SLC26A9 stimulates CFTR expression and function in human bronchial cell lines

The solute carrier family 26 member 9 modifies rapidly progressing cystic fibrosis associated with homozygous F508del CFTR mutation

BACKGROUND AND AIMS

Cystic fibrosis (CF) is an autosomal recessive disease caused by mutations to the CF transmembrane conductance regulator (CFTR). Symptoms and severity of the disease can be quite variable suggesting modifier genes play an important role.

MATERIALS AND METHODS

Exome sequencing was performed on six individuals carrying homozygous deltaF508 for CFTR genotype but present with rapidly progressing CF (RPCF). Data was analyzed using an unbiased genome-wide genetic burden test against 3076 controls. Single cell RNA sequencing data from LungMAP was utilized to evaluate unique and co-expression of candidate genes, and structural modeling to evaluate the deleterious effects of identified candidate variants.

RESULTS

We have identified solute carrier family 26 member 9 (SLC26A9) as a modifier gene to be associated with RPCF. Two rare missense SLC26A9 variants were discovered in three of six individuals deemed to have RPCF: c.229G > A; p.G77S (present in two patients), and c.1885C > T; p.P629S. Co-expression of SLC26A9 and CFTR mRNA is limited across different lung cell types, with the highest level of co-expression seen in human (6.3 %) and mouse (9.0 %) alveolar type 2 (AT2) cells. Structural modeling suggests deleterious effects of these mutations as they are in critical protein domains which might affect the anion transport capability of SLC26A9.

CONCLUSION

The enrichment of rare and potentially deleterious SLC26A9 mutations in patients with RPCF suggests SLC26A9 may act as an alternative anion transporter in CF and is a modifier gene associated with this lung phenotype.

Chloride/Multiple Anion Exchanger SLC26A Family: Systemic Roles of SLC26A4 in Various Organs

Solute carrier family 26 member 4 (SLC26A4) is a member of the SLC26A transporter family and is expressed in various tissues, including the airway epithelium, kidney, thyroid, and tumors. It transports various ions, including bicarbonate, chloride, iodine, and oxalate. As a multiple-ion transporter, SLC26A4 is involved in the maintenance of hearing function, renal function, blood pressure, and hormone and pH regulation. In this review, we have summarized the various functions of SLC26A4 in multiple tissues and organs. Moreover, the relationships between SLC26A4 and other channels, such as cystic fibrosis transmembrane conductance regulator, epithelial sodium channel, and sodium chloride cotransporter, are highlighted. Although the modulation of SLC26A4 is critical for recovery from malfunctions of various organs, development of specific inducers or agonists of SLC26A4 remains challenging. This review contributes to providing a better understanding of the role of SLC26A4 and development of therapeutic approaches for the SLC26A4-associated hearing loss and SLC26A4-related dysfunction of various organs.

SLC26A9 promotes colorectal tumorigenesis by modulating Wnt/β-catenin signaling

Solute carrier family 26 member 9 (SLC26A9) is a member of the Slc26a family of multifunctional anion transporters that functions as a Cl- channel in parietal cells during acid secretion. We explored the role of SLC26A9 in colorectal cancer (CRC) and its related mechanisms through clinical samples from CRC patients, CRC cell lines and mouse models. We observed that SLC26A9 was expressed at low levels in the cytoplasm of adjacent tissues, polyps and adenomas but was significantly increased in colorectal adenocarcinoma. Moreover, increased levels of SLC26A9 were associated with a high risk of disease and poor prognosis. In addition, downregulation of SLC26A9 in CRC cells induced cell cycle arrest and apoptosis but inhibited cell proliferation and xenograft tumor growth both in vitro and in vivo. Mechanistic analysis revealed that SLC26A9 was colocalized with β-catenin in the nucleus of CRC cells. The translocation of these two proteins from the cytoplasm to the nucleus reflected the activation of Wnt/β-catenin signaling, and promoted the transcription of downstream target proteins, including CyclinD1, c-Myc and Snail, but inhibited the expression of cytochrome C (Cyt-c), cleaved Caspase9, cleaved Caspase3 and apoptosis-inducing factor (AIF). CRC is accompanied by alteration of epithelial mesenchymal transition (EMT) markers. Meanwhile, further studies showed that in SW48 cells, overexpressing SLC26A9 was cocultured with the β-catenin inhibitor XAV-939, β-catenin was downregulated, and EMT was reversed. Our study demonstrated SLC26A9 may be responsible for alterations in the proliferative ability and aggressive potential of CRC by regulating the Wnt/β-catenin signaling pathway.

Dynamic regulation of airway surface liquid pH by TMEM16A and SLC26A4 in cystic fibrosis nasal epithelia with rare mutations

In cystic fibrosis (CF), defects in the CF transmembrane conductance regulator (CFTR) channel lead to an acidic airway surface liquid (ASL), which compromises innate defence mechanisms, predisposing to pulmonary failure. Restoring ASL pH is a potential therapy for people with CF, particularly for those who cannot benefit from current highly effective modulator therapy. However, we lack a comprehensive understanding of the complex mechanisms underlying ASL pH regulation. The calcium-activated chloride channel, TMEM16A, and the anion exchanger, SLC26A4, have been proposed as targets for restoring ASL pH, but current results are contradictory and often utilise nonphysiological conditions. To provide better evidence for a role of these two proteins in ASL pH homeostasis, we developed an efficient CRISPR-Cas9-based approach to knock-out (KO) relevant transporters in primary airway basal cells lacking CFTR and then measured dynamic changes in ASL pH under thin-film conditions in fully differentiated airway cultures, which better simulate the in vivo situation. Unexpectantly, we found that both proteins regulated steady-state as well as agonist-stimulated ASL pH, but only under inflammatory conditions. Furthermore, we identified two Food and Drug Administration (FDA)-approved drugs which raised ASL pH by activating SLC26A4. While we identified a role for SLC26A4 in fluid absorption, KO had no effect on cyclic adenosine monophosphate (cAMP)-stimulated fluid secretion in airway organoids. Overall, we have identified a role of TMEM16A in ASL pH homeostasis and shown that both TMEM16A and SLC26A4 could be important alternative targets for ASL pH therapy in CF, particularly for those people who do not produce any functional CFTR.

The revolution of personalized pharmacotherapies for cystic fibrosis: what does the future hold?

INTRODUCTION

Cystic fibrosis (CF), a potentially fatal genetic disease, is caused by loss-of-function mutations in the gene encoding for the CFTR chloride/bicarbonate channel. Modulator drugs rescuing mutant CFTR traffic and function are now in the clinic, providing unprecedented breakthrough therapies for people with CF (PwCF) carrying specific genotypes. However, several CFTR variants are unresponsive to these therapies.

AREA COVERED

We discussed several therapeutic approaches that are under development to tackle the fundamental cause of CF, including strategies targeting defective CFTR mRNA and/or protein expression and function. Alternatively, defective chloride secretion and dehydration in CF epithelia could be restored by exploiting pharmacological modulation of alternative targets, i.e., ion channels/transporters that concur with CFTR to maintain the airway surface liquid homeostasis (e.g., ENaC, TMEM16A, SLC26A4, SLC26A9, and ATP12A). Finally, we assessed progress and challenges in the development of gene-based therapies to replace or correct the mutant CFTR gene.

EXPERT OPINION

CFTR modulators are benefiting many PwCF responsive to these drugs, yielding substantial improvements in various clinical outcomes. Meanwhile, the CF therapy development pipeline continues to expand with the development of novel CFTR modulators and alternative therapeutic strategies with the ultimate goal of providing effective therapies for all PwCF in the foreseeable future.

Pathogenic Relationships in Cystic Fibrosis and Renal Diseases: CFTR, SLC26A9 and Anoctamins

The Cl--transporting proteins CFTR, SLC26A9, and anoctamin (ANO1; ANO6) appear to have more in common than initially suspected, as they all participate in the pathogenic process and clinical outcomes of airway and renal diseases. In the present review, we will therefore concentrate on recent findings concerning electrolyte transport in the airways and kidneys, and the role of CFTR, SLC26A9, and the anoctamins ANO1 and ANO6. Special emphasis will be placed on cystic fibrosis and asthma, as well as renal alkalosis and polycystic kidney disease. In essence, we will summarize recent evidence indicating that CFTR is the only relevant secretory Cl- channel in airways under basal (nonstimulated) conditions and after stimulation by secretagogues. Information is provided on the expressions of ANO1 and ANO6, which are important for the correct expression and function of CFTR. In addition, there is evidence that the Cl- transporter SLC26A9 expressed in the airways may have a reabsorptive rather than a Cl--secretory function. In the renal collecting ducts, bicarbonate secretion occurs through a synergistic action of CFTR and the Cl-/HCO3- transporter SLC26A4 (pendrin), which is probably supported by ANO1. Finally, in autosomal dominant polycystic kidney disease (ADPKD), the secretory function of CFTR in renal cyst formation may have been overestimated, whereas ANO1 and ANO6 have now been shown to be crucial in ADPKD and therefore represent new pharmacological targets for the treatment of polycystic kidney disease.

