characteristics by [3H]prazosin binding. Unlabeled prazosin had a Ki of 0.89 ± 0.1 nM. The α1A-adrenergic receptor-selective antagonists, 5-methylurapidil and WB-4101, bound with high affinity and had Ki values of 4.9 ± 1 and 1.0 ± 0.1 nM, respectively. BMY-7378, an α1D-adrenergic receptor-selective antagonist, bound with low affinity (260 ± 60 nM). The 91.6% amino acid sequence identity and Kis of the cloned gerbil α1a-adrenergic receptor are similar to those of the human α1a-adrenergic receptor clone. These results show that the gerbil α1a-adrenergic receptor is representative of the human α1a-adrenergic receptor, lending validity to the use of the gerbil spiral modiolar artery as a model in studies of vascular disorders of the cochlea.
Alpha1-adrenergic receptor subtype; Gerbil cochlea; Spiral modiolar artery
Several members of the SLC26 gene family have highly-restricted expression patterns in the auditory and vestibular periphery and mutations in mice of at least two of these (SLC26A4 and SLC26A5) lead to deficits in hearing and/or balance. A previous report pointed to SLC26A7 as a candidate gene important for cochlear function. In the present study, inner ears were assayed by immunostaining for Slc26a7 in neonatal and adult mice. Slc26a7 was detected in the basolateral membrane of Reissner’s membrane epithelial cells but not neighboring cells, with an onset of expression at P5; gene knockout resulted in the absence of protein expression in Reissner’s membrane. Whole-cell patch clamp recordings revealed anion currents and conductances that were elevated for NO3− over Cl− and inhibited by I− and NPPB. Elevated NO3− currents were absent in Slc26a7 knockout mice. There were, however, no major changes to hearing (auditory brainstem response) of knockout mice during early adult life under constitutive and noise exposure conditions. The lack of Slc26a7 protein expression found in the wild-type vestibular labyrinth was consistent with the observation of normal balance. We conclude that SLC26A7 participates in Cl− transport in Reissner’s membrane epithelial cells, but that either other anion pathways, such as ClC-2, possibly substitute satisfactorily under the conditions tested or that Cl− conductance in these cells is not critical to cochlear function. The involvement of SLC26A7 in cellular pH regulation in other epithelial cells leaves open the possibility that SLC26A7 is needed in Reissner’s membrane cells during local perturbations of pH.
Mutations of SLC26A4 are a common cause of human hearing loss associated with enlargement of the vestibular aqueduct. SLC26A4 encodes pendrin, an anion exchanger expressed in a variety of epithelial cells in the cochlea, the vestibular labyrinth and the endolymphatic sac. Slc26a4Δ/Δ mice are devoid of pendrin and develop a severe enlargement of the membranous labyrinth, fail to acquire hearing and balance, and thereby provide a model for the human phenotype. Here, we generated a transgenic mouse line that expresses human SLC26A4 controlled by the promoter of ATP6V1B1. Crossing this transgene into the Slc26a4Δ/Δ line restored protein expression of pendrin in the endolymphatic sac without inducing detectable expression in the cochlea or the vestibular sensory organs. The transgene prevented abnormal enlargement of the membranous labyrinth, restored a normal endocochlear potential, normal pH gradients between endolymph and perilymph in the cochlea, normal otoconia formation in the vestibular labyrinth and normal sensory functions of hearing and balance. Our study demonstrates that restoration of pendrin to the endolymphatic sac is sufficient to restore normal inner ear function. This finding in conjunction with our previous report that pendrin expression is required for embryonic development but not for the maintenance of hearing opens the prospect that a spatially and temporally limited therapy will restore normal hearing in human patients carrying a variety of mutations of SLC26A4.
Mutations of SLC26A4 are the most common cause for hearing loss associated with a swelling of the inner ear. This human disease is largely recapitulated in a mutant mouse model. Mutant mice lack Slc26a4 expression and their inner ears swell during embryonic development, which leads to failure of the cochlea and the vestibular organs resulting in deafness and loss of balance. SLC26A4 is normally found in the cochlea and vestibular organs of the inner ear as well as in the endolymphatic sac, which is a non-sensory part of the inner ear. The multitude of sites where SLC26A4 is located made the goal to restore function through restoration look futile, unless some sites were more important than others. Here, we generated a new mutant mouse that expresses SLC26A4 in the endolymphatic sac but not in the cochlea or the vestibular organs of the inner ear. Fantastically, this mouse did not develop the detrimental swelling of the inner ear and even more exciting, the mouse developed normal hearing and balance. Our study provides the proof-of-concept that a therapy aimed at repairing the endolymphatic sac during embryonic development is sufficient to restore a life-time of normal hearing and balance.
