Inhibitors of pendrin anion exchange identified in a small molecule screen increase airway surface liquid volume in cystic fibrosis

High potency 3-carboxy-2-methylbenzofuran pendrin inhibitors as novel diuretics

Pendrin (SLC26A4) is an anion exchanger expressed in epithelial cells of kidney and lung. Pendrin inhibition is a potential treatment approach for edema, hypertension and inflammatory lung diseases. We have previously identified first-in-class pendrin inhibitors by high-throughput screening, albeit with low potency for pendrin inhibition (IC50 ∼10 μM). Here, we performed a de novo small molecule screen with follow-on structure-activity studies to identify more potent pendrin inhibitors. Screening of 50,000 synthetic small molecules identified four novel classes of pendrin inhibitors with diverse scaffolds, including 5-benzyloxy-2-methylbenzofurans, N-aryl urea substituted 5-methyltryptamines, N-aryl urea substituted anthranilic acids, and substituted N-benzyl 3-carboxyindoles. The most potent inhibitor from the initial screen, a 3-carboxy-2-methylbenzofuran (1a), had IC50 of 4.1 μM. Structure-activity studies using 732 benzofuran analogs identified 1d with IC50 ∼ 0.5 μM for pendrin inhibition. Selectivity studies showed that 1d has minimal or no activity against related ion channels/transporters including SLC26A3, SLC26A6 and CFTR at high concentrations. 1d administration to mice at 10 mg/kg had no effect on urine volume when used alone, but potentiated the diuretic effect of furosemide by 45 %. In conclusion, we have identified novel pendrin inhibitors with greatly improved potency and good in vivo efficacy. These compounds can be used as pharmacological tools to study the roles of pendrin, and potentially developed as drug candidates for edema, hypertension and lung diseases.

Metabolic alkalosis treatment standard

The kidney is poised to defend against development of metabolic alkalosis through non-adaptive mechanisms in the proximal nephron and adaptive processes in the distal nephron. Despite a prodigious capacity to excrete base, metabolic alkalosis is the most common acid-base disturbance in hospitalized patients. Development of this disorder requires pathophysiologic changes leading to generation of new HCO3- combined with an augmentation in the capacity of the kidney to reclaim filtered HCO3-. The initial approach to these patients is careful assessment of effective arterial blood volume focusing on the physical examination and urine electrolytes. Identifying the mechanisms by which the kidney's ability to correct alkalosis are perturbed provides an understanding of the clinical approach to differential diagnosis and appropriate treatment. While metabolic alkalosis is frequently not dangerous, in certain settings, metabolic alkalosis may contribute to mortality and should be aggressively managed.

CFTR dysfunction leads to defective bacterial eradication on cystic fibrosis airways

Dysfunction of the cystic fibrosis transmembrane conductance regulator (CFTR) anion channel by genetic mutations causes the inherited disease cystic fibrosis (CF). CF lung disease that involves multiple disorders of epithelial function likely results from loss of CFTR function as an anion channel conducting chloride and bicarbonate ions and its function as a cellular regulator modulating the activity of membrane and cytosol proteins. In the absence of CFTR activity, abundant mucus accumulation, bacterial infection and inflammation characterize CF airways, in which inflammation-associated tissue remodeling and damage gradually destroys the lung. Deciphering the link between CFTR dysfunction and bacterial infection in CF airways may reveal the pathogenesis of CF lung disease and guide the development of new treatments. Research efforts towards this goal, including high salt, low volume, airway surface liquid acidosis and abnormal mucus hypotheses are critically reviewed.

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.

Pendrin: linking acid base to blood pressure

Pendrin (SLC26A4) is an anion exchanger from the SLC26 transporter family which is mutated in human patients affected by Pendred syndrome, an autosomal recessive disease characterized by sensoneurinal deafness and hypothyroidism. Pendrin is also expressed in the kidney where it mediates the exchange of internal HCO3- for external Cl- at the apical surface of renal type B and non-A non-B-intercalated cells. Studies using pendrin knockout mice have first revealed that pendrin is essential for renal base excretion. However, subsequent studies have demonstrated that pendrin also controls chloride absorption by the distal nephron and that this mechanism is critical for renal NaCl balance. Furthermore, pendrin has been shown to control vascular volume and ultimately blood pressure. This review summarizes the current knowledge about how pendrin is linking renal acid-base regulation to blood pressure control.