The SLC26A9 inhibitor S9-A13 provides no evidence for a role of SLC26A9 in airway chloride secretion but suggests a contribution to regulation of ASL pH and gastric proton secretion

The solute carrier 26 family member A9 (SLC26A9) is an epithelial anion transporter that is assumed to contribute to airway chloride secretion and surface hydration. Whether SLC26A9 or CFTR is responsible for airway Cl^- transport under basal conditions is still unclear, due to the lack of a specific inhibitor for SLC26A9. In the present study, we report a novel potent and specific inhibitor for SLC26A9, identified by screening of a drug-like molecule library and subsequent chemical modifications. The most potent compound S9-A13 inhibited SLC26A9 with an IC₅₀ of 90.9 ± 13.4 nM. S9-A13 did not inhibit other members of the SLC26 family and had no effects on Cl^- channels such as CFTR, TMEM16A, or VRAC. S9-A13 inhibited SLC26A9 Cl^- currents in cells that lack expression of CFTR. It also inhibited proton secretion by HGT-1 human gastric cells. In contrast, S9-A13 had minimal effects on ion transport in human airway epithelia and mouse trachea, despite clear expression of SLC26A9 in the apical membrane of ciliated cells. In both tissues, basal and stimulated Cl^- secretion was due to CFTR, while acidification of airway surface liquid by S9-A13 suggests a role of SLC26A9 for airway bicarbonate secretion.

SLC26A9 as a Potential Modifier and Therapeutic Target in Cystic Fibrosis Lung Disease

SLC26A9 belongs to the solute carrier family 26 (SLC26), which comprises membrane proteins involved in ion transport mechanisms. On the basis of different preliminary findings, including the phenotype of SlC26A9-deficient mice and its possible role as a gene modifier of the human phenotype and treatment response, SLC26A9 has emerged as one of the most interesting alternative targets for the treatment of cystic fibrosis (CF). However, despite relevant clues, some open issues and controversies remain. The lack of specific pharmacological modulators, the elusive expression reported in the airways, and its complex relationships with CFTR and the CF phenotype prevent us from conclusively understanding the contribution of SLC26A9 in human lung physiology and its real potential as a therapeutic target in CF. In this review, we summarized the various studies dealing with SLC26A9 expression, molecular structure, and function as an anion channel or transporter; its interaction and functional relationships with CFTR; and its role as a gene modifier and tried to reconcile them in order to highlight the current understanding and the gap in knowledge regarding the contribution of SLC26A9 to human lung physiology and CF disease and treatment.

SLC26A9 is selected for endoplasmic reticulum associated degradation (ERAD) via Hsp70-dependent targeting of the soluble STAS domain

SLC26A9, a member of the solute carrier protein family, transports chloride ions across various epithelia. SLC26A9 also associates with other ion channels and transporters linked to human health, and in some cases these heterotypic interactions are essential to support the biogenesis of both proteins. Therefore, understanding how this complex membrane protein is initially folded might provide new therapeutic strategies to overcome deficits in the function of SLC26A9 partners, one of which is associated with Cystic Fibrosis. To this end, we developed a novel yeast expression system for SLC26A9. This facile system has been used extensively with other ion channels and transporters to screen for factors that oversee protein folding checkpoints. As commonly observed for other channels and transporters, we first noted that a substantial fraction of SLC26A9 is targeted for endoplasmic reticulum associated degradation (ERAD), which destroys folding-compromised proteins in the early secretory pathway. We next discovered that ERAD selection requires the Hsp70 chaperone, which can play a vital role in ERAD substrate selection. We then created SLC26A9 mutants and found that the transmembrane-rich domain of SLC26A9 was quite stable, whereas the soluble cytosolic STAS domain was responsible for Hsp70-dependent ERAD. To support data obtained in the yeast model, we were able to recapitulate Hsp70-facilitated ERAD of the STAS domain in human tissue culture cells. These results indicate that a critical barrier to nascent membrane protein folding can reside within a specific soluble domain, one that is monitored by components associated with the ERAD machinery.

Synergy in Cystic Fibrosis Therapies: Targeting SLC26A9

SLC26A9, a constitutively active Cl^- transporter, has gained interest over the past years as a relevant disease modifier in several respiratory disorders including Cystic Fibrosis (CF), asthma, and non-CF bronchiectasis. SLC26A9 contributes to epithelial Cl^- secretion, thus preventing mucus obstruction under inflammatory conditions. Additionally, SLC26A9 was identified as a CF gene modifier, and its polymorphisms were shown to correlate with the response to drugs modulating CFTR, the defective protein in CF. Here, we aimed to investigate the relationship between SLC26A9 and CFTR, and its role in CF pathogenesis. Our data show that SLC26A9 expression contributes to enhanced CFTR expression and function. While knocking-down SLC26A9 in human bronchial cells leads to lower wt- and F508del-CFTR expression, function, and response to CFTR correctors, the opposite occurs upon its overexpression, highlighting SLC26A9 relevance for CF. Accordingly, F508del-CFTR rescue by the most efficient correctors available is further enhanced by increasing SLC26A9 expression. Interestingly, SLC26A9 overexpression does not increase the PM expression of non-F508del CFTR traffic mutants, namely those unresponsive to corrector drugs. Altogether, our data indicate that SLC26A9 stabilizes CFTR at the ER level and that the efficacy of CFTR modulator drugs may be further enhanced by increasing its expression.

SLC26A9 SNP rs7512462 is not associated with lung disease severity or lung function response to ivacaftor in cystic fibrosis patients with G551D-CFTR

BACKGROUND

The CFTR modulator ivacaftor has been variably effective in treating individuals with cystic fibrosis (CF) who harbor CFTR gating variants such as G551D, as well as other classes of CFTR variants when used with other modulators. Because CFTR genotype does not fully explain this variability, defining genetic modifiers of response to modulator therapy is of particular interest to the field of individualized CF drug therapy. Previous studies have proposed that a variant in SLC26A9 (rs7512462) is associated with lung disease severity and with response to treatment with ivacaftor in individuals with CF who carry G551D or gating variants.

METHODS

Given the implications for CF treatment, we re-examined the reported associations in three cohorts; patients enrolled in the Twin and Siblings study at Johns Hopkins University, the CF modifier study at the University of North Carolina at Chapel Hill, and the prospective G551D Observational (GOAL) study. The GOAL study was specifically designed to measure lung function response to ivacaftor.

RESULTS

We find no association between SLC26A9 (rs7512462) genotype and lung disease severity (n = 272) or change in lung function at one-, three-, and six-month intervals following ivacaftor treatment(n = 141) in individuals with CF who carry at least one G551D variant.

CONCLUSIONS

Our inability to replicate this association indicates that rs7512462 genotype should not be used in treatment decisions.

Choice of Differentiation Media Significantly Impacts Cell Lineage and Response to CFTR Modulators in Fully Differentiated Primary Cultures of Cystic Fibrosis Human Airway Epithelial Cells

In vitro cultures of primary human airway epithelial cells (hAECs) grown at air–liquid interface have become a valuable tool to study airway biology under normal and pathologic conditions, and for drug discovery in lung diseases such as cystic fibrosis (CF). An increasing number of different differentiation media, are now available, making comparison of data between studies difficult. Here, we investigated the impact of two common differentiation media on phenotypic, transcriptomic, and physiological features of CF and non-CF epithelia. Cellular architecture and density were strongly impacted by the choice of medium. RNA-sequencing revealed a shift in airway cell lineage; one medium promoting differentiation into club and goblet cells whilst the other enriched the growth of ionocytes and multiciliated cells. Pathway analysis identified differential expression of genes involved in ion and fluid transport. Physiological assays (intracellular/extracellular pH, Ussing chamber) specifically showed that ATP12A and CFTR function were altered, impacting pH and transepithelial ion transport in CF hAECs. Importantly, the two media differentially affected functional responses to CFTR modulators. We argue that the effect of growth conditions should be appropriately determined depending on the scientific question and that our study can act as a guide for choosing the optimal growth medium for specific applications.

Structural insights into the gating mechanism of human SLC26A9 mediated by its C-terminal sequence

The human SLC26 transporter family exhibits various transport characteristics, and family member SLC26A9 performs multiple roles, including acting as Cl-/HCO3- exchangers, Cl- channels, and Na+ transporters. Some mutations of SLC26A9 are correlated with abnormalities in respiration and digestion systems. As a potential target colocalizing with CFTR in cystic fibrosis patients, SLC26A9 is of great value in drug development. Here, we present a cryo-EM structure of the human SLC26A9 dimer at 2.6 Å resolution. A segment at the C-terminal end is bound to the entry of the intracellular vestibule of the putative transport pathway, which has been proven by electrophysiological experiments to be a gating modulator. Multiple chloride and sodium ions are resolved in the high-resolution structure, identifying novel ion-binding pockets for the first time. Together, our structure takes important steps in elucidating the structural features and regulatory mechanism of SLC26A9, with potential significance in the treatment of cystic fibrosis.

TMEM16A: An Alternative Approach to Restoring Airway Anion Secretion in Cystic Fibrosis?

