Fast fluorometric method for measuring pendrin (SLC26A4) Cl-/I- transport activity

Asymmetric pendrin homodimer reveals its molecular mechanism as anion exchanger

Pendrin (SLC26A4) is an anion exchanger expressed in the apical membranes of selected epithelia. Pendrin ablation causes Pendred syndrome, a genetic disorder associated with sensorineural hearing loss, hypothyroid goiter, and reduced blood pressure. However its molecular structure has remained unknown, limiting our understanding of the structural basis of transport. Here, we determine the cryo-electron microscopy structures of mouse pendrin with symmetric and asymmetric homodimer conformations. The asymmetric homodimer consists of one inward-facing protomer and the other outward-facing protomer, representing coincident uptake and secretion- a unique state of pendrin as an electroneutral exchanger. The multiple conformations presented here provide an inverted alternate-access mechanism for anion exchange. The structural and functional data presented here disclose the properties of an anion exchange cleft and help understand the importance of disease-associated variants, which will shed light on the pendrin exchange mechanism.

Molecular Features of SLC26A4 Common Variant p.L117F

The SLC26A4 gene, which encodes the anion exchanger pendrin, is involved in determining syndromic (Pendred syndrome) and non-syndromic (DFNB4) autosomal recessive hearing loss. SLC26A4 c.349C>T, p.L117F is a relatively common allele in the Ashkenazi Jewish community, where its minor allele frequency is increased compared to other populations. Although segregation and allelic data support the pathogenicity of this variant, former functional tests showed characteristics that were indistinguishable from those of the wild-type protein. Here, we applied a triad of cell-based assays, i.e., measurement of the ion transport activity by a fluorometric method, determination of the subcellular localization by confocal microscopy, and assessment of protein expression levels, to conclusively assign or exclude the pathogenicity of SLC26A4 p.L117F. This protein variant showed a moderate, but significant, reduction in ion transport function, a partial retention in the endoplasmic reticulum, and a strong reduction in expression levels as a consequence of an accelerated degradation by the Ubiquitin Proteasome System, all supporting pathogenicity. The functional and molecular features of human pendrin p.L117F were recapitulated by the mouse ortholog, thus indicating that a mouse carrying this variant might represent a good model of Pendred syndrome/DFNB4.

[Molecular and functional testing in case of hereditary hearing loss associated with the SLC26A4 gene]

Due to development of molecular techniques at hand, the number of genomic sequence variants detected in patient investigations is rising constantly. The number of potentially involved genes in hereditary hearing loss is rising simultaneously.In this overview, current methods for diagnostic workup on a molecular and functional level for variants of the SLC26A4 gene are described. Based on the description of the physiological function of the resulting protein Pendrin, molecular investigations for interpretation of the function are explained. Based on these investigations, the potential clinical consequences of a variant may be predicted more precisely and simplify routine reporting of a proven genotype and a phenotype, at hand. Finally, subsequent clinical investigations necessary, such as perchlorate discharge test, as well as therapeutic options are discussed.

Comparative analysis of functional assay evidence use by ClinGen Variant Curation Expert Panels

BACKGROUND

The 2015 American College of Medical Genetics and Genomics (ACMG) and the Association for Molecular Pathology (AMP) guidelines for clinical sequence variant interpretation state that "well-established" functional studies can be used as evidence in variant classification. These guidelines articulated key attributes of functional data, including that assays should reflect the biological environment and be analytically sound; however, details of how to evaluate these attributes were left to expert judgment. The Clinical Genome Resource (ClinGen) designates Variant Curation Expert Panels (VCEPs) in specific disease areas to make gene-centric specifications to the ACMG/AMP guidelines, including more specific definitions of appropriate functional assays. We set out to evaluate the existing VCEP guidelines for functional assays.

METHODS

We evaluated the functional criteria (PS3/BS3) of six VCEPs (CDH1, Hearing Loss, Inherited Cardiomyopathy-MYH7, PAH, PTEN, RASopathy). We then established criteria for evaluating functional studies based on disease mechanism, general class of assay, and the characteristics of specific assay instances described in the primary literature. Using these criteria, we extensively curated assay instances cited by each VCEP in their pilot variant classification to analyze VCEP recommendations and their use in the interpretation of functional studies.