Slc26a4Δ/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 Slc26a4Δ/+, which develop normal hearing, and of Slc26a4Δ/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 Slc26a4Δ/+ and Slc26a4Δ/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 Slc26a4Δ/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 Slc26a4Δ/+ and Slc26a4Δ/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.
Cochlear blood flow regulation is important to prevent hearing loss caused by ischemia and oxidative stress. Cochlear blood supply is provided by the spiral modiolar artery (SMA). The myogenic tone of the SMA is enhanced by the nitric oxide synthase (NOS) blocker L-NG-Nitro-Arginine (LNNA) in males, but not in females. Here, we investigated whether this gender difference is based on differences in the cytosolic Ca2+ concentration and/or the Ca2+ sensitivity of the myofilaments. Vascular diameter, myogenic tone, cytosolic Ca2+, and Ca2+ sensitivity were evaluated in pressurized SMA segments isolated from male and female gerbils using laser-scanning microscopy and microfluorometry. The gender difference of the LNNA-induced tone was compared, in the same vessel segments, to tone induced by 150 mM K+ and endothelin-1, neither of which showed an apparent gender-difference. Interestingly, LNNA-induced tone in male SMAs was observed in protocols that included changes in intramural pressure, but not when the intramural pressure was held constant. LNNA in male SMAs did not increase the global Ca2+ concentration in smooth muscle cells but increased the Ca2+ sensitivity. This increase in the Ca2+ sensitivity was abolished in the presence of the guanylyl cyclase inhibitor ODQ or by extrinsic application of either the nitric oxide (NO)-donor DEA-NONOate or the cGMP analog 8-pCPT-cGMP. The rho-kinase blocker Y27632 decreased the basal Ca2+ sensitivity and abolished the LNNA-induced increase in Ca2+ sensitivity in male SMAs. Neither LNNA nor Y27632 changed the Ca2+ sensitivity in female SMAs. The data suggest that the gender difference in LNNA-induced tone is based on a gender difference in the regulation of rho-kinase mediated Ca2+ sensitivity. Rho-kinase and NO thus emerge as critical factors in the regulation of cochlear blood flow. The larger role of NO-dependent mechanisms in male SMAs predicts greater restrictions on cochlear blood flow under conditions of impaired endothelial cell function.
Ameloblasts need to regulate pH during formation of enamel crystals, a process that generates protons. Solute carrier family 26A member 4 (SLC26A4, or pendrin) is an anion exchanger for chloride, bicarbonate, iodine and formate. It is expressed in apical membranes of ion-transporting epithelia in kidney, inner ear and thyroid where it regulates luminal pH and fluid transport. We hypothesized that maturation ameloblasts express SLC26A4 to neutralize acidification of enamel fluid in forming enamel. In rodents, secretory and maturation ameloblasts were immunopositive for SLC26A4. Staining was particularly strong in apical membranes of maturation ameloblasts facing forming enamel. RT-PCR confirmed the presence of mRNA transcripts for Slc26a4 in enamel organs. SLC26A4 immunostaining was also found in mineralizing connective tissues including odontoblasts, osteoblasts, osteocytes, osteoclasts, bone lining cells, cellular cementoblasts and cementocytes. However, Slc26a4-null mutant mice had no overt dental phenotype. The presence of SLC26A4 in apical plasma membranes of maturation ameloblasts is consistent with a potential function as pH regulator. SLC26A4 does not appear critical for ameloblast functioning and is likely compensated by other pH regulators.
ameloblasts; mineralization; pH regulation; immunolocalization; null mutant; bone cells; odontoblasts
Enlargement of the vestibular aqueduct (EVA) is one of the most common inner ear malformations associated with sensorineural hearing loss in children. The delayed onset and progressive nature of this phenotype offer a window of opportunity to prevent or retard progression of hearing loss. EVA is not the direct cause of hearing loss in these patients, but rather is a radiologic marker for some underlying pathogenetic defect. Mutations of the SLC26A4 gene are a common cause of EVA. Studies of an Slc26a4 knockout mouse demonstrate that enlargement of the scala media is a key event in the pathogenesis of deafness. The enlargement is driven by fluid secretion in the vestibular labyrinth and a failure of fluid absorption in the embryonic endolymphatic sac. Elucidating the mechanism of hearing loss may offer clues to potential therapeutic strategies.