Mechanism of anion exchange and small-molecule inhibition of pendrin

Pendrin (SLC26A4) is an anion exchanger that mediates bicarbonate (HCO3-) exchange for chloride (Cl-) and is crucial for maintaining pH and salt homeostasis in the kidney, lung, and cochlea. Pendrin also exports iodide (I-) in the thyroid gland. Pendrin mutations in humans lead to Pendred syndrome, causing hearing loss and goiter. Inhibition of pendrin is a validated approach for attenuating airway hyperresponsiveness in asthma and for treating hypertension. However, the mechanism of anion exchange and its inhibition by drugs remains poorly understood. We applied cryo-electron microscopy to determine structures of pendrin from Sus scrofa in the presence of either Cl-, I-, HCO3- or in the apo-state. The structures reveal two anion-binding sites in each protomer, and functional analyses show both sites are involved in anion exchange. The structures also show interactions between the Sulfate Transporter and Anti-Sigma factor antagonist (STAS) and transmembrane domains, and mutational studies suggest a regulatory role. We also determine the structure of pendrin in a complex with niflumic acid (NFA), which uncovers a mechanism of inhibition by competing with anion binding and impeding the structural changes necessary for anion exchange. These results reveal directions for understanding the mechanisms of anion selectivity and exchange and their regulations by the STAS domain. This work also establishes a foundation for analyzing the pathophysiology of mutations associated with Pendred syndrome.

SLC26 Anion Transporters

Solute carrier family 26 (SLC26) is a family of functionally diverse anion transporters found in all kingdoms of life. Anions transported by SLC26 proteins include chloride, bicarbonate, and sulfate, but also small organic dicarboxylates such as fumarate and oxalate. The human genome encodes ten functional homologs, several of which are causally associated with severe human diseases, highlighting their physiological importance. Here, we review novel insights into the structure and function of SLC26 proteins and summarize the physiological relevance of human members.

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.

Cross-species applicability of an adverse outcome pathway network for thyroid hormone system disruption

Thyroid hormone system disrupting compounds are considered potential threats for human and environmental health. Multiple adverse outcome pathways (AOPs) for thyroid hormone system disruption (THSD) are being developed in different taxa. Combining these AOPs results in a cross-species AOP network for THSD which may provide an evidence-based foundation for extrapolating THSD data across vertebrate species and bridging the gap between human and environmental health. This review aimed to advance the description of the taxonomic domain of applicability (tDOA) in the network to improve its utility for cross-species extrapolation. We focused on the molecular initiating events (MIEs) and adverse outcomes (AOs) and evaluated both their plausible domain of applicability (taxa they are likely applicable to) and empirical domain of applicability (where evidence for applicability to various taxa exists) in a THSD context. The evaluation showed that all MIEs in the AOP network are applicable to mammals. With some exceptions, there was evidence of structural conservation across vertebrate taxa and especially for fish and amphibians, and to a lesser extent for birds, empirical evidence was found. Current evidence supports the applicability of impaired neurodevelopment, neurosensory development (eg, vision) and reproduction across vertebrate taxa. The results of this tDOA evaluation are summarized in a conceptual AOP network that helps prioritize (parts of) AOPs for a more detailed evaluation. In conclusion, this review advances the tDOA description of an existing THSD AOP network and serves as a catalog summarizing plausible and empirical evidence on which future cross-species AOP development and tDOA assessment could build.

Inflammation as a Regulator of the Airway Surface Liquid pH in Cystic Fibrosis

The airway surface liquid (ASL) is a thin sheet of fluid that covers the luminal aspect of the airway epithelium. The ASL is a site of several first-line host defenses, and its composition is a key factor that determines respiratory fitness. Specifically, the acid-base balance of ASL has a major influence on the vital respiratory defense processes of mucociliary clearance and antimicrobial peptide activity against inhaled pathogens. In the inherited disorder cystic fibrosis (CF), loss of cystic fibrosis transmembrane conductance regulator (CFTR) anion channel function reduces HCO3- secretion, lowers the pH of ASL (pHASL), and impairs host defenses. These abnormalities initiate a pathologic process whose hallmarks are chronic infection, inflammation, mucus obstruction, and bronchiectasis. Inflammation is particularly relevant as it develops early in CF and persists despite highly effective CFTR modulator therapy. Recent studies show that inflammation may alter HCO3- and H+ secretion across the airway epithelia and thus regulate pHASL. Moreover, inflammation may enhance the restoration of CFTR channel function in CF epithelia exposed to clinically approved modulators. This review focuses on the complex relationships between acid-base secretion, airway inflammation, pHASL regulation, and therapeutic responses to CFTR modulators. These factors have important implications for defining optimal ways of tackling CF airway inflammation in the post-modulator era.