RESULTS

Unsurprisingly, our analysis highlighted the breadth of VCEP-approved assays, reflecting the diversity of disease mechanisms among VCEPs. We also noted substantial variability between VCEPs in the method used to select these assays and in the approach used to specify strength modifications, as well as differences in suggested validation parameters. Importantly, we observed discrepancies between the parameters VCEPs specified as required for approved assay instances and the fulfillment of these requirements in the individual assays cited in pilot variant interpretation.

CONCLUSIONS

Interpretation of the intricacies of functional assays often requires expert-level knowledge of the gene and disease, and current VCEP recommendations for functional assay evidence are a useful tool to improve the accessibility of functional data by providing a starting point for curators to identify approved functional assays and key metrics. However, our analysis suggests that further guidance is needed to standardize this process and ensure consistency in the application of functional evidence.

Expert specification of the ACMG/AMP variant interpretation guidelines for genetic hearing loss

Due to the high genetic heterogeneity of hearing loss (HL), current clinical testing includes sequencing large numbers of genes, which often yields a significant number of novel variants. Therefore, the standardization of variant interpretation is crucial to provide consistent and accurate diagnoses. The Hearing Loss Variant Curation Expert Panel was created within the Clinical Genome Resource to provide expert guidance for standardized genomic interpretation in the context of HL. As one of its major tasks, our Expert Panel has adapted the American College of Medical Genetics and Genomics/Association for Molecular Pathology (ACMG/AMP) guidelines for the interpretation of sequence variants in HL genes. Here, we provide a comprehensive illustration of the newly specified ACMG/AMP HL rules. Three rules remained unchanged, four rules were removed, and the remaining 21 rules were specified. These rules were further validated and refined using a pilot set of 51 variants assessed by curators and disease experts. Of the 51 variants evaluated in the pilot, 37% (19/51) changed category based upon application of the expert panel specified rules and/or aggregation of evidence across laboratories. These HL-specific ACMG/AMP rules will help standardize variant interpretation, ultimately leading to better care for individuals with HL.

Functional Testing of SLC26A4 Variants-Clinical and Molecular Analysis of a Cohort with Enlarged Vestibular Aqueduct from Austria

The prevalence and spectrum of sequence alterations in the SLC26A4 gene, which codes for the anion exchanger pendrin, are population-specific and account for at least 50% of cases of non-syndromic hearing loss associated with an enlarged vestibular aqueduct. A cohort of nineteen patients from Austria with hearing loss and a radiological alteration of the vestibular aqueduct underwent Sanger sequencing of SLC26A4 and GJB2, coding for connexin 26. The pathogenicity of sequence alterations detected was assessed by determining ion transport and molecular features of the corresponding SLC26A4 protein variants. In this group, four uncharacterized sequence alterations within the SLC26A4 coding region were found. Three of these lead to protein variants with abnormal functional and molecular features, while one should be considered with no pathogenic potential. Pathogenic SLC26A4 sequence alterations were only found in 12% of patients. SLC26A4 sequence alterations commonly found in other Caucasian populations were not detected. This survey represents the first study on the prevalence and spectrum of SLC26A4 sequence alterations in an Austrian cohort and further suggests that genetic testing should always be integrated with functional characterization and determination of the molecular features of protein variants in order to unequivocally identify or exclude a causal link between genotype and phenotype.

Iodide excess regulates its own efflux: a possible involvement of pendrin

Adequate iodide supply and metabolism are essential for thyroid hormones synthesis. In thyrocytes, iodide uptake is mediated by the sodium-iodide symporter, but several proteins appear to be involved in iodide efflux. Previous studies demonstrated that pendrin is able to mediate apical efflux of iodide in thyrocytes. Acute iodide excess transiently impairs thyroid hormone synthesis, a phenomenon known as the Wolff-Chaikoff effect. Although the escape from this inhibitory effect is not completely understood, it has been related to the inhibition of sodium-iodide symporter-mediated iodide uptake. However, the effects of iodide excess on iodide efflux have not been characterized. Herein, we investigated the consequences of iodide excess on pendrin abundance, subcellular localization, and iodide efflux in rat thyroid PCCl3 cells. Our results indicate that iodide excess increases pendrin abundance and plasma membrane insertion after 24 h of treatment. Moreover, iodide excess increases pendrin half-life. Finally, iodide exposure also increases iodide efflux from PCCl3 cells. In conclusion, these data suggest that pendrin may have an important role in mediating iodide efflux in thyrocytes, especially under conditions of iodide excess.