Enlargement of the vestibular aqueduct (EVA) is the most common inner ear anomaly detected in ears of children with sensorineural hearing loss. Pendred syndrome (PS) is an autosomal recessive disorder characterized by bilateral sensorineural hearing loss with EVA and an iodine organification defect that can lead to thyroid goiter. Pendred syndrome is caused by mutations of the SLC26A4 gene. SLC26A4 mutations may also be identified in some patients with nonsyndromic EVA (NSEVA). The presence of two mutant alleles of SLC26A4 is correlated with bilateral EVA and Pendred syndrome, whereas unilateral EVA and NSEVA are correlated with one (M1) or zero (M0) mutant alleles of SLC26A4. Thyroid gland enlargement (goiter) appears to be primarily dependent on the presence of two mutant alleles of SLC26A4 in pediatric patients, but not in older patients. In M1 families, EVA may be associated with a second, undetected SLC26A4 mutation or epigenetic modifications. In M0 families, there is probably etiologic heterogeneity that includes causes other than, or in addition to, monogenic inheritance.
SLC26A4; Pendred syndrome; Hearing; Deafness; Inner ear; Genotype-phenotype correlation
To demonstrate that TNFα, via sphingosine-1-phosphate (S1P) signaling, has the potential to alter cochlear blood flow and thus, cause ischemic hearing loss.
Methods and Results
TNFα induced a pro-constrictive state throughout the cochlear microvasculature, which reduced capillary diameter and cochlear blood flow in vivo. In vitro isolated preparations of the spiral modiolar artery and spiral ligament capillaries confirmed these observations. Antagonizing S1P receptor 2 subtype signaling (1µmol/L JTE013) attenuated the effects of TNFα in all models. TNFα activated Sk1 and induced its translocation to the smooth muscle cell membrane. Expression of a dominant-negative Sk1 mutant (Sk1G82D) eliminated both baseline spiral modiolar artery calcium sensitivity and TNFα effects, while a non-phosphorylatable Sk1 mutant (Sk1S225A) only blocked the effects of TNFα. A small group of etanercept-treated hearing loss patients recovered with a one-phase exponential decay (t½=1.56±0.20 weeks), which matched a kinetic predicted for a vascular origin.
TNFα indeed reduces cochlear blood flow via the activation of vascular S1P signaling. This integrates hearing loss into the family of ischemic microvascular pathologies, with implications for risk stratification, diagnosis and treatment.
Signal transduction; transfection; etanercept; sphingosine kinase 1; cochlear microcirculation
Strial marginal cells (SMC) and vestibular dark cells (VDC) are known to secrete K+ into endolymph. Slowly-activating, voltage-dependent K+ channels (KCNQ1/KCNE1; IsK; min K) have been identified in the apical membrane of these cells. Several experimental maneuvers known to increase or decrease transepithelial K+ secretion have been found in VDC to change the current through these channels in the same ways. In both SMC and VDC the kinetics of activation and deactivation resemble those of the IsK channel exogenously expressed in Xenopus oocytes and endogenous to heart myocytes. The present study sought evidence that this current is indeed carried by IsK channels and that this current is the basis for transepithelial K+ secretion. Both on-cell macro-patch recordings of the apical membrane and perforated-patch whole-cell recordings were made on SMC from gerbil in order to measure macroscopic cell currents. The on-cell current was found to 1) be K+-selective, 2) have a cation permeability sequence of K+ ~ Rb+ > Cs+ >> Li+ = Na+, 3) be activated with a time constant of 1764 ± 413 ms by voltage steps from 0 to +40 mV, 4) be deactivated with a time constant of 324 ± 57 ms by voltage steps from 0 to -40 mV and 5) be reduced 84 ± 5% by bumetanide (10-5 M), an inhibitor of K+ secretion. The single-channel conductance of the apical currents in the homologous VDC was estimated by fluctuation analysis to be 1.6 pS. The potent inhibitor of IsK channels, chromanol 293B (10-5 M), reduced the whole-cell current in SMC by 72 ± 10 %. Clofilium (10-4 M), a putative IsK channel inhibitor known to have additional non-specific effects, led to a stimulation of both on-cell (by 598 ± 177%) and whole-cell (by 162 ± 18%) currents in gerbil SMC but to a decrease of whole-cell currents (by 39 ± 12%) in rat SMC. Taken together with other findings reviewed here, these results strongly argue that the slowly-activating, voltage-dependent conductance in the apical membrane of SMC is the IsK channel and provide additional evidence for the poor specificity of clofilium.