Small molecule inhibitors of intestinal epithelial anion exchanger SLC26A3 (DRA) with a luminal, extracellular site of action

The anion exchanger protein SLC26A3 (down-regulated in adenoma, DRA) is expressed in the luminal membrane of intestinal epithelial cells in colon, where it facilitates the absorption of Cl^- and oxalate. We previously identified a 4,8-dimethylcoumarin class of SLC26A3 inhibitors that act from the SLC26A3 cytoplasmic surface, and demonstrated their efficacy in mouse models of constipation and hyperoxaluria. Here, screening of 50,000 new compounds and 1740 chemical analogs of active compounds from the primary screen produced five novel classes of SLC26A3-selective inhibitors (1,3-dioxoisoindoline-amides; N-(5-sulfamoyl-1,3,4-thiadiazol-2-yl)acetamides; thiazolo-pyrimidin-5-ones; 3-carboxy-2-phenylbenzofurans and benzoxazin-4-ones) with IC₅₀ down to 100 nM. Kinetic washout and onset of action studies revealed an extracellular site of action for the thiazolo-pyrimidin-5-one and 3-carboxy-2-phenylbenzofuran inhibitors. Molecular docking computations revealed putative binding sites for these inhibitors. In a loperamide model of constipation in mice, orally administered 7-(2-chloro-phenoxymethyl)-3-phenyl-thiazolo [3,2-a]pyrimidin-5-one (3a) significantly increased stool weight, pellet number and water content. SLC26A3 inhibitors with an extracellular site of action offer the possibility of creating non-absorbable, luminally acting inhibitors with minimal systemic exposure following oral administration. Our findings also suggest that inhibitors of related SLC26 anion transporters with an extracellular site of action might be identified for pharmacological modulation of selected epithelial ion transport processes.

Airway surface hyperviscosity and defective mucociliary transport by IL-17/TNF-α are corrected by β-adrenergic stimulus

The fluid covering the surface of airway epithelia represents a first barrier against pathogens. The chemical and physical properties of the airway surface fluid are controlled by the activity of ion channels and transporters. In cystic fibrosis (CF), loss of CFTR chloride channel function causes airway surface dehydration, bacterial infection, and inflammation. We investigated the effects of IL-17A plus TNF-α, 2 cytokines with relevant roles in CF and other chronic lung diseases. Transcriptome analysis revealed a profound change with upregulation of several genes involved in ion transport, antibacterial defense, and neutrophil recruitment. At the functional level, bronchial epithelia treated in vitro with the cytokine combination showed upregulation of ENaC channel, ATP12A proton pump, ADRB2 β-adrenergic receptor, and SLC26A4 anion exchanger. The overall result of IL-17A/TNF-α treatment was hyperviscosity of the airway surface, as demonstrated by fluorescence recovery after photobleaching (FRAP) experiments. Importantly, stimulation with a β-adrenergic agonist switched airway surface to a low-viscosity state in non-CF but not in CF epithelia. Our study suggests that CF lung disease is sustained by a vicious cycle in which epithelia cannot exit from the hyperviscous state, thus perpetuating the proinflammatory airway surface condition.

Chloride channels in the lung: Challenges and perspectives for viral infections, pulmonary arterial hypertension, and cystic fibrosis

Fine control over chloride homeostasis in the lung is required to maintain membrane excitability, transepithelial transport as well as intra- and extracellular ion and water homeostasis. Over the last decades, a growing number of chloride channels and transporters have been identified in the cells of the pulmonary vasculature and the respiratory tract. The importance of these proteins is underpinned by the fact that impairment of their physiological function is associated with functional dysregulation, structural remodeling, or hereditary diseases of the lung. This paper reviews the field of chloride channels and transporters in the lung and discusses chloride channels in disease processes such as viral infections including SARS-CoV- 2, pulmonary arterial hypertension, cystic fibrosis and asthma. Although chloride channels have become a hot research topic in recent years, remarkably few of them have been targeted by pharmacological agents. As such, we complement the putative pathophysiological role of chloride channels here with a summary of their therapeutic potential.

Regulation of Blood Pressure and Salt Balance By Pendrin-Positive Intercalated Cells: Donald Seldin Lecture 2020

Intercalated cells make up about a third of all cells within the connecting tubule and the collecting duct and are subclassified as type A, type B and non-A, non-B based on the subcellular distribution of the H+-ATPase, which dictates whether it secretes H+ or HCO3-. Type B intercalated cells mediate Cl- absorption and HCO3- secretion, which occurs largely through the anion exchanger pendrin. Pendrin is stimulated by angiotensin II via the angiotensin type 1a receptor and by aldosterone through MR (mineralocorticoid receptor). Aldosterone stimulates pendrin expression and function, in part, through the alkalosis it generates. Pendrin-mediated HCO3- secretion increases in models of metabolic alkalosis, which attenuates the alkalosis. However, pendrin-positive intercalated cells also regulate blood pressure, at least partly, through pendrin-mediated Cl- absorption and through their indirect effect on the epithelial Na+ channel, ENaC. This aldosterone-induced increase in pendrin secondarily stimulates ENaC, thereby contributing to the aldosterone pressor response. This review describes the contribution of pendrin-positive intercalated cells to Na+, K+, Cl- and acid-base balance.