Reduction of Cellular Expression Levels Is a Common Feature of Functionally Affected Pendrin (SLC26A4) Protein Variants

Sequence alterations in the pendrin gene (SLC26A4) leading to functionally affected protein variants are frequently involved in the pathogenesis of syndromic and nonsyndromic deafness. Considering the high number of SLC26A4 sequence alterations reported to date, discriminating between functionally affected and unaffected pendrin protein variants is essential in contributing to determine the genetic cause of deafness in a given patient. In addition, identifying molecular features common to the functionally affected protein variants can be extremely useful to design future molecule-directed therapeutic approaches. Here we show the functional and molecular characterization of six previously uncharacterized pendrin protein variants found in a cohort of 58 Brazilian deaf patients. Two variants (p.T193I and p.L445W) were undetectable in the plasma membrane, completely retained in the endoplasmic reticulum and showed no transport function; four (p.P142L, p.G149R, p.C282Y and p.Q413R) showed reduced function and significant, although heterogeneous, expression levels in the plasma membrane. Importantly, total expression levels of all of the functionally affected protein variants were significantly reduced with respect to the wild-type and a fully functional variant (p.R776C), regardless of their subcellular localization. Interestingly, reduction of expression may also reduce the transport activity of variants with an intrinsic gain of function (p.Q413R). As reduction of overall cellular abundance was identified as a common molecular feature of pendrin variants with affected function, the identification of strategies to prevent reduction in expression levels may represent a crucial step of potential future therapeutic interventions aimed at restoring the transport activity of dysfunctional pendrin variants.

IL-17A induces Pendrin expression and chloride-bicarbonate exchange in human bronchial epithelial cells

The epithelium plays an active role in the response to inhaled pathogens in part by responding to signals from the immune system. Epithelial responses may include changes in chemokine expression, increased mucin production and antimicrobial peptide secretion, and changes in ion transport. We previously demonstrated that interleukin-17A (IL-17A), which is critical for lung host defense against extracellular bacteria, significantly raised airway surface pH in vitro, a finding that is common to a number of inflammatory diseases. Using microarray analysis of normal human bronchial epithelial (HBE) cells treated with IL-17A, we identified the electroneutral chloride-bicarbonate exchanger Pendrin (SLC26A4) as a potential mediator of this effect. These data were verified by real-time, quantitative PCR that demonstrated a time-dependent increase in Pendrin mRNA expression in HBE cells treated with IL-17A up to 48 h. Using immunoblotting and immunofluorescence, we confirmed that Pendrin protein expression is increased in IL-17 treated HBE cells and that it is primarily localized to the mucosal surface of the cells. Functional studies using live-cell fluorescence to measure intracellular pH demonstrated that IL-17A induced chloride-bicarbonate exchange in HBE cells that was not present in the absence of IL-17A. Furthermore, HBE cells treated with short interfering RNA against Pendrin showed substantially reduced chloride-bicarbonate exchange. These data suggest that Pendrin is part of IL-17A-dependent epithelial changes and that Pendrin may therefore be a therapeutic target in IL-17A-dependent lung disease.

American Thyroid Association Guide to investigating thyroid hormone economy and action in rodent and cell models

BACKGROUND

An in-depth understanding of the fundamental principles that regulate thyroid hormone homeostasis is critical for the development of new diagnostic and treatment approaches for patients with thyroid disease.

SUMMARY

Important clinical practices in use today for the treatment of patients with hypothyroidism, hyperthyroidism, or thyroid cancer are the result of laboratory discoveries made by scientists investigating the most basic aspects of thyroid structure and molecular biology. In this document, a panel of experts commissioned by the American Thyroid Association makes a series of recommendations related to the study of thyroid hormone economy and action. These recommendations are intended to promote standardization of study design, which should in turn increase the comparability and reproducibility of experimental findings.

CONCLUSIONS

It is expected that adherence to these recommendations by investigators in the field will facilitate progress towards a better understanding of the thyroid gland and thyroid hormone dependent processes.