The concept that increasing airway hydration leads to improvements in mucus clearance and lung function in cystic fibrosis has been clinically validated with osmotic agents such as hypertonic saline and more convincingly with cystic fibrosis transmembrane conductance regulator (CFTR) repair therapies. Although rapidly becoming the standard of care in cystic fibrosis (CF), current CFTR modulators do not treat all patients nor do they restore the rate of decline in lung function to normal levels. As such, novel approaches are still required to ensure all with CF have effective therapies. Although CFTR plays a fundamental role in the regulation of fluid secretion across the airway mucosa, there are other ion channels and transporters that represent viable targets for future therapeutics. In this review article we will summarise the current progress with CFTR-independent approaches to restoring mucosal hydration, including epithelial sodium channel (ENaC) blockade and modulators of SLC26A9. A particular emphasis is given to modulation of the airway epithelial calcium-activated chloride channel (CaCC), TMEM16A, as there is controversy regarding whether it should be positively or negatively modulated. This is discussed in light of a recent report describing for the first time bona fide TMEM16A potentiators and their positive effects upon epithelial fluid secretion and mucus clearance.

Genetic variation in CFTR and modifier loci may modulate cystic fibrosis disease severity

In patients with cystic fibrosis (CF), genetic variants within and outside the CFTR locus contribute to the variability of the disease severity. CFTR transcription is tightly regulated by cis-regulatory elements (CREs) that control the three-dimensional structure of the locus, chromatin accessibility and transcription factor recruitment. Variants within these CREs may contribute to the pathophysiology and to the phenotypic heterogeneity by altering CFTR transcript abundance. In addition to the CREs, variants outside the CFTR locus, namely "modifiers genes", may also be associated with the clinical variability. This review addresses variants at the CFTR locus itself and CFTR CREs, together with the outcomes of the latest modifier gene studies with respect to the different CF phenotypes.

Pendrin Mediates Bicarbonate Secretion and Enhances Cystic Fibrosis Transmembrane Conductance Regulator Function in Airway Surface Epithelia

Bicarbonate facilitates mucin unpacking and bacterial killing; however, its transport mechanisms in the airways are not well understood. cAMP stimulates anion efflux through the cystic fibrosis (CF) transmembrane conductance regulator (CFTR; ABCC7) anion channel, and this is defective in CF. The anion exchanger pendrin (SLC26A4) also mediates HCO₃^- efflux and is upregulated by proinflammatory cytokines. Here, we examined pendrin and CFTR expression and their contributions to HCO₃^- secretion by human nasal and bronchial epithelia. In native tissue, both proteins were most abundant at the apical pole of ciliated surface cells with little expression in submucosal glands. In well-differentiated primary nasal and bronchial cell cultures, IL-4 dramatically increased pendrin mRNA levels and apical immunostaining. Exposure to low-Cl^- apical solution caused intracellular alkalinization (ΔpH_i) that was enhanced fourfold by IL-4 pretreatment. ΔpH_i was unaffected by 4,4'-diisothiocyano-2,2'-stilbenedisulfonic acid (DIDS) or CFTR inhibitor CFTR_inh-172, but was reduced by adenoviral shRNA targeting pendrin. Forskolin increased ΔpH_i, and this stimulation was prevented by CFTR_inh-172, implicating CFTR, yet forskolin only increased ΔpH_i after pendrin expression had been induced by IL-4. The dependence of ΔpH_i on pendrin suggests there is minimal electrical coupling between Cl^- and HCO₃^- fluxes and that CFTR activation increases anion exchange-mediated HCO₃^- influx. Conversely, inducing pendrin expression increased forskolin-stimulated, CFTR_inh-172-sensitive current by approximately twofold in epithelial and nonepithelial cells. We conclude that pendrin mediates most HCO₃^- secretion across airway surface epithelium during inflammation and enhances electrogenic Cl^- secretion via CFTR, as described for other SLC26A transporters.

The anion transporter SLC26A9 localizes to tight junctions and is degraded by the proteasome when co-expressed with F508del-CFTR

Mutations in the gene encoding the cystic fibrosis transmembrane conductance regulator (CFTR) disrupt epithelial secretion and cause cystic fibrosis (CF). Available CFTR modulators provide only modest clinical benefits, so alternative therapeutic targets are being explored. The anion-conducting transporter solute carrier family 26 member 9 (SLC26A9) is a promising candidate, but its functional expression is drastically reduced in cells that express the most common CF-associated CFTR variant, F508del-CFTR, through mechanisms that remain incompletely understood. Here, we examined the metabolic stability and location of SLC26A9 and its relationship to CFTR. Compared with SLC26A9 levels in BHK cells expressing SLC26A9 alone or with WT-CFTR, co-expression of SLC26A9 with F508del-CFTR reduced total and plasma membrane levels of SLC26A9. Proteasome inhibitors increased SLC26A9 immunofluorescence in primary human bronchial epithelial cells (pHBEs) homozygous for F508del-CFTR but not in non-CF pHBEs, suggesting that F508del-CFTR enhances proteasomal SLC26A9 degradation. Apical SLC26A9 expression increased when F508del-CFTR trafficking was partially corrected by low temperature or with the CFTR modulator VX-809. The immature glycoforms of SLC26A9 and CFTR co-immunoprecipitated, consistent with their interaction in the endoplasmic reticulum (ER). Transfection with increasing amounts of WT-CFTR cDNA progressively increased SLC26A9 levels in F508del-CFTR-expressing cells, suggesting that WT-CFTR competes with F508del-CFTR for SLC26A9 binding. Immunofluorescence staining of endogenous SLC26A9 and transfection of a 3HA-tagged construct into well-differentiated cells revealed that SLC26A9 is mostly present at tight junctions. We conclude that SLC26A9 interacts with CFTR in both the ER and Golgi and that its interaction with F508del-CFTR increases proteasomal SLC26A9 degradation.

Emerging Concepts and Therapies for Mucoobstructive Lung Disease

A spectrum of intrapulmonary airway diseases, for example, cigarette smoke-induced bronchitis, cystic fibrosis, primary ciliary dyskinesia, and non-cystic fibrosis bronchiectasis, can be categorized as "mucoobstructive" airway diseases. A common theme for these diseases appears to be the failure to properly regulate mucus concentration, producing mucus hyperconcentration that slows mucus transport and, importantly, generates plaque/plug adhesion to airway surfaces. These mucus plaques/plugs generate long diffusion distances for oxygen, producing hypoxic niches within adherent airway mucus and subjacent epithelia. Data suggest that concentrated mucus plaques/plugs are proinflammatory, in part mediated by release of IL-1α from hypoxic cells. The infectious component of mucoobstructive diseases may be initiated by anaerobic bacteria that proliferate within the nutrient-rich hypoxic mucus environment. Anaerobes ultimately may condition mucus to provide the environment for a succession to classic airway pathogens, including Staphylococcus aureus, Haemophilus influenzae, and ultimately Pseudomonas aeruginosa. Novel therapies to treat mucoobstructive diseases focus on restoring mucus concentration. Strategies to rehydrate mucus range from the inhalation of osmotically active solutes, designed to draw water into airway surfaces, to strategies designed to manipulate the relative rates of sodium absorption versus chloride secretion to endogenously restore epithelial hydration. Similarly, strategies designed to reduce the mucin burden in the airways, either by reducing mucin production/secretion or by clearing accumulated mucus (e.g., reducing agents), are under development. Thus, the new insights into a unifying process, that is, mucus hyperconcentration, that drives a significant component of the pathogenesis of mucoobstructive diseases promise multiple new therapeutic strategies to aid patients with this syndrome.

Alternative chloride transport pathways as pharmacological targets for the treatment of cystic fibrosis

Cystic fibrosis is a hereditary disease that originates from mutations in the epithelial chloride channel CFTR. Whereas established therapies for the treatment of cystic fibrosis target CFTR to repair its function, alternative therapeutic strategies aim for the restoration of chloride transport by the activation of other chloride transport proteins such as TMEM16A or SLC26A9 or by the application of synthetic anionophores. TMEM16A is an anion-selective channel that is activated by the binding of Ca²⁺ from the cytoplasm. Pharmacological efforts aim for the increase of its open probability at resting Ca²⁺ concentrations. SLC26 is an uncoupled chloride transporter, which shuttles chloride across the membrane by an alternate-access mechanism. Its activation requires its mobilization from intracellular stores. Finally, anionophores are small synthetic molecules that bind chloride to form lipid-soluble complexes, which shuttle the anion across the membrane. All three approaches are currently pursued and have provided promising initial results.

pH and male fertility: making sense on pH homeodynamics throughout the male reproductive tract

In the male reproductive tract, ionic equilibrium is essential to maintain normal spermatozoa production and, hence, the reproductive potential. Among the several ions, HCO3- and H+ have a central role, mainly due to their role on pH homeostasis. In the male reproductive tract, the major players in pH regulation and homeodynamics are carbonic anhydrases (CAs), HCO3- membrane transporters (solute carrier 4-SLC4 and solute carrier 26-SLC26 family transporters), Na+-H+ exchangers (NHEs), monocarboxylate transporters (MCTs) and voltage-gated proton channels (Hv1). CAs and these membrane transporters are widely distributed throughout the male reproductive tract, where they play essential roles in the ionic balance of tubular fluids. CAs are the enzymes responsible for the production of HCO3- which is then transported by membrane transporters to ensure the maturation, storage, and capacitation of the spermatozoa. The transport of H+ is carried out by NHEs, Hv1, and MCTs and is essential for the electrochemical balance and for the maintenance of the pH within the physiological limits along the male reproductive tract. Alterations in HCO3- production and transport of ions have been associated with some male reproductive dysfunctions. Herein, we present an up-to-date review on the distribution and role of the main intervenient on pH homeodynamics in the fluids throughout the male reproductive tract. In addition, we discuss their relevance for the establishment of the male reproductive potential.