Chloride transport modulators as drug candidates

Chloride transport across cell membranes is broadly involved in epithelial fluid transport, cell volume and pH regulation, muscle contraction, membrane excitability, and organellar acidification. The human genome encodes at least 53 chloride-transporting proteins with expression in cell plasma or intracellular membranes, which include chloride channels, exchangers, and cotransporters, some having broad anion specificity. Loss-of-function mutations in chloride transporters cause a wide variety of human diseases, including cystic fibrosis, secretory diarrhea, kidney stones, salt-wasting nephropathy, myotonia, osteopetrosis, hearing loss, and goiter. Although impactful advances have been made in the past decade in drug treatment of cystic fibrosis using small molecule modulators of the defective cystic fibrosis transmembrane conductance regulator (CFTR) chloride channel, other chloride channels and solute carrier proteins (SLCs) represent relatively underexplored target classes for drug discovery. New opportunities have emerged for the development of chloride transport modulators as potential therapeutics for secretory diarrheas, constipation, dry eye disorders, kidney stones, polycystic kidney disease, hypertension, and osteoporosis. Approaches to chloride transport-targeted drug discovery are reviewed herein, with focus on chloride channel and exchanger classes in which recent preclinical advances have been made in the identification of small molecule modulators and in proof of concept testing in experimental animal models.

SLC26A6-selective inhibitor identified in a small-molecule screen blocks fluid absorption in small intestine

SLC26A6 (also known as putative anion transporter 1 [PAT1]) is a Cl-/HCO3- exchanger expressed at the luminal membrane of enterocytes where it facilitates intestinal Cl- and fluid absorption. Here, high-throughput screening of 50,000 synthetic small molecules in cells expressing PAT1 and a halide-sensing fluorescent protein identified several classes of inhibitors. The most potent compound, the pyrazolo-pyrido-pyrimidinone PAT1inh-B01, fully inhibited PAT1-mediated anion exchange (IC50 ~350 nM), without inhibition of the related intestinal transporter SLC26A3 (also known as DRA). In closed midjejunal loops in mice, PAT1inh-B01 inhibited fluid absorption by 50%, which increased to >90% when coadministered with DRA inhibitor DRAinh-A270. In ileal loops, PAT1inh-B01 blocked fluid absorption by >80%, whereas DRAinh-A270 was without effect. In colonic loops, PAT1inh-B01 was without effect, whereas DRAinh-A270 completely blocked fluid absorption. In a loperamide constipation model, coadministration of PAT1inh-B01 with DRAinh-A270 increased stool output compared with DRAinh-A270 alone. These results provide functional evidence for complementary and region-specific roles of PAT1 and DRA in intestinal fluid absorption, with PAT1 as the predominant anion exchanger in mouse ileum. We believe that PAT1inh-B01 is a novel tool to study intestinal ion and fluid transport and perhaps a drug candidate for small intestinal hyposecretory disorders such as cystic fibrosis-related meconium ileus and distal intestinal obstruction syndrome.

Airway Surface Liquid pH Regulation in Airway Epithelium Current Understandings and Gaps in Knowledge

Knowledge on the mechanisms of acid and base secretion in airways has progressed recently. The aim of this review is to summarize the known mechanisms of airway surface liquid (ASL) pH regulation and their implication in lung diseases. Normal ASL is slightly acidic relative to the interstitium, and defects in ASL pH regulation are associated with various respiratory diseases, such as cystic fibrosis. Basolateral bicarbonate (HCO3-) entry occurs via the electrogenic, coupled transport of sodium (Na+) and HCO3-, and, together with carbonic anhydrase enzymatic activity, provides HCO3- for apical secretion. The latter mainly involves CFTR, the apical chloride/bicarbonate exchanger pendrin and paracellular transport. Proton (H+) secretion into ASL is crucial to maintain its relative acidity compared to the blood. This is enabled by H+ apical secretion, mainly involving H+/K+ ATPase and vacuolar H+-ATPase that carry H+ against the electrochemical potential gradient. Paracellular HCO3- transport, the direction of which depends on the ASL pH value, acts as an ASL protective buffering mechanism. How the transepithelial transport of H+ and HCO3- is coordinated to tightly regulate ASL pH remains poorly understood, and should be the focus of new studies.

Modern trends in diuretics development

Diuretics are the first-line therapy for widespread cardiovascular and non-cardiovascular diseases. Traditional diuretics are commonly prescribed for treatment in patients with hypertension, edema and heart failure, as well as with a number of kidney problems. They are diseases with high mortality, and the number of patients suffering from heart and kidney diseases is increasing year by year. The use of several classes of diuretics currently available for clinical use exhibits an overall favorable risk/benefit balance. However, they are not devoid of side effects. Hence, pharmaceutical researchers have been making efforts to develop new drugs with a better pharmacological profile. High-throughput screening, progress in protein structure analysis and modern methods of chemical modification have opened good possibilities for identification of new promising agents for preclinical and clinical testing. In this review, we provide an overview of the medicinal chemistry approaches toward the development of small molecule compounds showing diuretic activity that have been discovered over the past decade and are interesting drug candidates. We have discussed promising natriuretics/aquaretics/osmotic diuretics from such classes as: vasopressin receptor antagonists, SGLT2 inhibitors, urea transporters inhibitors, aquaporin antagonists, adenosine receptor antagonists, natriuretic peptide receptor agonists, ROMK inhibitors, WNK-SPAK inhibitors, and pendrin inhibitors.