Molecular and functional studies of 4 candidate loci in Pendred syndrome and nonsyndromic hearing loss

Patients with PS or non-syndromic deafness were submitted to genetic/functional analyzes of SLC26A4, of its binding domain for FOXI1 (FOXI1-DBD), of the transcription activator FOXI1, and of the potassium channel KCNJ10. SLC26A4 was the most frequently mutated gene. An altered intracellular localization with immunocytochemistry, and a hampered maturation process were demonstrated for two novel SLC26A4 variants. Biochemical and immunocytochemical analyzes led to the development of a more sensitive fluorometric functional assay able to reveal the partial loss-of-function of SLC26A4 mutations. A novel missense variant was found in FOXI1 gene, though functional analysis showed no significant impairment in the transcriptional activation of SLC26A4. Finally, 3 patients were found to harbor a variant in KCNJ10, which was classified as polymorphism. The novelty of the study resides in the analysis of all the 4 candidate genetic loci linked to PS/non-syndromic deafness, and in the precise definition of the thyroid phenotype. PS was invariably associated with biallelic mutations of SLC26A4, whereas the genetic origin of non-syndromic deafness remained largely undetermined, since monoallelic SLC26A4 variants accounted for one fourth of the cases and FOXI1 and KCNJ10 were not involved in this series.

Iodide treatment acutely increases pendrin (SLC26A4) mRNA expression in the rat thyroid and the PCCl3 thyroid cell line by transcriptional mechanisms

Iodine is a critical element involved in thyroid hormone synthesis. Its efflux into the follicular lumen is thought to occur, in part, through pendrin at the apical membrane of thyrocytes. This study attempted to investigate whether iodide administration affects SLC26A4 mRNA expression in rat thyroid and in PCCl3 cells. Rats and cells were treated or not with NaI from 30 min up to 48 h. One group was concomitantly treated with sodium perchlorate. SLC26A4 mRNA expression was also investigated in PCCl3 cells treated with actinomycin D prior to NaI treatment. Iodide administration significantly increased SLC26A4 mRNA content in both models. The simultaneous administration of NaI and perchlorate, as well as the treatment of PCCl3 cells with actinomycin D prevented this effect, indicating that intracellular iodide is essential for this event, which appears to be triggered by transcriptional mechanisms. These data show that intracellular iodide rapidly upregulates SLC26A4 mRNA expression.

Molecular and functional characterization of human pendrin and its allelic variants

Pendrin (SLC26A4, PDS) is an electroneutral anion exchanger transporting I(-), Cl(-), HCO(3)(-), OH(-), SCN(-) and formate. In the thyroid, pendrin is expressed at the apical membrane of the follicular epithelium and may be involved in mediating apical iodide efflux into the follicle; in the inner ear, it plays a crucial role in the conditioning of the pH and ion composition of the endolymph; in the kidney, it may exert a role in pH homeostasis and regulation of blood pressure. Mutations of the pendrin gene can lead to syndromic and non-syndromic hearing loss with EVA (enlarged vestibular aqueduct). Functional tests of mutated pendrin allelic variants found in patients with Pendred syndrome or non-syndromic EVA (ns-EVA) revealed that the pathological phenotype is due to the reduction or loss of function of the ion transport activity. The diagnosis of Pendred syndrome and ns-EVA can be difficult because of the presence of phenocopies of Pendred syndrome and benign polymorphisms occurring in the general population. As a consequence, defining whether or not an allelic variant is pathogenic is crucial. Recently, we found that the two parameters used so far to assess the pathogenic potential of a mutation, i.e. low incidence in the control population, and substitution of evolutionary conserved amino acids, are not always reliable for predicting the functionality of pendrin allelic variants; actually, we identified mutations occurring with the same frequency in the cohort of hearing impaired patients and in the control group of normal hearing individuals. Moreover, we identified functional polymorphisms affecting highly conserved amino acids. As a general rule however, we observed a complete loss of function for all truncations and amino acid substitutions involving a proline. In this view, clinical and radiological studies should be combined with genetic and molecular studies for a definitive diagnosis. In performing genetic studies, the possibility that the mutation could affect regions other than the pendrin coding region, such as its promoter region and/or the coding regions of functionally related genes (FOXI1, KCNJ10), should be taken into account. The presence of benign polymorphisms in the population suggests that genetic studies should be corroborated by functional studies; in this context, the existence of hypo-functional variants and possible differences between the I(-)/Cl(-) and Cl(-)/HCO(3)(-) exchange activities should be carefully evaluated.