K+ channel; perforated-patch whole-cell voltage clamp; fluctuation analysis; KCNQ1/KCNE1 K channel; gerbil; rat
Vestibular dark cell epithelium was isolated from the semicircular canal of gerbils to test the proposal that the sulfhydryl alkylating agent N-ethylmaleimide (NEM) inhibits K+ secretion by this tissue and does so by reacting with a site in or near the apical membrane. Dark cell epithelium was mounted in a micro-Ussing chamber for measurements of transepithelial voltage (Vt) and resistance (Rt) or in a perfused bath on the stage of a microscope for measurement of cell height as an index of cell volume. Perfusion of the apical or basolateral side with 10−3 M NEM caused an increase in Vt superimposed upon a slower decrease of Vt, resulting in a triphasic response. There were only small changes in Rt. Under this condition, Vt is proportional to short circuit current and to K+ secretion. Both the stimulatory and the inhibitory responses of Vt were dose-dependent between 10−6 and 10−3 M NEM and the inhibition was irreversible. The specificity of the reaction of NEM with sulfhydryl groups was confirmed by the use of the reducing agent dithiothreitol (DTT). Perfusion of 5×10−4 M DTT on the apical side caused no significant changes in Vt but completely prevented both stimulation and inhibition of Vt by NEM (10−3 M). The amplitudes of the stimulation and the inhibition of Vt were greater for basolateral than for apical perfusion of NEM. Similarly, the response times for each effect were faster from the basolateral side, suggesting that the primary sites of action are at or near the basolateral membrane. The site of action of NEM was further explored by subjecting the tissue to a membrane-impermeant sulfhydryl reagent, stilbenedisulfonate maleimide (SDM). Apical perfusion of 10−3 M SDM had no effect on Vt or Rt, whereas basolateral perfusion caused a reversible increase of Vt (5.2 ± 0.5 to initially 6.8 ± 0.5 mV which relaxed after 60 s to 5.8 ± 0.5 mV) and to an initial decrease in Rt by 4%. No inhibitory phase was observed. Elevation of basolateral [K+] from 3.6 to 25 mM is known to increase Vt and reduce Rt via direct stimulation of basolateral K+ uptake and indirect stimulation of the apical membrane conductance. Basolateral perfusion of 10−3 M NEM fully inhibited the increase of Vt due to 25 mM K+. Elevation of basolateral [K+] from 3.6 to 25 mM is known to increase reversibly cell volume. NEM was found to inhibit cell swelling in a dose-dependent manner but did not initially affect the rate of shrinking after K+-induced swelling, pointing to action only on basolateral transport pathways. The effects of NEM on K+-induced cell swelling were completely prevented by 5×10−4 M DTT, demonstrating that the inhibitory effect of NEM was on sulfhydryl groups. In contrast to interpretations of NEM action in frog semicircular canal, we have found that NEM appears to stimulate an ion transport process in mammalian dark cells at an extracellular site in the basolateral membrane and inhibits another ion transport process in the basolateral membrane at another site. Inhibition by NEM from the apical side occurs most likely by diffusion of the agent to a site at or near the cytosolic side of the basolateral membrane.