Paracellular bicarbonate flux across human cystic fibrosis airway epithelia tempers changes in airway surface liquid pH

KEY POINTS

Cl- and HCO3- had similar paracellular permeabilities in human airway epithelia. PCl /PNa of airway epithelia was unaltered by pH 7.4 vs. pH 6.0 solutions. Under basal conditions, calculated paracellular HCO3- flux was secretory. Cytokines that increased airway surface liquid pH decreased or reversed paracellular HCO3- flux. HCO3- flux through the paracellular pathway may counterbalance effects of cellular H+ and HCO3- secretion.

ABSTRACT

Airway epithelia control the pH of airway surface liquid (ASL), thereby optimizing respiratory defences. Active H+ and HCO3- secretion by airway epithelial cells produce an ASL that is acidic compared with the interstitial space. The paracellular pathway could provide a route for passive HCO3- flux that also modifies ASL pH. However, there is limited information about paracellular HCO3- flux, and it remains uncertain whether an acidic pH produced by loss of cystic fibrosis transmembrane conductance regulator anion channels or proinflammatory cytokines might alter the paracellular pathway function. To investigate paracellular HCO3- transport, we studied differentiated primary cultures of human cystic fibrosis (CF) and non-CF airway epithelia. The paracellular pathway was pH-insensitive at pH 6.0 vs. pH 7.4 and was equally permeable to Cl- and HCO3- . Under basal conditions at pH ∼6.6, calculated paracellular HCO3- flux was weakly secretory. Treating epithelia with IL-17 plus TNFα alkalinized ASL pH to ∼7.0, increased paracellular HCO3- permeability, and paracellular HCO3- flux was negligible. Applying IL-13 increased ASL pH to ∼7.4 without altering paracellular HCO3- permeability, and calculated paracellular HCO3- flux was absorptive. These results suggest that HCO3- flux through the paracellular pathway counterbalances, in part, changes in the ASL pH produced via cellular mechanisms. As the pH of ASL increases towards that of basolateral liquid, paracellular HCO3- flux becomes absorptive, tempering the alkaline pH generated by transcellular HCO3- secretion.

Antisense oligonucleotide-mediated correction of CFTR splicing improves chloride secretion in cystic fibrosis patient-derived bronchial epithelial cells

Cystic fibrosis (CF) is an autosomal recessive disorder caused by mutations in the CF transmembrane conductance regulator (CFTR) gene, encoding an anion channel that conducts chloride and bicarbonate across epithelial membranes. Mutations that disrupt pre-mRNA splicing occur in >15% of CF cases. One common CFTR splicing mutation is CFTR c.3718-2477C>T (3849+10 kb C>T), which creates a new 5′ splice site, resulting in splicing to a cryptic exon with a premature termination codon. Splice-switching antisense oligonucleotides (ASOs) have emerged as an effective therapeutic strategy to block aberrant splicing. We test an ASO targeting the CFTR c.3718-2477C>T mutation and show that it effectively blocks aberrant splicing in primary bronchial epithelial (hBE) cells from CF patients with the mutation. ASO treatment results in long-term improvement in CFTR activity in hBE cells, as demonstrated by a recovery of chloride secretion and apical membrane conductance. We also show that the ASO is more effective at recovering chloride secretion in our assay than ivacaftor, the potentiator treatment currently available to these patients. Our findings demonstrate the utility of ASOs in correcting CFTR expression and channel activity in a manner expected to be therapeutic in patients.

Inhibition of Pendrin by a small molecule reduces Lipopolysaccharide-induced acute Lung Injury