Identification of allelic variants of pendrin (SLC26A4) with loss and gain of function

BACKGROUND

Pendrin is a multifunctional anion transporter that exchanges chloride and iodide in the thyroid, as well as chloride and bicarbonate in the inner ear, kidney and airways. Loss or reduction in the function of pendrin results in both syndromic (Pendred syndrome) and non-syndromic (non-syndromic enlarged vestibular aqueduct (ns-EVA)) hearing loss. Factors inducing an up-regulation of pendrin in the kidney and the lung may have an impact on the pathogenesis of hypertension, chronic obstructive pulmonary disease (COPD) and asthma. Here we characterize the ion transport activity of wild-type (WT) pendrin and seven of its allelic variants selected among those reported in the single nucleotide polymorphisms data base (dbSNPs), some of which were previously identified in a cohort of individuals with normal hearing or deaf patients belonging to the Spanish population.

METHODS

WT and mutated pendrin allelic variants were functionally characterized in a heterologous over-expression system by means of fluorometric methods evaluating the I(-)/Cl(-) and Cl(-)/OH(-) exchange and an assay evaluating the efflux of radiolabeled iodide.

RESULTS

The transport activity of pendrin P70L, P301L and F667C is completely abolished; pendrin V609G and D687Y allelic variants are functionally impaired but retain significant transport. Pendrin F354S activity is indistinguishable from WT, while pendrin V88I and G740S exhibit a gain of function.

CONCLUSION

Amino acid substitutions involving a proline always result in a severe loss of function of pendrin. Two hyperfunctional allelic variants (V88I, G740S) have been identified, and they may have a contributing role in the pathogenesis of hypertension, COPD and asthma.

Pendrin overexpression affects cell volume recovery, intracellular pH and chloride concentration after hypotonicity-induced cell swelling

The pendrin (SLC26A4 or PDS) gene is responsible, when mutated, for the Pendred syndrome, a recessive disorder characterized by sensorineural hearing loss often accompanied by thyroid dysfunctions. Pendrin protein is an anion exchanger and we focused on a still unexplored function that it might play in view of its importance in the inner ear: Cl(-) fluxes regulation during cellular volume control. We challenged HEK-293 Phoenix cells over-expressing wild type pendrin (PDS HEK cells) together with the EYFP (Enhanced Yellow Fluorescent Protein) or over-expressing the EYFP alone (control HEK cells) with hypo-osmolar solutions. Taking advantage of the confocal optical sectioning we measured the cell volume. In addition, we determined the intracellular pH and chloride concentration with fluorescent probes (EYFP and seminaphthorhodafluor-5F, SNARF-5F). Consequently, we could estimate simultaneously Cl(-) fluxes, cellular volume and intracellular pH variations. Cl(-) movements markedly differed between PDS and control HEK cells upon hypotonic shock and are accompanied by an attenuation of the swelling induced pH drop in PDS HEK cells. The contemporary measurements of the three variables not yet reported in living cells, allowed to assess a possible influence of pendrin upregulation in volume homeostasis and evidenced its participation to Cl(-) fluxes.

Functional characterization of pendrin mutations found in the Israeli and Palestinian populations

BACKGROUND

Pendrin is a transport protein exchanging chloride for other anions, such as iodide in the thyroid gland or bicarbonate in the inner ear. Mutations in the SLC26A4 gene encoding for pendrin are responsible for both syndromic (Pendred syndrome) and non-syndromic (non-syndromic enlarged vestibular aqueduct, EVA) hearing loss. Besides clinical and radiological assessments, molecular and functional studies are essential for the correct diagnosis of Pendred syndrome and non-syndromic EVA. While a broad spectrum of mutations found in the Caucasian population has been functionally characterized, little is known about mutations specifically occurring in the populations of the Middle East. Here we show the characterization of the ion transport activity of three pendrin mutations previously found in deaf patients with EVA in the Israeli Jewish and Palestinian Arab populations, i.e. V239D, G334V X335 and I487Y FSX39.

METHODS

Wild type and mutated pendrin allelic variants were functionally characterized in a heterologous over-expression system. The Cl(-)/I(-) and Cl(-)/OH(-) exchange activities were assessed by fluorometric methods suitable for measuring iodide fluxes and the intracellular pH.

RESULTS

Both the Cl(-)/I(-) and the Cl(-)/OH(-) exchange activities of pendrin V239D, G334V X335 and I487Y FSX39 were significantly reduced with respect to the wild type, with V239D displaying a residual iodide transport.