Vestibular labyrinth; epithelial transport; cell volume; dark cells; sulfhydryl reagents
Mutations in human SLC26A4 are a common cause of hearing loss associated with enlarged vestibular aqueducts (EVA). SLC26A4 encodes pendrin, an anion-base exchanger expressed in inner ear epithelial cells that secretes HCO3– into endolymph. Studies of Slc26a4-null mice indicate that pendrin is essential for inner ear development, but have not revealed whether pendrin is specifically necessary for homeostasis. Slc26a4-null mice are profoundly deaf, with severe inner ear malformations and degenerative changes that do not model the less severe human phenotype. Here, we describe studies in which we generated a binary transgenic mouse line in which Slc26a4 expression could be induced with doxycycline. The transgenes were crossed onto the Slc26a4-null background so that all functional pendrin was derived from the transgenes. Varying the temporal expression of Slc26a4 revealed that E16.5 to P2 was the critical interval in which pendrin was required for acquisition of normal hearing. Lack of pendrin during this period led to endolymphatic acidification, loss of the endocochlear potential, and failure to acquire normal hearing. Doxycycline initiation at E18.5 or discontinuation at E17.5 resulted in partial hearing loss approximating the human EVA auditory phenotype. These data collectively provide mechanistic insight into hearing loss caused by SLC26A4 mutations and establish a model for further studies of EVA-associated hearing loss.
Regulation of cochlear blood flow is critical for hearing due to its exquisite sensitivity to ischemia and oxidative stress. Many forms of hearing loss such as sensorineural hearing loss and presbyacusis may involve or be aggravated by blood flow disorders. Animal experiments and clinical outcomes further suggest that there is a gender preference in hearing loss, with males being more susceptible. Autoregulation of cochlear blood flow has been demonstrated in some animal models in vivo, suggesting that similar to the brain, blood vessels supplying the cochlea have the ability to control flow within normal limits, despite variations in systemic blood pressure. Here, we investigated myogenic regulation in the cochlear blood supply of the Mongolian gerbil, a widely used animal model in hearing research. The cochlear blood supply originates at the basilar artery, followed by the anterior inferior cerebellar artery, and inside the inner ear, by the spiral modiolar artery and the radiating arterioles that supply the capillary beds of the spiral ligament and stria vascularis. Arteries from male and female gerbils were isolated and pressurized using a concentric pipette system. Diameter changes in response to increasing luminal pressures were recorded by laser scanning microscopy. Our results show that cochlear vessels from male and female gerbils exhibit myogenic regulation but with important differences. Whereas in male gerbils, both spiral modiolar arteries and radiating arterioles exhibited pressure-dependent tone, in females, only radiating arterioles had this property. Male spiral modiolar arteries responded more to L-NNA than female spiral modiolar arteries, suggesting that NO-dependent mechanisms play a bigger role in the myogenic regulation of male than female gerbil cochlear vessels.
Calcium sparks are ryanodine receptor mediated transient calcium signals that have been shown to hyperpolarize the membrane potential by activating large conductance calcium activated potassium (BK) channels in vascular smooth muscle cells. Along with voltage-dependent calcium channels, they form a signaling unit that has a vasodilatory influence on vascular diameter and regulation of myogenic tone. The existence and role of calcium sparks has hitherto been unexplored in the spiral modiolar artery, the end artery that controls blood flow to the cochlea. The goal of the present study was to determine the presence and properties of calcium sparks in the intact gerbil spiral modiolar artery.
Calcium sparks were recorded from smooth muscle cells of intact arteries loaded with fluo-4 AM. Calcium sparks occurred with a frequency of 2.6 Hz, a rise time of 17 ms and a time to half-decay of 20 ms. Ryanodine reduced spark frequency within 3 min from 2.6 to 0.6 Hz. Caffeine (1 mM) increased spark frequency from 2.3 to 3.3 Hz and prolonged rise and half-decay times from 17 to 19 ms and from 20 to 23 ms, respectively. Elevation of potassium (3.6 to 37.5 mM), presumably via depolarization, increased spark frequency from 2.4 to 3.2 Hz. Neither ryanodine nor depolarization changed rise or decay times.
This is the first characterization of calcium sparks in smooth muscle cells of the spiral modiolar artery. The results suggest that calcium sparks may regulate the diameter of the spiral modiolar artery and cochlear blood flow.
Motivation: Mass spectrometry (MS) has become the method of choice for protein/peptide sequence and modification analysis. The technology employs a two-step approach: ionized peptide precursor masses are detected, selected for fragmentation, and the fragment mass spectra are collected for computational analysis. Current precursor selection schemes are based on data- or information-dependent acquisition (DDA/IDA), where fragmentation mass candidates are selected by intensity and are subsequently included in a dynamic exclusion list to avoid constant refragmentation of highly abundant species. DDA/IDA methods do not exploit valuable information that is contained in the fractional mass of high-accuracy precursor mass measurements delivered by current instrumentation.