Cryo-EM structures and functional characterization of murine Slc26a9 reveal mechanism of uncoupled chloride transport

The epithelial anion transporter SLC26A9 contributes to airway surface hydration and gastric acid production. Colocalizing with CFTR, SLC26A9 has been proposed as a target for the treatment of cystic fibrosis. To provide molecular details of its transport mechanism, we present cryo-EM structures and a functional characterization of murine Slc26a9. These structures define the general architecture of eukaryotic SLC26 family members and reveal an unusual mode of oligomerization which relies predominantly on the cytosolic STAS domain. Our data illustrates conformational transitions of Slc26a9, supporting a rapid alternate-access mechanism which mediates uncoupled chloride transport with negligible bicarbonate or sulfate permeability. The characterization of structure-guided mutants illuminates the properties of the ion transport path, including a selective anion binding site located in the center of a mobile module within the transmembrane domain. This study thus provides a structural foundation for the understanding of the entire SLC26 family and potentially facilitates their therapeutic exploitation.

Many processes in the human body are regulated by chloride and other charged particles (known as ions) moving in and out of cells. Each cell is surrounded by a membrane barrier, which prevents ions from entering or exiting. Therefore, to control the levels of ions inside the cell, specific proteins in the membrane act as channels or transporters to provide routes for the ions to pass through the membrane.

Channel proteins form pores that, when open, allow a steady stream of ions to pass through the membrane. Transporter proteins, on the other hand, generally contain a pocket that is only accessible from one side of the membrane. When individual ions enter this pocket the transporter changes shape. This causes the entrance of the pocket to close and then re-open on the other side of the membrane.

Inside the lung, an ion channel known as CFTR provides a route for chloride ions to move out of cells, which helps clear harmful material from the airways. Mutations affecting this protein cause the mucus lining the airways to become very sticky, leading to a severe disease known as cystic fibrosis. CFTR works together with another protein that is also found in the membrane, called SLC26A9. Previous studies have suggested that SLC26A9 also allows chloride ions to pass through the membrane. It was not clear, however, if SLC26A9 operates as an ion channel or a transporter protein, or how the protein is arranged in the membrane.

Now, Walter, Sawicka and Dutzler combined two techniques known as cryo-electron microscopy and patch-clamp electrophysiology to reveal the detailed three-dimensional structure of the mouse version of SLC26A9, which is highly similar to the human form. The experiments found that mouse SLC26A9 proteins form pairs in the membrane referred to as homodimers, which arranged themselves in an unexpected way. Further investigation into the structure of these homodimers suggests that despite having many channel-like properties, SLC26A9 operates as a fast transporter, rather than a true channel.

These findings help us understand the role of SLC26A9 and other similar proteins in the lung and other parts of the body. In the future it may be possible to develop drugs that target SLC26A9 to treat cystic fibrosis and other severe lung diseases.

Genetic association and transcriptome integration identify contributing genes and tissues at cystic fibrosis modifier loci

Cystic Fibrosis (CF) exhibits morbidity in several organs, including progressive lung disease in all patients and intestinal obstruction at birth (meconium ileus) in ~15%. Individuals with the same causal CFTR mutations show variable disease presentation which is partly attributed to modifier genes. With >6,500 participants from the International CF Gene Modifier Consortium, genome-wide association investigation identified a new modifier locus for meconium ileus encompassing ATP12A on chromosome 13 (min p = 3.83x10(-10)); replicated loci encompassing SLC6A14 on chromosome X and SLC26A9 on chromosome 1, (min p<2.2x10(-16), 2.81x10(-11), respectively); and replicated a suggestive locus on chromosome 7 near PRSS1 (min p = 2.55x10(-7)). PRSS1 is exclusively expressed in the exocrine pancreas and was previously associated with non-CF pancreatitis with functional characterization demonstrating impact on PRSS1 gene expression. We thus asked whether the other meconium ileus modifier loci impact gene expression and in which organ. We developed and applied a colocalization framework called the Simple Sum (SS) that integrates regulatory and genetic association information, and also contrasts colocalization evidence across tissues or genes. The associated modifier loci colocalized with expression quantitative trait loci (eQTLs) for ATP12A (p = 3.35x10(-8)), SLC6A14 (p = 1.12x10(-10)) and SLC26A9 (p = 4.48x10(-5)) in the pancreas, even though meconium ileus manifests in the intestine. The meconium ileus susceptibility locus on chromosome X appeared shifted in location from a previously identified locus for CF lung disease severity. Using the SS we integrated the lung disease association locus with eQTLs from nasal epithelia of 63 CF participants and demonstrated evidence of colocalization with airway-specific regulation of SLC6A14 (p = 2.3x10(-4)). Cystic Fibrosis is realizing the promise of personalized medicine, and identification of the contributing organ and understanding of tissue specificity for a gene modifier is essential for the next phase of personalizing therapeutic strategies.

A Neutralizing Aptamer to TGFBR2 and miR-145 Antagonism Rescue Cigarette Smoke- and TGF-β-Mediated CFTR Expression

Transforming growth factor β (TGF-β), signaling induced by cigarette smoke (CS), plays an important role in the progression of airway diseases, like chronic bronchitis associated with chronic obstructive pulmonary disease (COPD), and in smokers. Chronic bronchitis is characterized by reduced mucociliary clearance (MCC). Cystic fibrosis transmembrane conductance regulator (CFTR) plays an important role in normal MCC. TGF-β and CS (via TGF-β) promote acquired CFTR dysfunction by suppressing CFTR biogenesis and function. Understanding the mechanism by which CS promotes CFTR dysfunction can identify therapeutic leads to reverse CFTR suppression and rescue MCC. TGF-β alters the microRNAome of primary human bronchial epithelium. TGF-β and CS upregulate miR-145-5p expression to suppress CFTR and the CFTR modifier, SLC26A9. miR-145-5p upregulation with a concomitant CFTR and SLC26A9 suppression was validated in CS-exposed mouse models. While miR-145-5p antagonism rescued the effects of TGF-β in bronchial epithelial cells following transfection, an aptamer to block TGF-β signaling rescues CS- and TGF-β-mediated suppression of CFTR biogenesis and function in the absence of any transfection reagent. These results demonstrate that miR-145-5p plays a significant role in acquired CFTR dysfunction by CS, and they validate a clinically feasible strategy for delivery by inhalation to locally modulate TGF-β signaling in the airway and rescue CFTR biogenesis and function.

Role of the SLC26A9 Chloride Channel as Disease Modifier and Potential Therapeutic Target in Cystic Fibrosis

The solute carrier family 26, member 9 (SLC26A9) is an epithelial chloride channel that is expressed in several organs affected in patients with cystic fibrosis (CF) including the lungs, the pancreas, and the intestine. Emerging evidence suggests SLC26A9 as a modulator of wild-type and mutant CFTR function, and as a potential alternative target to circumvent the basic ion transport defect caused by deficient CFTR-mediated chloride transport in CF. In this review, we summarize in vitro studies that revealed multifaceted molecular and functional interactions between SLC26A9 and CFTR that may be implicated in normal transepithelial chloride secretion in health, as well as impaired chloride/fluid transport in CF. Further, we focus on recent genetic association studies and investigations utilizing genetically modified mouse models that identified SLC26A9 as a disease modifier and supported an important role of this alternative chloride channel in the pathophysiology of several organ manifestations in CF, as well as other chronic lung diseases such as asthma and non-CF bronchiectasis. Collectively, these findings and the overlapping endogenous expression with CFTR suggest SLC26A9 an attractive novel therapeutic target that may be exploited to restore epithelial chloride secretion in patients with CF irrespective of their CFTR genotype. In addition, pharmacological activation of SLC26A9 may help to augment the effect of CFTR modulator therapies in patients with CF carrying responsive mutations such as the most common disease-causing mutation F508del-CFTR. However, future research and development including the identification of compounds that activate SLC26A9-mediated chloride transport are needed to explore this alternative chloride channel as a therapeutic target in CF and potentially other muco-obstructive lung diseases.