CONCLUSION

Functional assays confirmed the diagnosis of non-syndromic EVA due to SLC26A4 mutations performed by radiological and molecular tests in deaf patients belonging to the Israeli Jewish and Palestinian Arab populations. The new finding that the V239D mutation displays residual function suggests that the symptoms caused by this mutation could be ameliorated by a pendrin 'activator', if available.

The ESF meeting on "The proteomics, epigenetics and pharmacogenetics of pendrin"

Human pendrin (SCL26A4, PDS) is a 780 amino acid integral membrane protein with transport function. It acts as an electroneutral, sodium-independent anion exchanger for a wide range of anions, such as iodide, chloride, formate, bicarbonate, hydroxide and thiocyanate. Pendrin expression was originally described in the thyroid gland, kidney and inner ear. Accordingly, pendrin mutations with reduction or loss of transport function result in thyroid and inner ear abnormalities, manifested as syndromic (Pendred syndrome) and non-syndromic hearing loss with an enlarged vestibular aqueduct (ns-EVA). Pendred syndrome, the most common form of syndromic deafness, is an autosomal recessive disease characterized by sensorineural deafness due to inner ear malformations and a partial iodide organification defect that may lead to thyroid goiter. Later, it became evident that not only pendrin loss of function, but also up-regulation could participate in the pathogenesis of human diseases. Indeed, despite the absence of kidney dysfunction in Pendred syndrome patients, evidence exists that pendrin also plays a crucial role in this organ, with a potential involvement in the pathogenesis of hypertension. In addition, recent data underscore the role of pendrin in exacerbations of respiratory distresses including bronchial asthma and chronic obstructive pulmonary disease (COPD). Pendrin expression in other organs such as mammary gland, testis, placenta, endometrium and liver point to new, underscored pendrin functions that deserve to be further investigated.

Pendrin function and regulation in Xenopus oocytes

SLC26A4/PDS mutations cause Pendred Syndrome and non-syndromic deafness. but some aspects of function and regulation of the SLC26A4 polypeptide gene product, pendrin, remain controversial or incompletely understood. We have therefore extended the functional analysis of wildtype and mutant pendrin in Xenopus oocytes, with studies of isotopic flux, electrophysiology, and protein localization. Pendrin mediated electroneutral, pH-insensitive, DIDS-insensitive anion exchange, with extracellular K((1/2)) (in mM) of 1.9 (Cl(-)), 1.8 (I(-)), and 0.9 (Br(-)). The unusual phenotype of Pendred Syndrome mutation E303Q (loss-of-function with normal surface expression) prompted systematic mutagenesis at position 303. Only mutant E303K exhibited loss-of-function unrescued by forced overexpression. Mutant E303C was insensitive to charge modification by methanethiosulfonates. The corresponding mutants SLC26A2 E336Q, SLC26A3 E293Q, and SLC26A6 E298Q exhibited similar loss-of-function phenotypes, with wildtype surface expression also documented for SLC26A2 E336Q. The strong inhibition of wildtype SLC26A2, SLC26A3, and SLC26A6 by phorbol ester contrasts with its modest inhibition of pendrin. Phorbol ester inhibition of SLC26A2, SLC26A3, and SLC26A6 was blocked by coexpressed kinase-dead PKCδ but was without effect on pendrin. Mutation of SLC26A2 serine residues conserved in PKCδ -sensitive SLC26 proteins but absent from pendrin failed to reduce PKCδ sensitivity of SLC26A2 (190).

Salicylate restores transport function and anion exchanger activity of missense pendrin mutations