Results: We extend previous contributions that suggest that fractional mass information allows targeted fragmentation of analytes of interest. We introduce a non-linear Random Forest classification and a discrete mapping approach, which can be trained to discriminate among arbitrary fractional mass patterns for an arbitrary number of classes of analytes. These methods can be used to increase fragmentation efficiency for specific subsets of analytes or to select suitable fragmentation technologies on-the-fly. We show that theoretical generalization error estimates transfer into practical application, and that their quality depends on the accuracy of prior distribution estimate of the analyte classes. The methods are applied to two real-world proteomics datasets.
Availability: All software used in this study is available from http://software.steenlab.org/fmf
Supplementary information: Supplementary data are available at Bioinformatics online.
Loss-of-function mutations of SLC26A4/pendrin are among the most prevalent causes of deafness. Deafness and vestibular dysfunction in the corresponding mouse model, Slc26a4−/−, are associated with an enlargement and acidification of the membranous labyrinth. Here we relate the onset of expression of the HCO3− transporter pendrin to the luminal pH and to enlargement-associated epithelial cell stretching. We determined expression with immunocytochemistry, cell stretching by digital morphometry and pH with double-barreled ion-selective electrodes. Pendrin was first expressed in the endolymphatic sac at embryonic day (E) 11.5, in the cochlear hook-region at E13.5, in the utricle and saccule at E14.5, in ampullae at E16.5, and in the upper turn of the cochlea at E17.5. Epithelial cell stretching in Slc26a4−/− mice began at E14.5. pH changes occurred first in the cochlea at E15.5 and in the endolymphatic sac at E17.5. At postnatal day 2, stria vascularis, outer sulcus and Reissner's membrane epithelial cells, and utricular and saccular transitional cells were stretched, whereas sensory cells in the cochlea, utricle and saccule did not differ between Slc26a4+/− and Slc26a4−/− mice. Structural development of stria vascularis, including vascularization, was retarded in Slc26a4−/− mice. In conclusion, the data demonstrate that the enlargement and stretching of non-sensory epithelial cells precedes luminal acidification in the cochlea and the endolymphatic sac. Stretching and luminal acidification may alter cell-to-cell communication and lead to the observed retarded development of stria vascularis, which may be an important step on the path to deafness in Slc26a4−/− mice, and possibly in humans, lacking functional pendrin expression.
Mutations of SLC26A4 are among the most prevalent causes of hereditary deafness. Deafness in the corresponding mouse model, Slc26a4−/−, results from an abnormally enlarged cochlear lumen. The goal of this study was to determine whether the cochlear enlargement originates with defective cochlear fluid transport or with a malfunction of fluid transport in the connected compartments, which are the vestibular labyrinth and the endolymphatic sac. Embryonic inner ears from Slc26a4+/− and Slc26a4−/− mice were examined by confocal microscopy ex vivo or after 2 days of organ culture. Culture allowed observations of intact, ligated or partially resected inner ears. Cochlear lumen formation was found to begin at the base of the cochlea between embryonic day (E) 13.5 and 14.5. Enlargement was immediately evident in Slc26a4−/− compared to Slc26a4+/− mice. In Slc26a4+/− and Slc26a4−/− mice, separation of the cochlea from the vestibular labyrinth by ligation at E14.5 resulted in a reduced cochlear lumen. Resection of the endolymphatic sacs at E14.5 led to an enlarged cochlear lumen in Slc26a4+/− mice but caused no further enlargement of the already enlarged cochlear lumen in Slc26a4−/− mice. Ligation or resection performed later, at E17.5, did not alter the cochlea lumen. In conclusion, the data suggest that cochlear lumen formation is initiated by fluid secretion in the vestibular labyrinth and temporarily controlled by fluid absorption in the endolymphatic sac. Failure of fluid absorption in the endolymphatic sac due to lack of Slc26a4 expression appears to initiate cochlear enlargement in mice, and possibly humans, lacking functional Slc26a4 expression.
The low luminal Ca2+ concentration of mammalian endolymph in the inner ear is required for normal hearing and balance. We recently reported the expression of mRNA for a Ca2+-absorptive transport system in primary cultures of semicircular canal duct (SCCD) epithelium.