Rationale: Pendrin is encoded by SLC26A4 and its mutation leads to congenital hearing loss. Additionally, pendrin is up-regulated in inflammatory airway diseases such as chronic obstructive pulmonary disease, allergic rhinitis, and asthma. In this study, the effects of a novel pendrin inhibitor, YS-01, were investigated in an LPS-induced acute lung injury (ALI) mice model, and the mechanism underlying the effect of YS-01 was examined. Methods: Lipopolysaccharide (LPS, 10 mg/kg) was intranasally instilled in wild type (WT) and pendrin-null mice. YS-01 (10 mg/kg) was administered intra-peritoneally before or after LPS inhalation. Lung injury parameters were assessed in the lung tissue and bronchoalveolar lavage fluid (BALF). Pendrin levels in the BALF of 41 patients with acute respiratory distress syndrome (ARDS) due to pneumonia and 25 control (solitary pulmonary nodule) patients were also measured. Results: LPS instillation induced lung injury in WT mice but not in pendrin-null mice. Pendrin expression was increased by LPS stimulation both in vitro and in vivo. YS-01 treatment dramatically attenuated lung injury and reduced BALF cell counts and protein concentration after LPS instillation in WT mice. Proinflammatory cytokines and NF-κB activation were suppressed by YS-01 treatment in LPS-induced ALI mice. In BALF of patients whose ARDS was caused by pneumonia, pendrin expression was up-regulated compared to that in controls (mean, 24.86 vs. 6.83 ng/mL, P < 0.001). Conclusions: A novel pendrin inhibitor, YS-01, suppressed lung injury in LPS-induced ALI mice and our data provide a new strategy for the treatment of inflammatory airway diseases including sepsis-induced ALI.

Transport properties in CFTR-/- knockout piglets suggest normal airway surface liquid pH and enhanced amiloride-sensitive Na+ absorption

Previous analysis of CFTR-knockout (CFTR-/-) in piglets has provided important insights into the pathology of cystic fibrosis. However, controversies exist as to the true contribution of CFTR to the pH balance in airways and intestine. We therefore compared ion transport properties in newborn wild-type (CFTR+/+) and CFTR-knockout (CFTR-/- piglets). Tracheas of CFTR-/- piglets demonstrated typical cartilage malformations and muscle cell bundles. CFTR-/- airway epithelial cells showed enhanced lipid peroxidation, suggesting inflammation early in life. CFTR was mainly expressed in airway submucosal glands and was absent in lungs of CFTR-/- piglets, while expression of TMEM16A was uncompromised. mRNA levels for TMEM16A, TMEM16F, and αβγENaC were unchanged in CFTR-/- airways, while mRNA for SLC26A9 appeared reduced. CFTR was undetectable in epithelial cells of CFTR-/- airways and intestine. Small intestinal epithelium of CFTR-/- piglets showed mucus accumulation. Secretion of both electrolytes and mucus was activated by stimulation with prostaglandin E2 and ATP in the intestine of CFTR+/+, but not of CFTR-/- animals. pH was measured inside small bronchi using a pH microelectrode and revealed no difference between CFTR+/+ and CFTR-/- piglets. Intracellular pH in porcine airway epithelial cells revealed only a small contribution of CFTR to bicarbonate secretion, which was absent in cells from CFTR-/- piglets. In contrast to earlier reports, our data suggest a minor impact of CFTR on ASL pH. In contrast, enhanced amiloride-sensitive Na⁺ absorption may contribute to lung pathology in CFTR-/- piglets, along with a compromised CFTR- and TMEM16A-dependent Cl^- transport.

The Renal Physiology of Pendrin-Positive Intercalated Cells

Intercalated cells (ICs) are found in the connecting tubule and the collecting duct. Of the three IC subtypes identified, type B intercalated cells are one of the best characterized and known to mediate Cl- absorption and HCO3- secretion, largely through the anion exchanger pendrin. This exchanger is thought to act in tandem with the Na+-dependent Cl-/HCO3- exchanger, NDCBE, to mediate net NaCl absorption. Pendrin is stimulated by angiotensin II and aldosterone administration via the angiotensin type 1a and the mineralocorticoid receptors, respectively. It is also stimulated in models of metabolic alkalosis, such as with NaHCO3 administration. In some rodent models, pendrin-mediated HCO3- secretion modulates acid-base balance. However, of probably more physiological or clinical significance is the role of these pendrin-positive ICs in blood pressure regulation, which occurs, at least in part, through pendrin-mediated renal Cl- absorption, as well as their effect on the epithelial Na+ channel, ENaC. Aldosterone stimulates ENaC directly through principal cell mineralocorticoid hormone receptor (ligand) binding and also indirectly through its effect on pendrin expression and function. In so doing, pendrin contributes to the aldosterone pressor response. Pendrin may also modulate blood pressure in part through its action in the adrenal medulla, where it modulates the release of catecholamines, or through an indirect effect on vascular contractile force. In addition to its role in Na+ and Cl- balance, pendrin affects the balance of other ions, such as K+ and I-. This review describes how aldosterone and angiotensin II-induced signaling regulate pendrin and the contribution of pendrin-positive ICs in the kidney to distal nephron function and blood pressure.