The SLC26A4 gene encodes the transmembrane protein pendrin, which is involved in the homeostasis of the ion concentration of the endolymph of the inner ear, most likely by acting as a chloride/bicarbonate transporter. Mutations in the SLC26A4 gene cause sensorineuronal hearing loss. However, the mechanisms responsible for such loss have remained unknown. Therefore, in this study, we focused on the function of ten missense pendrin mutations (p.P123S (Pendred syndrome), p.M147V (NSEVA), p.K369E (NSEVA), p.A372V (Pendred syndrome/NSEVA), p.N392Y (Pendred syndrome), p.C565Y (NSEVA), p.S657N (NSEVA), p.S666F (NSEVA), p.T721M (NSEVA) and p.H723R (Pendred syndrome/NSEVA)) reported in Japanese patients, and analyzed their cellular localization and anion exchanger activity using HEK293 cells transfected with each mutant gene. Immunofluorescent staining of the cellular localization of the pendrin mutants revealed that p.K369E and p.C565Y, as well as wild-type pendrin, were transported to the plasma membrane, while 8 other mutants were retained in the cytoplasm. Furthermore, we analyzed whether salicylate, as a pharmacological chaperone, restores normal plasma membrane localization of 8 pendrin mutants retained in the cytoplasm to the plasma membrane. Incubation with 10 mM of salicylate of the cells transfected with the mutants induced the transport of 4 pendrin mutants (p.P123S, p.M147V, p.S657Y and p.H723R) from the cytoplasm to the plasma membrane and restored the anion exchanger activity. These findings suggest that salicylate might contribute to development of a new method of medical treatment for sensorineuronal hearing loss caused by the mutation of the deafness-related proteins, including pendrin.

Calcium oxalate stone formation in the inner ear as a result of an Slc26a4 mutation

Calcium oxalate stone formation occurs under pathological conditions and accounts for more than 80% of all types of kidney stones. In the current study, we show for the first time that calcium oxalate stones are formed in the mouse inner ear of a genetic model for hearing loss and vestibular dysfunction in humans. The vestibular system within the inner ear is dependent on extracellular tiny calcium carbonate minerals for proper function. Thousands of these biominerals, known as otoconia, are associated with the utricle and saccule sensory maculae and are vital for mechanical stimulation of the sensory hair cells. We show that a missense mutation within the Slc26a4 gene abolishes the transport activity of its encoded protein, pendrin. As a consequence, dramatic changes in mineral composition, size, and shape occur within the utricle and saccule in a differential manner. Although abnormal giant carbonate minerals reside in the utricle at all ages, in the saccule, a gradual change in mineral composition leads to a formation of calcium oxalate in adult mice. By combining imaging and spectroscopy tools, we determined the profile of mineral composition and morphology at different time points. We propose a novel mechanism for the accumulation and aggregation of oxalate crystals in the inner ear.

Functional assessment of allelic variants in the SLC26A4 gene involved in Pendred syndrome and nonsyndromic EVA

Pendred syndrome is an autosomal recessive disorder characterized by sensorineural hearing loss, with malformations of the inner ear, ranging from enlarged vestibular aqueduct (EVA) to Mondini malformation, and deficient iodide organification in the thyroid gland. Nonsyndromic EVA (ns-EVA) is a separate type of sensorineural hearing loss showing normal thyroid function. Both Pendred syndrome and ns-EVA seem to be linked to the malfunction of pendrin (SLC26A4), a membrane transporter able to exchange anions between the cytosol and extracellular fluid. In the past, the pathogenicity of SLC26A4 missense mutations were assumed if the mutations fulfilled two criteria: low incidence of the mutation in the control population and substitution of evolutionary conserved amino acids. Here we show that these criteria are insufficient to make meaningful predictions about the effect of these SLC26A4 variants on the pendrin-induced ion transport. Furthermore, we functionally characterized 10 missense mutations within the SLC26A4 ORF, and consistently found that on the protein level, an addition or omission of a proline or a charged amino acid in the SLC26A4 sequence is detrimental to its function. These types of changes may be adequate for predicting SLC26A4 functionality in the absence of direct functional tests.

Pendred syndrome and iodide transport in the thyroid

Pendred syndrome is an autosomal recessive disorder characterized by sensorineural hearing impairment, presence of goiter, and a partial defect in iodide organification, which may be associated with insufficient thyroid hormone synthesis. Goiter development and development of hypothyroidism are variable and depend on nutritional iodide intake. Pendred syndrome is caused by biallelic mutations in the SLC26A4 gene, which encodes pendrin, a transporter of chloride, bicarbonate and iodide. This review discusses the controversies surrounding the potential role of pendrin in mediating apical iodide efflux into the lumen of thyroid follicles, and discusses its functional role in the kidney and the inner ear.