Slippery When Wet: Airway Surface Liquid Homeostasis and Mucus Hydration

The ability to regulate cell volume is crucial for normal physiology; equally the regulation of extracellular fluid homeostasis is of great importance. Alteration of normal extracellular fluid homeostasis contributes to the development of several diseases including cystic fibrosis. With regard to the airway surface liquid (ASL), which lies apically on top of airway epithelia, ion content, pH, mucin and protein abundance must be tightly regulated. Furthermore, airway epithelia must be able to switch from an absorptive to a secretory state as required. A heterogeneous population of airway epithelial cells regulate ASL solute and solvent composition, and directly secrete large mucin molecules, antimicrobials, proteases and soluble mediators into the airway lumen. This review focuses on how epithelial ion transport influences ASL hydration and ASL pH, with a specific focus on the roles of anion and cation channels and exchangers. The role of ions and pH in mucin expansion is also addressed. With regard to fluid volume regulation, we discuss the roles of nucleotides, adenosine and the short palate lung and nasal epithelial clone 1 (SPLUNC1) as soluble ASL mediators. Together, these mechanisms directly influence ciliary beating and in turn mucociliary clearance to maintain sterility and to detoxify the airways. Whilst all of these components are regulated in normal airways, defective ion transport and/or mucin secretion proves detrimental to lung homeostasis as such we address how defective ion and fluid transport, and a loss of homeostatic mechanisms, contributes to the development of pathophysiologies associated with cystic fibrosis.

Physiological and Pathophysiological Relevance of the Anion Transporter Slc26a9 in Multiple Organs

Transepithelial Cl^- and HCO₃^- transport is crucial for the function of all epithelia, and HCO₃^- is a biological buffer that maintains acid-base homeostasis. In most epithelia, a series of Cl^-/HCO₃^- exchangers and Cl^- channels that mediate Cl^- absorption and HCO₃^- secretion have been detected in the luminal and basolateral membranes. Slc26a9 belongs to the solute carrier 26 (Slc26) family of anion transporters expressed in the epithelia of multiple organs. This review summarizes the expression pattern and functional diversity of Slc26a9 in different systems based on all investigations performed thus far. Furthermore, the physical and functional interactions between Slc26a9 and cystic fibrosis transmembrane conductance regulator (CFTR) are discussed due to their overlapping expression pattern in multiple organs. Finally, we focus on the relationship between slc26a9 mutations and disease onset. An understanding of the physiological and pathophysiological relevance of Slc26a9 in multiple organs offers new possibilities for disease therapy.

Recent advances in developing therapeutics for cystic fibrosis

Despite hope that a cure was imminent when the causative gene was cloned nearly 30 years ago, cystic fibrosis (CF [MIM: 219700]) remains a life-shortening disease affecting more than 70 000 individuals worldwide. However, within the last 6 years the Food and Drug Administration's approval of Ivacaftor, the first drug that corrects the defective cystic fibrosis transmembrane conductance regulator protein [CFTR (MIM: 602421)] in patients with the G551D mutation, marks a watershed in the development of novel therapeutics for this devastating disease. Here we review recent progress in diverse research areas, which all focus on curing CF at the genetic, biochemical or physiological level. In the near future it seems probable that development of mutation-specific therapies will be the focus, since it is unlikely that any one approach will be efficient in correcting the more than 2000 disease-associated variants. We discuss the new drugs and combinations of drugs that either enhance delivery of misfolded CFTR protein to the cell membrane, where it functions as an ion channel, or that activate channel opening. Next we consider approaches to correct the causative genetic lesion at the DNA or RNA level, through repressing stop mutations and nonsense-mediated decay, modulating splice mutations, fixing errors by gene editing or using novel routes to gene replacement. Finally, we explore how modifier genes, loci elsewhere in the genome that modify CF disease severity, may be used to restore a normal phenotype. Progress in all of these areas has been dramatic, generating enthusiasm that CF may soon become a broadly treatable disease.

SLC26A9 Gene Is Associated With Lung Function Response to Ivacaftor in Patients With Cystic Fibrosis

Ivacaftor is a drug used to treat cystic fibrosis (CF) patients carrying specific gating CFTR mutations. Interpatient variability in the lung response has been shown to be partly explained by rs7512462 in the Solute Carrier Family 26 Member 9 (SLC26A9) gene. In an independent and larger cohort, we aimed to evaluate whether SLC26A9 variants contribute to the variability of the lung phenotype and if they influence the lung response to ivacaftor. We genotyped the French CF Gene Modifier Study cohort (n = 4,840) to investigate whether SLC26A9 variants were involved in the lung phenotype heterogeneity. Their influence in the response to ivacaftor was tested in the 30 treated patients who met the inclusion criteria: older than 6 years of age, percent-predicted forced expiratory volume measured in 1 s (FEV_1pp) in the 3 months before treatment initiation ranging between 40 and 90%. Response to treatment was determined by the change in FEV_1pp from baseline, averaged in 15–75 days, and the 1st-year post-treatment. We observed that SLC26A9 variants were not associated with lung function variability in untreated patients and that gain of lung function in patients treated with ivacaftor was similar to clinical trials. We confirmed that rs7512462 was associated with variability in ivacaftor-lung response, with a significant reduction in lung function improvement for patients with the C allele. Other SLC26A9 SNPs also contributed to the ivacaftor-response. Interindividual variability in lung response to ivacaftor is associated with SLC26A9 variants in French CF patients. Pharmacogenomics and personalized medicine will soon be part of CF patient care.

Extracellular pH and lung infections in cystic fibrosis

Cystic fibrosis (CF) is an autosomal recessive disease caused by CFTR mutations. It is characterized by high NaCl concentration in sweat and the production of a thick and sticky mucus, occluding secretory ducts, intestine and airways, accompanied by chronic inflammation and infections of the lungs. This causes a progressive and lethal decline in lung function. Therefore, finding the mechanisms driving the high susceptibility to lung infections has been a key issue. For decades the prevalent hypothesis was that a reduced airway surface liquid (ASL) volume and composition, and the consequent increased mucus concentration (dehydration), create an environment favoring infections. However, a few years ago, in a pig model of CF, the Na⁺/K⁺ concentrations and the ASL volume were found intact. Immediately a different hypothesis arose, postulating a reduced ASL pH as the cause for the increased susceptibility to infections, due to a diminished bicarbonate secretion through CFTR. Noteworthy, a recent report found normal ASL pH values in CF children and in cultured primary airway cells, challenging the ASL pH hypothesis. On the other hand, recent evidences revitalized the hypothesis of a reduced ASL secretion. Thus, the role of the ASL pH in the CF is still a controversial matter. In this review we discuss the basis that sustain the role of CFTR in modulating the extracellular pH, and the recent results sustaining the different points of view. Finding the mechanisms of CFTR signaling that determine the susceptibility to infections is crucial to understand the pathophysiology of CF and related lung diseases.

Lipophilicity of the Cystic Fibrosis Drug, Ivacaftor (VX-770), and Its Destabilizing Effect on the Major CF-causing Mutation: F508del

Deletion of phenylalanine at position 508 (F508del) in cystic fibrosis transmembrane conductance regulator (CFTR) is the most common cystic fibrosis (CF)-causing mutation. Recently, ORKAMBI, a combination therapy that includes a corrector of the processing defect of F508del-CFTR (lumacaftor or VX-809) and a potentiator of channel activity (ivacaftor or VX-770), was approved for CF patients homozygous for this mutation. However, clinical studies revealed that the effect of ORKAMBI on lung function is modest and it was proposed that this modest effect relates to a negative impact of VX-770 on the stability of F508del-CFTR. In the current studies, we showed that this negative effect of VX-770 at 10 μM correlated with its inhibitory effect on VX-809-mediated correction of the interface between the second membrane spanning domain and the first nucleotide binding domain bearing F508del. Interestingly, we found that VX-770 exerted a similar negative effect on the stability of other membrane localized solute carriers (SLC26A3, SLC26A9, and SLC6A14), suggesting that this negative effect is not specific for F508del-CFTR. We determined that the relative destabilizing effect of a panel of VX-770 derivatives on F508del-CFTR correlated with their predicted lipophilicity. Polarized total internal reflection fluorescence microscopy on a supported lipid bilayer model shows that VX-770, and not its less lipophilic derivative, increased the fluidity of and reorganized the membrane. In summary, our findings show that there is a potential for nonspecific effects of VX-770 on the lipid bilayer and suggest that this effect may account for its destabilizing effect on VX-809- rescued F508del-CFTR.

Translational research to enable personalized treatment of cystic fibrosis

Translational research efforts in cystic fibrosis (CF) aim to develop therapies for all subjects with CF. To reach this goal new therapies need to be developed that target multiple aspects of the disease. To enable individuals to benefit maximally from these treatments will require improved methods to tailor these therapies specifically to individuals who suffer from CF. This report highlights current examples of translational CF research efforts to reach this goal. The use of intestinal organoids and genetics to better understand individual assessment of CFTR modulator treatment effects to ultimately enable a better personalized treatment for CF subjects will be discussed. In addition, development of viral vectors and non-viral synthetic nanoparticles for delivery of mRNA, sgRNA and DNA will be highlighted. New approaches to restore function of CFTR with early premature termination codons using nanoparticle delivery of suppressor tRNAs and new insights into mechanisms of airway epithelial repair will be reviewed as well. The state-of-the-art approaches that are discussed in this review demonstrate significant progress towards the development of optimal individual therapies for CF patients, but also reveal that remaining challenges still lie ahead.