TNFα and IL-17 alkalinize airway surface liquid through CFTR and pendrin

The pH of airway surface liquid (ASL) is a key factor that determines respiratory host defense; ASL acidification impairs and alkalinization enhances key defense mechanisms. Under healthy conditions, airway epithelia secrete base ([Formula: see text]) and acid (H⁺) to control ASL pH (pH_ASL). Neutrophil-predominant inflammation is a hallmark of several airway diseases, and TNFα and IL-17 are key drivers. However, how these cytokines perturb pH_ASL regulation is uncertain. In primary cultures of differentiated human airway epithelia, TNFα decreased and IL-17 did not change pH_ASL. However, the combination (TNFα+IL-17) markedly increased pH_ASL by increasing [Formula: see text] secretion. TNFα+IL-17 increased expression and function of two apical [Formula: see text] transporters, CFTR anion channels and pendrin Cl^-/[Formula: see text] exchangers. Both were required for maximal alkalinization. TNFα+IL-17 induced pendrin expression primarily in secretory cells where it was coexpressed with CFTR. Interestingly, significant pendrin expression was not detected in CFTR-rich ionocytes. These results indicate that TNFα+IL-17 stimulate [Formula: see text] secretion via CFTR and pendrin to alkalinize ASL, which may represent an important defense mechanism in inflamed airways.

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.

4,8-Dimethylcoumarin Inhibitors of Intestinal Anion Exchanger slc26a3 (Downregulated in Adenoma) for Anti-Absorptive Therapy of Constipation

The chloride/bicarbonate exchanger SLC26A3 (downregulated in adenoma) is expressed mainly in colonic epithelium, where it dehydrates the stool by facilitating the final step of chloride and fluid absorption. SLC26A3 inhibition has predicted efficacy in various types of constipation including that associated with cystic fibrosis. We previously identified, by high-throughput screening, 4,8-dimethylcoumarin inhibitors of murine slc26a3 with IC50 down to ∼150 nM. Here, we synthesized a focused library of forty-three 4,8-dimethylcoumarin analogues. Structure-activity studies revealed the requirement of 4,8-dimethylcoumarin-3-acetic acid for activity. The most potent inhibitors were produced by replacements at C7, including 3-iodo- (4az) and 3-trifluoromethyl- (4be), with IC50 of 40 and 25 nM, respectively. Pharmacokinetics in mice showed predicted therapeutic concentrations of 4az for >72 h following a single 10 mg/kg oral dose. 4az at 10 mg/kg fully normalized stool water content in a loperamide-induced mouse model of constipation. The favorable inhibition potency, selectivity within the SLC26 family, and pharmacological properties of 4az support its further preclinical development.

Synthesis and evaluation of tetrahydropyrazolopyridine inhibitors of anion exchange protein SLC26A4 (pendrin)

Pendrin is a transmembrane chloride/anion antiporter that is strongly upregulated in the airways in rhinoviral infection, asthma, cystic fibrosis and chronic rhinosinusitis. Based on its role in the regulation of airway surface liquid depth, pendrin inhibitors have potential indications for treatment of inflammatory airways diseases. Here, a completely regioselective route to tetrahydro-pyrazolopyridine pendrin inhibitors based on 1,3-diketone and substituted hydrazine condensation was been developed. Structure-activity relationships at the tetrahydropyridyl nitrogen were investigated using a focused library, establishing the privileged nature of N-phenyl ureas and improving inhibitor potency by greater than 2-fold.

Airway surface liquid acidification initiates host defense abnormalities in Cystic Fibrosis

Cystic fibrosis (CF) is caused by defective Cystic Fibrosis Transmembrane Conductance Regulator (CFTR) protein. Morbidity is mainly due to early airway infection. We hypothesized that S. aureus clearance during the first hours of infection was impaired in CF human Airway Surface Liquid (ASL) because of a lowered pH. The ASL pH of human bronchial epithelial cell lines and primary respiratory cells from healthy controls (WT) and patients with CF was measured with a pH microelectrode. The antimicrobial capacity of airway cells was studied after S. aureus apical infection by counting surviving bacteria. ASL was significantly more acidic in CF than in WT respiratory cells. This was consistent with a defect in bicarbonate secretion involving CFTR and SLC26A4 (pendrin) and a persistent proton secretion by ATP12A. ASL demonstrated a defect in S. aureus clearance which was improved by pH normalization. Pendrin inhibition in WT airways recapitulated the CF airway defect and increased S. aureus proliferation. ATP12A inhibition by ouabain decreased bacterial proliferation. Antimicrobial peptides LL-37 and hBD1 demonstrated a pH-dependent activity. Normalizing ASL pH might improve innate airway defense in newborns with CF during onset of S. aureus infection. Pendrin activation and ATP12A inhibition could represent novel therapeutic strategies to normalize pH in CF airways.