Pendred syndrome

Pendred syndrome (PDS) is an autosomal recessive disorder clinically characterized by sensorineural hearing loss and goiter. PDS is mainly caused by mutations in the SLC26A4 gene, although a few cases are due to mutations in the FOXI1 gene. SLC26A4 encodes pendrin, a sodium-independent transporter of iodide/chloride, chloride/formate and bicarbonate, that is expressed in the inner ear, thyroid gland, syncytiotrophoblast cells, endometrium and kidney. FOXI1 encodes a transcription factor necessary for pendrin expression. Patients with PDS show a bilateral and severe-to-profound hearing loss, although some cases present with a slowly progressive and fluctuating course. Temporal bone abnormalities with enlargement of the vestibular aqueduct, alone or with Mondini dysplasia, are common. Goiter appears most frequently in the second decade of life with a range of variations in size, depending on the amount of iodide intake and the effect that the mutation causes in pendrin function in any individual patient. A standard thyroid hormone-replacement regimen should be given to PDS patients with hypothyroidism to re-establish euthyroidism and prevent or decrease goiter growth. Total or partial thyroidectomy is occasionally the treatment of choice. Hearing aids and proper educational programs should also be offered to patients.

Fluorescence quantitation of thyrocyte iodide accumulation with the yellow fluorescent protein variant YFP-H148Q/I152L

The thyroid gland accumulates iodide for the synthesis of thyroid hormones. The aim of the current study was to quantify iodide accumulation in cultured thyroid cells by live cell imaging using the halide-sensitive yellow fluorescent protein (YFP) variant YFP-H148Q/I152L. In vivo calibrations were performed in FRTL-5 thyrocytes to determine the sensitivity of YFP-H148Q/I152L to iodide. In the presence of ion-selective ionophores, YFP-H148Q/I152L fluorescence was suppressed by halides in a pH-dependent manner with 20-fold selectivity for iodide versus chloride and competition between the two halides. At a physiological pH of 7 and a chloride concentration of 15mM, the affinity constant of YFP-H148Q/I152L for iodide was 3.5mM. In intact FRTL-5 cells, iodide induced a reversible decrease in YFP-H148Q/I152L fluorescence. FRTL-5 cells concentrated iodide to 60 times the extracellular concentration. Iodide influx exhibited saturation kinetics with respect to extracellular iodide with a K(m) of 35 microM and a V(max) of 55 microM/s. Iodide efflux exhibited saturation kinetics with respect to intracellular iodide concentration with a K(m) of 2.2mM and a V(max) of 43 microM/s. The results of this study demonstrate the utility of YFP-H148Q/I152L as a sensitive and selective biosensor for the quantification of iodide accumulation in thyroid cells.

SLC26A9 is a Cl(-) channel regulated by the WNK kinases

SLC26A9 is a member of the SLC26 family of anion transporters, which is expressed at high levels in airway and gastric surface epithelial cells. The transport properties and regulation of SLC26A9, and thus its physiological function, are not known. Here we report that SLC26A9 is a highly selective Cl(-) channel with minimal OH(-)/HCO(3)(-) permeability that is regulated by the WNK kinases. Expression in Xenopus oocytes and simultaneous measurement of membrane potential or current, intracellular pH (pH(i)) and intracellular Cl(-) (Cl(-)(i)) revealed that expression of SLC26A9 resulted in a large Cl(-) current. SLC26A9 displays a selectivity sequence of I(-) > Br(-) > NO(3)(-) > Cl(-) > Glu(-), but it conducts Br(-) > Cl(-) > I(-) > NO(3)(-) > Glu(-), with NO(3)(-) and I(-) inhibiting the Cl(-) conductance. Similarly, expression of SLC26A9 in HEK cells resulted in a large Cl(-) current. Although detectable, OH(-) and HCO(3)(-) fluxes in oocytes expressing SLC26A9 were very small. Moreover, HCO(3)(-) had no discernable effect on the Cl(-) current, the reversal potential in the presence or absence of Cl(-)(o) and, importantly, HCO(3)(-) had no effect on Cl(-) fluxes. These findings indicate that SLC26A9 is a Cl(-) channel with minimal OH(-)/HCO(3)(-) permeability. Co-expression of SLC26A9 with the WNK kinases WNK1, WNK3 or WNK4 inhibited SLC26A9 activity, and the inhibition was independent of WNK kinase activity. Immunolocalization in oocytes and cell surface biotinylation in HEK cells indicated that the WNK-mediated inhibition of SLC26A9 activity is caused by reduced SLC26A9 surface expression. Expression of SLC26A9 in the airway and the response of the WNKs to homeostatic stress raise the possibility that SLC26A9 serves to mediate the response of the airway to stress.