A missense mutation in SLC26A3 is associated with human male subfertility and impaired activation of CFTR

Chloride absorption and bicarbonate excretion through exchange by the solute carrier family 26 member 3 (SLC26A3) and cystic fibrosis transmembrane conductance regulator (CFTR) are crucial for many tissues including sperm and epithelia of the male reproductive tract. Homozygous SLC26A3 mutations cause congenital chloride diarrhea with male subfertility, while homozygous CFTR mutations cause cystic fibrosis with male infertility. Some homozygous or heterozygous CFTR mutations only manifest as male infertility. Accordingly, we studied the influence of SLC26A3 on idiopathic infertility by sequencing exons of SLC26A3 in 283 infertile and 211 control men. A heterozygous mutation c.2062 G > C (p.Asp688His) appeared in nine (3.2%) infertile men, and additionally, in two (0.9%) control men, whose samples revealed a sperm motility defect. The p.Asp688His mutation is localized in the CFTR-interacting STAS domain of SLC26A3 and enriched in Finland, showing a significant association with male infertility in comparison with 6,572 Finnish (P < 0.05) and over 120,000 global alleles (P < 0.0001) (ExAC database). Functional studies showed that while SLC26A3 is a strong activator of CFTR-dependent anion transport, SLC26A3-p.Asp688His mutant retains normal Cl⁻/HCO₃⁻ exchange activity but suppresses CFTR, despite unaffected domain binding and expression. These results suggest a novel mechanism for human male infertility─impaired anion transport by the coupled SLC26A3 and CFTR.

Bypassing CFTR dysfunction in cystic fibrosis with alternative pathways for anion transport

One therapeutic strategy for cystic fibrosis (CF) seeks to restore anion transport to affected epithelia by targeting other apical membrane Cl^- channels to bypass dysfunction of the cystic fibrosis transmembrane conductance regulator (CFTR) Cl^- channel. The properties and regulation of the Ca²⁺-activated Cl^- channel TMEM16A argue that long-acting small molecules which target directly TMEM16A are required to overcome CFTR loss. Through genetic studies of lung diseases, SLC26A9, a member of the solute carrier 26 family of anion transporters, has emerged as a promising target to bypass CFTR dysfunction. An alternative strategy to circumvent CFTR dysfunction is to deliver to CF epithelia artificial anion transporters that shuttle Cl^- across the apical membrane. Recently, powerful, non-toxic, biologically-active artificial anion transporters have emerged.

Genetic abnormalities leading to qualitative defects of sperm morphology or function

Infertility, defined by the inability of conceiving a child after 1 year is estimated to concern approximately 50 million couples worldwide. As the male gamete is readily accessible and can be studied by a simple spermogram it is easier to subcategorize male than female infertility. Subjects with a specific sperm phenotype are more likely to have a common origin thus facilitating the search for causal factors. Male infertility is believed to be often multifactorial and caused by both genetic and extrinsic factors, but severe cases of male infertility are likely to have a predominant genetic etiology. Patients presenting with a monomorphic teratozoospermia such as globozoospermia or macrospermia with more than 85% of the spermatozoa presenting this specific abnormality have been analyzed permitting to identify several key genes for spermatogenesis such as AURKC and DPY19L2. The study of patients with other specific sperm anomalies such as severe alteration of sperm motility, in particular multiple morphological anomalies of the sperm flagella (MMAF) or sperm unability to fertilize the oocyte (oocyte activation failure syndrome) has also enable the identification of new infertility genes. Here we review the recent works describing the identification and characterization of gene defects having a direct qualitative effect on sperm morphology or function.

Cystic fibrosis gene modifier SLC26A9 modulates airway response to CFTR-directed therapeutics

Cystic fibrosis is realizing the promise of personalized medicine. Recent advances in drug development that target the causal CFTR directly result in lung function improvement, but variability in response is demanding better prediction of outcomes to improve management decisions. The genetic modifier SLC26A9 contributes to disease severity in the CF pancreas and intestine at birth and here we assess its relationship with disease severity and therapeutic response in the airways. SLC26A9 association with lung disease was assessed in individuals from the Canadian and French CF Gene Modifier consortia with CFTR-gating mutations and in those homozygous for the common Phe508del mutation. Variability in response to a CFTR-directed therapy attributed to SLC26A9 genotype was assessed in Canadian patients with gating mutations. A primary airway model system determined if SLC26A9 shows modification of Phe508del CFTR function upon treatment with a CFTR corrector. In those with gating mutations that retain cell surface-localized CFTR we show that SLC26A9 modifies lung function while this is not the case in individuals homozygous for Phe508del where cell surface expression is lacking. Treatment response to ivacaftor, which aims to improve CFTR-channel opening probability in patients with gating mutations, shows substantial variability in response, 28% of which can be explained by rs7512462 in SLC26A9 (P = 0.0006). When homozygous Phe508del primary bronchial cells are treated to restore surface CFTR, SLC26A9 likewise modifies treatment response (P = 0.02). Our findings indicate that SLC26A9 airway modification requires CFTR at the cell surface, and that a common variant in SLC26A9 may predict response to CFTR-directed therapeutics.

Generation and functional characterization of epithelial cells with stable expression of SLC26A9 Cl- channels

Recent studies identified the SLC26A9 Cl(-) channel as a modifier and potential therapeutic target in cystic fibrosis (CF). However, understanding of the regulation of SLC26A9 in epithelia remains limited and cellular models with stable expression for biochemical and functional studies are missing. We, therefore, generated Fisher rat thyroid (FRT) epithelial cells with stable expression of HA-tagged SLC26A9 via retroviral transfection and characterized SLC26A9 expression and function using Western blotting, immunolocalization, whole cell patch-clamp, and transepithelial bioelectric studies in Ussing chambers. We demonstrate stable expression of SLC26A9 in transfected FRT (SLC26A9-FRT) cells on the mRNA and protein level. Immunolocalization and Western blotting detected SLC26A9 in different intracellular compartments and to a lesser extent at the cell surface. Whole cell patch-clamp recordings demonstrated significantly increased constitutive Cl(-) currents in SLC26A9-FRT compared with control-transduced FRT (Control-FRT) cells (P < 0.01). Similar, transepithelial measurements showed that the basal short circuit current was significantly increased in SLC26A9-FRT vs. Control-FRT cell monolayers (P < 0.01). SLC26A9-mediated Cl(-) currents were increased by cAMP-dependent stimulation (IBMX and forskolin) and inhibited by GlyH-101, niflumic acid, DIDS, and 5-nitro-2-(3-phenylpropylamino) benzoic acid (NPPB), as well as RNAi knockdown of WNK1 implicated in epithelial osmoregulation. Our results support that these novel epithelial cells with stable expression of SLC26A9 will be a useful model for studies of pharmacological regulation including the identification of activators of SLC26A9 Cl(-) channels that may compensate deficient cystic fibrosis transmembrane regulator (CFTR)-mediated Cl(-) secretion and serve as an alternative therapeutic target in patients with CF and potentially other muco-obstructive lung diseases.

Novel Roles for Chloride Channels, Exchangers, and Regulators in Chronic Inflammatory Airway Diseases

Chloride transport proteins play critical roles in inflammatory airway diseases, contributing to the detrimental aspects of mucus overproduction, mucus secretion, and airway constriction. However, they also play crucial roles in contributing to the innate immune properties of mucus and mucociliary clearance. In this review, we focus on the emerging novel roles for a chloride channel regulator (CLCA1), a calcium-activated chloride channel (TMEM16A), and two chloride exchangers (SLC26A4/pendrin and SLC26A9) in chronic inflammatory airway diseases.

Targeting ion channels in cystic fibrosis

Mutations in the cystic fibrosis transmembrane conductance regulator (CFTR) gene cause a characteristic defect in epithelial ion transport that plays a central role in the pathogenesis of cystic fibrosis (CF). Hence, pharmacological correction of this ion transport defect by targeting of mutant CFTR, or alternative ion channels that may compensate for CFTR dysfunction, has long been considered as an attractive approach to a causal therapy of this life-limiting disease. The recent introduction of the CFTR potentiator ivacaftor into the therapy of a subgroup of patients with specific CFTR mutations was a major milestone and enormous stimulus for seeking effective ion transport modulators for all patients with CF. In this review, we discuss recent breakthroughs and setbacks with CFTR modulators designed to rescue mutant CFTR including the common mutation F508del. Further, we examine the alternative chloride channels TMEM16A and SLC26A9, as well as the epithelial sodium channel ENaC as alternative targets in CF lung disease, which remains the major cause of morbidity and mortality in patients with CF. Finally, we will focus on the hurdles that still need to be overcome to make effective ion transport modulation therapies available for all patients with CF irrespective of their CFTR genotype.

Variants in Solute Carrier SLC26A9 Modify Prenatal Exocrine Pancreatic Damage in Cystic Fibrosis

OBJECTIVES

To test the hypothesis that multiple constituents of the apical plasma membrane residing alongside the causal cystic fibrosis (CF) transmembrane conductance regulator protein, including known CF modifiers SLC26A9, SLC6A14, and SLC9A3, would be associated with prenatal exocrine pancreatic damage as measured by newborn screened (NBS) immunoreactive trypsinogen (IRT) levels.