We now identify this system in native vestibular and cochlear tissues by qRT-PCR, immunoblots and confocal immunolocalization. Transcripts were found and quantified for several isoforms of epithelial calcium channels (TRPV5, TRPV6), calcium buffer proteins (calbindin-D9K, calbindin-D28K), sodium-calcium exchangers (NCX1, NCX2, NCX3) and plasma membrane Ca2+-ATPase (PMCA1, PMCA2, PMCA3, and PMCA4) in native SCCD, cochlear lateral wall (LW) and stria vascularis (SV) of adult rat as well as Ca2+ channels in neonatal SCCD. All components were expressed except TRPV6 in SV and PMCA2 in SCCD. 1,25-(OH)2vitamin D3 (VitD) significantly up-regulated transcripts of TRPV5 in SCCD, calbindin-D9K in SCCD and LW, NCX2 in LW, while PMCA4 in SCCD and PMCA3 in LW were down-regulated. The expression of TRPV5 relative to TRPV6 was in the sequence SV > Neonatal SCCD > Adult SCCD > LW > primary culture SCCD. Expression of TRPV5 protein from primary culture of SCCD did not increase significantly when cells were incubated with VitD (1.2 times control; P > 0.05). Immunolocalization showed the distribution of TRPV5 and TRPV6. TRPV5 was found near the apical membrane of strial marginal cells and both TRPV5 and TRPV6 in outer and inner sulcus cells of the cochlea and in the SCCD of the vestibular system.
These findings demonstrate for the first time the expression of a complete Ca2+ absorptive system in native cochlear and vestibular tissues. Regulation by vitamin D remains equivocal since the results support the regulation of this system at the transcript level but evidence for control of the TRPV5 channel protein was lacking.
Hereditary hearing loss is one of the most common birth defects, yet the majority of genes required for audition is thought to remain unidentified. Ethylnitrosourea (ENU)–mutagenesis has been a valuable approach for generating new animal models of deafness and discovering previously unrecognized gene functions. Here we report on the characterization of a new ENU–induced mouse mutant (nmf329) that exhibits recessively inherited deafness. We found a widespread loss of sensory hair cells in the hearing organs of nmf329 mice after the second week of life. Positional cloning revealed that the nmf329 strain carries a missense mutation in the claudin-9 gene, which encodes a tight junction protein with unknown biological function. In an epithelial cell line, heterologous expression of wild-type claudin-9 reduced the paracellular permeability to Na+ and K+, and the nmf329 mutation eliminated this ion barrier function without affecting the plasma membrane localization of claudin-9. In the nmf329 mouse line, the perilymphatic K+ concentration was found to be elevated, suggesting that the cochlear tight junctions were dysfunctional. Furthermore, the hair-cell loss in the claudin-9–defective cochlea was rescued in vitro when the explanted hearing organs were cultured in a low-K+ milieu and in vivo when the endocochlear K+-driving force was diminished by deletion of the pou3f4 gene. Overall, our data indicate that claudin-9 is required for the preservation of sensory cells in the hearing organ because claudin-9–defective tight junctions fail to shield the basolateral side of hair cells from the K+-rich endolymph. In the tight-junction complexes of hair cells, claudin-9 is localized specifically to a subdomain that is underneath more apical tight-junction strands formed by other claudins. Thus, the analysis of claudin-9 mutant mice suggests that even the deeper (subapical) tight-junction strands have biologically important ion barrier function.
Hereditary deafness is a common birth defect in the human population, yet the majority of genes required for audition is thought to be unidentified. Genetic approaches in the mouse have greatly contributed to our understanding of the molecular mechanisms that underlie hearing. Random mutagenesis of mice, identification of deaf mutants, and subsequent analysis of the deafness-causing gene defects has led to the discovery of several previously unrecognized gene functions. Here, we report on the characterization of a new mutant mouse line (nmf329) that exhibits profound hearing loss and loss of sensory cells in the auditory organ. Genetic analysis reveals that these animals carry a mutation in the claudin-9 gene, which encodes a protein with hitherto unknown biological function. We have found that normal claudin-9—but not a mutant form—inhibits the paracellular movement of certain ions. The lack of the claudin-9 ion barrier in the inner ear leads to changes in ionic conditions that can account for the loss of sensory cells in the mutant mice. Within the cell–cell junctions, the claudin-9 layer is located basal to those of other claudins. Thus, our analysis of claudin-9–deficient animals suggests that even the deeper layers of claudins have important ion barrier function.