STUDY DESIGN

NBS IRT measures and genome-wide genotype data were available on 111 subjects from Colorado, 37 subjects from Wisconsin, and 80 subjects from France. Multiple linear regression was used to determine whether any of 8 single nucleotide polymorphisms (SNPs) in SLC26A9, SLC6A14, and SLC9A3 were associated with IRT and whether other constituents of the apical plasma membrane contributed to IRT.

RESULTS

In the Colorado sample, 3 SLC26A9 SNPs were associated with NBS IRT (min P=1.16×10(-3); rs7512462), but no SLC6A14 or SLC9A3 SNPs were associated (P>.05). The rs7512462 association replicated in the Wisconsin sample (P=.03) but not in the French sample (P=.76). Furthermore, rs7512462 was the top-ranked apical membrane constituent in the combined Colorado and Wisconsin sample.

CONCLUSIONS

NBS IRT is a biomarker of prenatal exocrine pancreatic disease in patients with CF, and a SNP in SLC26A9 accounts for significant IRT variability. This work suggests SLC26A9 as a potential therapeutic target to ameliorate exocrine pancreatic disease.

Bicarbonate transport in health and disease

Bicarbonate (HCO3(-)) has a central place in human physiology as the waste product of mitochondrial energy production and for its role in pH buffering throughout the body. Because bicarbonate is impermeable to membranes, bicarbonate transport proteins are necessary to enable control of bicarbonate levels across membranes. In humans, 14 bicarbonate transport proteins, members of the SLC4 and SLC26 families, function by differing transport mechanisms. In addition, some anion channels and ZIP metal transporters contribute to bicarbonate movement across membranes. Defective bicarbonate transport leads to diseases, including systemic acidosis, brain dysfunction, kidney stones, and hypertension. Altered expression levels of bicarbonate transporters in patients with breast, colon, and lung cancer suggest an important role of these transporters in cancer.

Loss of Slc26a9 anion transporter alters intestinal electrolyte and HCO3(-) transport and reduces survival in CFTR-deficient mice

Slc26a9 is an anion transporter that is strongly expressed in the stomach and lung. Slc26a9 variants were recently found associated with a higher incidence of meconium ileus in cystic fibrosis (CF) infants, raising the question whether Slc26a9 is expressed in the intestine and what its functional role is. Slc26a9 messenger RNA (mRNA) was found highly expressed in the mucosae of the murine and human upper gastrointestinal tract, with an abrupt decrease in expression levels beyond the duodenum. Absence of SLC26a9 expression strongly increased the intestinally related mortality in cystic fibrosis transmembrane conductance regulator (CFTR)-deficient mice. Proximal duodenal JHCO3(-) and fluid secretion were reduced in the absence of Slc26a9 expression. In the proximal duodenum of young Slc26a9 KO mice, the glands and villi/crypts were elongated and proliferation was enhanced. This difference was lost with ageing, as were the alterations in fluid movement, whereas the reduction in JHCO3(-) remained. Laser dissection followed by qPCR suggested Slc26a9 expression to be crypt-predominant in the duodenum. In summary, deletion of Slc26a9 caused bicarbonate secretory and fluid absorptive changes in the proximal duodenal mucosa and increased the postweaning death rates in CFTR-deficient mice. Functional alterations in the duodenum were most prominent at young ages. We assume that the association of meconium ileus and Slc26a9 variants may be related to maldigestion and impaired downstream signaling caused by loss of upper GI tract digestive functions, aggravating the situation of lack of secretion and sticky mucus at the site of obstruction in CF intestine.

Functional interaction of the cystic fibrosis transmembrane conductance regulator with members of the SLC26 family of anion transporters (SLC26A8 and SLC26A9): physiological and pathophysiological relevance

The solute carrier 26 (SLC26) proteins are transmembrane proteins located at the plasma membrane of the cells and transporting a variety of monovalent and divalent anions, including chloride, bicarbonate, sulfate and oxalate. In humans, 11 members have been identified (SLC26A1 to SLC26A11) and although part of them display a very restricted tissue expression pattern, altogether they are widely expressed in the epithelial cells of the body where they contribute to the composition and the pH regulation of the secreted fluids. Importantly, mutations in SLC26A2, A3, A4, and A5 have been associated with distinct human genetic recessive disorders (i.e. diastrophic dysplasia, congenital chloride diarrhea, Pendred syndrome and deafness, respectively), demonstrating their essential and non-redundant functions in many tissues. During the last decade, physical and functional interactions of SLC26 members with the cystic fibrosis transmembrane conductance regulator (CFTR) have been highly documented, leading to the model of a crosstalk based on the binding of the SLC26 STAS domain to the CFTR regulatory domain. In this review, we will focus on the functional interaction of SLC26A8 and SLC26A9 with the CFTR channel. In particular we will highlight the newly published studies indicating that mutations in SLC26A8 and SLC26A9 proteins are associated with a deregulation of the CFTR anion transport activity in the pathophysiological context of the sperm and the pulmonary cells. These studies confirm the physiological relevance of SLC26 and CFTR cross-regulation, opening new gates for the treatment of cystic fibrosis.

Mechanism and synergism in epithelial fluid and electrolyte secretion

A central function of epithelia is the control of the volume and electrolyte composition of bodily fluids through vectorial transport of electrolytes and the obligatory H2O. In exocrine glands, fluid and electrolyte secretion is carried out by both acinar and duct cells, with the portion of fluid secreted by each cell type varying among glands. All acinar cells secrete isotonic, plasma-like fluid, while the duct determines the final electrolyte composition of the fluid by absorbing most of the Cl(-) and secreting HCO3 (-). The key transporters mediating acinar fluid and electrolyte secretion are the basolateral Na(+)/K(+) /2Cl(-) cotransporter, the luminal Ca(2+)-activated Cl(-) channel ANO1 and basolateral and luminal Ca(2+)-activated K(+) channels. Ductal fluid and HCO3 (-) secretion are mediated by the basolateral membrane Na(+)-HCO3 (-) cotransporter NBCe1-B and the luminal membrane Cl(-)/HCO3 (-) exchanger slc26a6 and the Cl(-) channel CFTR. The function of the transporters is regulated by multiple inputs, which in the duct include major regulation by the WNK/SPAK pathway that inhibit secretion and the IRBIT/PP1 pathway that antagonize the effects of the WNK/SPAK pathway to both stimulate and coordinate the secretion. The function of these regulatory pathways in secretory glands acinar cells is yet to be examined. An important concept in biology is synergism among signaling pathways to generate the final physiological response that ensures regulation with high fidelity and guards against cell toxicity. While synergism is observed in all epithelial functions, the molecular mechanism mediating the synergism is not known. Recent work reveals a central role for IRBIT as a third messenger that integrates and synergizes the function of the Ca(2+) and cAMP signaling pathways in activation of epithelial fluid and electrolyte secretion. These concepts are discussed in this review using secretion by the pancreatic and salivary gland ducts as model systems.

Genetic modifiers of cystic fibrosis-related diabetes

Diabetes is a common age-dependent complication of cystic fibrosis (CF) that is strongly influenced by modifier genes. We conducted a genome-wide association study in 3,059 individuals with CF (644 with CF-related diabetes [CFRD]) and identified single nucleotide polymorphisms (SNPs) within and 5' to the SLC26A9 gene that associated with CFRD (hazard ratio [HR] 1.38; P = 3.6 × 10(-8)). Replication was demonstrated in 694 individuals (124 with CFRD) (HR, 1.47; P = 0.007), with combined analysis significant at P = 9.8 × 10(-10). SLC26A9 is an epithelial chloride/bicarbonate channel that can interact with the CF transmembrane regulator (CFTR), the protein mutated in CF. We also hypothesized that common SNPs associated with type 2 diabetes also might affect risk for CFRD. A previous association of CFRD with SNPs in TCF7L2 was replicated in this study (P = 0.004; combined analysis P = 3.8 × 10(-6)), and type 2 diabetes SNPs at or near CDKAL1, CDKN2A/B, and IGF2BP2 were associated with CFRD (P < 0.004). These five loci accounted for 8.3% of the phenotypic variance in CFRD onset and had a combined population-attributable risk of 68%. Diabetes is a highly prevalent complication of CF, for which susceptibility is determined in part by variants at SLC26A9 (which mediates processes proximate to the CF disease-causing gene) and at four susceptibility loci for type 2 diabetes in the general population.

Unraveling the complex genetic model for cystic fibrosis: pleiotropic effects of modifier genes on early cystic fibrosis-related morbidities

The existence of pleiotropy in disorders with multi-organ involvement can suggest therapeutic targets that could ameliorate overall disease severity. Here we assessed pleiotropy of modifier genes in cystic fibrosis (CF). CF, caused by mutations in the cystic fibrosis transmembrane conductance regulator (CFTR) gene, affects the lungs, liver, pancreas and intestines. However, modifier genes contribute to variable disease severity across affected organs, even in individuals with the same CFTR genotype. We sought to determine whether SLC26A9, SLC9A3 and SLC6A14, that contribute to meconium ileus in CF, are pleiotropic for other early-affecting CF co-morbidities. In the Canadian CF population, we assessed evidence for pleiotropic effects on (1) pediatric lung disease severity (n = 815), (2) age at first acquisition of Pseudomonas aeruginosa (P. aeruginosa) (n = 730), and (3) prenatal pancreatic damage measured by immunoreactive trypsinogen (n = 126). A multiple-phenotype analytic strategy assessed evidence for pleiotropy in the presence of phenotypic correlation. We required the same alleles to be associated with detrimental effects. SLC26A9 was pleiotropic for meconium ileus and pancreatic damage (p = 0.002 at rs7512462), SLC9A3 for meconium ileus and lung disease (p = 1.5 × 10(-6) at rs17563161), and SLC6A14 for meconium ileus and both lung disease and age at first P. aeruginosa infection (p = 0.0002 and p = 0.006 at rs3788766, respectively). The meconium ileus risk alleles in SLC26A9, SLC9A3 and SLC6A14 are pleiotropic, increasing risk for other early CF co-morbidities. Furthermore, co-morbidities affecting the same organ tended to associate with the same genes. The existence of pleiotropy within this single disorder suggests that complementary therapeutic strategies to augment solute transport will benefit multiple CF-associated tissues.

The SLC26 gene family of anion transporters and channels

The phylogenetically ancient SLC26 gene family encodes multifunctional anion exchangers and anion channels transporting a broad range of substrates, including Cl(-), HCO3(-), sulfate, oxalate, I(-), and formate. SLC26 polypeptides are characterized by N-terminal cytoplasmic domains, 10-14 hydrophobic transmembrane spans, and C-terminal cytoplasmic STAS domains, and appear to be homo-oligomeric. SLC26-related SulP proteins of marine bacteria likely transport HCO3(-) as part of oceanic carbon fixation. SulP genes present in antibiotic operons may provide sulfate for antibiotic biosynthetic pathways. SLC26-related Sultr proteins transport sulfate in unicellular eukaryotes and in plants. Mutations in three human SLC26 genes are associated with congenital or early onset Mendelian diseases: chondrodysplasias for SLC26A2, chloride diarrhea for SLC26A3, and deafness with enlargement of the vestibular aqueduct for SLC26A4. Additional disease phenotypes evident only in mouse knockout models include oxalate urolithiasis for Slc26a6 and Slc26a1, non-syndromic deafness for Slc26a5, gastric hypochlorhydria for Slc26a7 and Slc26a9, distal renal tubular acidosis for Slc26a7, and male infertility for Slc26a8. STAS domains are required for cell surface expression of SLC26 proteins, and contribute to regulation of the cystic fibrosis transmembrane regulator in complex, cell- and tissue-specific ways. The protein interactomes of SLC26 polypeptides are under active investigation.

Molecular transport machinery involved in orchestrating luminal acid-induced duodenal bicarbonate secretion in vivo

The duodenal villus brush border membrane expresses several ion transporters and/or channels, including the solute carrier 26 anion transporters Slc26a3 (DRA) and Slc26a6 (PAT-1), the Na(+)/H(+) exchanger isoform 3 (NHE3), as well as the anion channels cystic fibrosis transmembrane conductance regulator (CFTR) and Slc26a9. Using genetically engineered mouse models lacking Scl26a3, Slc26a6, Slc26a9 or Slc9a3 (NHE3), the study was carried out to assess the role of these transporters in mediating the protective duodenal bicarbonate secretory response (DBS-R) to luminal acid; and to compare it to their role in DBS-R elicited by the adenylyl cyclase agonist forskolin. While basal DBS was reduced in the absence of any of the three Slc26 isoforms, the DBS-R to forskolin was not altered. In contrast, the DBS-R to a 5 min exposure to luminal acid (pH 2.5) was strongly reduced in the absence of Slc26a3 or Slc26a9, but not Slc26a6. CFTR inhibitor [CFTR(Inh)-172] reduced the first phase of the acid-induced DBS-R, while NHE3 inhibition (or knockout) abolished the sustained phase of the DBS-R. Luminal acid exposure resulted in the activation of multiple intracellular signalling pathways, including SPAK, AKT and p38 phosphorylation. It induced a biphasic trafficking of NHE3, first rapidly into the brush border membrane, followed by endocytosis in the later stage. We conclude that the long-lasting DBS-R to luminal acid exposure activates multiple duodenocyte signalling pathways and involves changes in trafficking and/or activity of CFTR, Slc26 isoforms Slc26a3 and Slc26a9, and NHE3.

Characterization of SLC26A9 in patients with CF-like lung disease

Diffuse bronchiectasis is a common problem in respiratory clinics. We hypothesized that mutations in the solute carrier 26A9 (SLC26A9) gene, encoding for a chloride (Cl(-)) transporter mainly expressed in lungs, may lead to defects in mucociliary clearance. We describe two missense variants in the SLC26A9 gene in heterozygote patients presenting with diffuse idiopathic bronchiectasis : p.Arg575Trp, identified in a patient also heterozygote for p.Phe508del in the CFTR gene; and p.Val486Ile. Expression of both mutants in Xenopus laevis oocytes abolished SLC26A9-mediated Cl(-) conductance without decreasing protein membrane expression. Coexpression of CFTR with SLC26A9-p.Val486Ile resulted in a significant increase in the Cl(-) current induced by PKA stimulation, similar to that obtained in oocytes expressing CFTR and SLC26A9-WT. In contrast, coexpression of CFTR with SLC26A9-p.Arg575Trp inhibited SLC26A9-enhanced CFTR activation upon PKA. Further structure-function analyses led us to propose a site encompassing Arg575 in the SLC26A9-STAS domain for CFTR-SLC26A9 interaction. We hypothesize that SLC26A9-p.Arg575Trp prevented SLC26A9-mediated functional activation of CFTR by altering SLC26A9-CFTR interaction. Although we cannot confirm that these mutations by themselves are deleterious, we propose that they trigger the pathogenic role of a single CFTR mutation and provide insight into a novel mechanism of Cl(-) transport alteration across the respiratory mucosa, based on functional inhibition of CFTR.

SLC26A6 and NaDC-1 transporters interact to regulate oxalate and citrate homeostasis

The combination of hyperoxaluria and hypocitraturia can trigger Ca(2+)-oxalate stone formation, even in the absence of hypercalciuria, but the molecular mechanisms that control urinary oxalate and citrate levels are not understood completely. Here, we examined the relationship between the oxalate transporter SLC26A6 and the citrate transporter NaDC-1 in citrate and oxalate homeostasis. Compared with wild-type mice, Slc26a6-null mice exhibited increased renal and intestinal sodium-dependent succinate uptake, as well as urinary hyperoxaluria and hypocitraturia, but no change in urinary pH, indicating enhanced transport activity of NaDC-1. When co-expressed in Xenopus oocytes, NaDC-1 enhanced Slc26a6 transport activity. In contrast, Slc26a6 inhibited NaDC-1 transport activity in an activity dependent manner to restricted tubular citrate absorption. Biochemical and physiologic analysis revealed that the STAS domain of Slc26a6 and the first intracellular loop of NaDC-1 mediated both the physical and functional interactions of these transporters. These findings reveal a molecular pathway that senses and tightly regulates oxalate and citrate levels and may control Ca(2+)-oxalate stone formation.

Missense mutations in SLC26A8, encoding a sperm-specific activator of CFTR, are associated with human asthenozoospermia

The cystic fibrosis transmembrane conductance regulator (CFTR) is present in mature sperm and is required for sperm motility and capacitation. Both these processes are controlled by ions fluxes and are essential for fertilization. We have shown that SLC26A8, a sperm-specific member of the SLC26 family of anion exchangers, associates with the CFTR channel and strongly stimulates its activity. This suggests that the two proteins cooperate to regulate the anion fluxes required for correct sperm motility and capacitation. Here, we report on three heterozygous SLC26A8 missense mutations identified in a cohort of 146 men presenting with asthenozoospermia: c.260G>A (p.Arg87Gln), c.2434G>A (p.Glu812Lys), and c.2860C>T (p.Arg954Cys). These mutations were not present in 121 controls matched for ethnicity, and statistical analysis on a control population of 8,600 individuals (from dbSNP and 1000 Genomes) showed them to be associated with asthenozoospermia with a power > 95%. By cotransfecting Chinese hamster ovary (CHO)-K1 cells with SLC26A8 variants and CFTR, we showed that the physical interaction between the two proteins was partly conserved but that the capacity to activate CFTR-dependent anion transport was completely abolished for all mutants. Biochemical studies revealed the presence of much smaller amounts of protein for all variants, but these amounts were restored to wild-type levels upon treatment with the proteasome inhibitor MG132. Immunocytochemistry also showed the amounts of SLC26A8 in sperm to be abnormally small in individuals carrying the mutations. These mutations might therefore impair formation of the SLC26A8-CFTR complex, principally by affecting SLC26A8 stability, consistent with an impairment of CFTR-dependent sperm-activation events in affected individuals.