Simultaneous expression of ClopHensor and SLC26A3 reveals the nature of endogenous oxalate transport in CHO cells

ClopHensor, a fluorescent fusion protein, is a dual function biosensor that has been utilized as a tool for the simultaneous measurement of intracellular chloride and pH in cells. ClopHensor has traditionally been used in conjunction with fluorescence microscopy for single cell measurements. Here, we present a promising multi-well format advancement for the use of ClopHensor as a potential high-throughput method capable of measuring fluorescence signal intensity across a well of confluent cells with highly reproducible results. Using this system, we gained mechanistic insight into an endogenous oxalate transporter in Chinese hamster ovary (CHO) cells expressing ClopHensor and the human chloride transporter, SLC26A3. SLC26A3, a known anion exchanger, has been proposed to play a role in colonic oxalate absorption in humans. Our attempt to study the role of SLC26A3 in oxalate transport revealed the presence of an endogenous oxalate transporter in CHO cells. This transporter was strongly inhibited by niflumate, and exhibited clear saturability. Use of ClopHensor in a multi-well cell assay allowed us to quickly demonstrate that the endogenous oxalate transporter was unable to exchange chloride for bicarbonate, unlike SLC26A3.

Interleukin-Mediated Pendrin Transcriptional Regulation in Airway and Esophageal Epithelia

Pendrin (SLC26A4), a Cl⁻/anion exchanger, is expressed at high levels in kidney, thyroid, and inner ear epithelia, where it has an essential role in bicarbonate secretion/chloride reabsorption, iodide accumulation, and endolymph ion balance, respectively. Pendrin is expressed at lower levels in other tissues, such as airways and esophageal epithelia, where it is transcriptionally regulated by the inflammatory cytokines interleukin (IL)-4 and IL-13 through a signal transducer and activator of transcription 6 (STAT6)-mediated pathway. In the airway epithelium, increased pendrin expression during inflammatory diseases leads to imbalances in airway surface liquid thickness and mucin release, while, in the esophageal epithelium, dysregulated pendrin expression is supposed to impact the intracellular pH regulation system. In this review, we discuss some of the recent findings on interleukin-mediated transcriptional regulation of pendrin and how this dysregulation impacts airway and esophagus epithelial homeostasis during inflammatory diseases.

Increased expression of ATP12A proton pump in cystic fibrosis airways

Proton secretion mediated by ATP12A protein on the surface of the airway epithelium may contribute to cystic fibrosis (CF) lung disease by favoring bacterial infection and airway obstruction. We studied ATP12A in fresh bronchial samples and in cultured epithelial cells. In vivo, ATP12A expression was found almost exclusively at the apical side of nonciliated cells of airway epithelium and in submucosal glands, with much higher expression in CF samples. This could be due to bacterial infection and inflammation, since treating cultured cells with bacterial supernatants or with IL-4 (a cytokine that induces goblet cell hyperplasia) increased the expression of ATP12A in nonciliated cells. This observation was associated with upregulation and translocation of ATP1B1 protein from the basal to apical epithelial side, where it colocalizes with ATP12A. ATP12A function was evaluated by measuring the pH of the apical fluid in cultured epithelia. Under resting conditions, CF epithelia showed more acidic values. This abnormality was minimized by inhibiting ATP12A with ouabain. Following treatment with IL-4, ATP12A function was markedly increased, as indicated by strong acidification occurring under bicarbonate-free conditions. Our study reveals potentially novel aspects of ATP12A and remarks its importance as a possible therapeutic target in CF and other respiratory 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.

Ivacaftor-induced sweat chloride reductions correlate with increases in airway surface liquid pH in cystic fibrosis

BACKGROUND

Disruption of cystic fibrosis transmembrane conductance regulator (CFTR) anion channel function causes cystic fibrosis (CF), and lung disease produces most of the mortality. Loss of CFTR-mediated HCO3- secretion reduces the pH of airway surface liquid (ASL) in vitro and in neonatal humans and pigs in vivo. However, we previously found that, in older children and adults, ASL pH does not differ between CF and non-CF. Here, we tested whether the pH of CF ASL increases with time after birth. Finding that it did suggested that adaptations by CF airways increase ASL pH. This conjecture predicted that increasing CFTR activity in CF airways would further increase ASL pH and also that increasing CFTR activity would correlate with increases in ASL pH.

METHODS

To test for longitudinal changes, we measured ASL pH in newborns and then at 3-month intervals. We also studied people with CF (bearing G551D or R117H mutations), in whom we could acutely stimulate CFTR activity with ivacaftor. To gauge changes in CFTR activity, we measured changes in sweat Cl- concentration immediately before and 48 hours after starting ivacaftor.

RESULTS

Compared with that in the newborn period, ASL pH increased by 6 months of age. In people with CF bearing G551D or R117H mutations, ivacaftor did not change the average ASL pH; however reductions in sweat Cl- concentration correlated with elevations of ASL pH. Reductions in sweat Cl- concentration also correlated with improvements in pulmonary function.

CONCLUSIONS

Our results suggest that CFTR-independent mechanisms increase ASL pH in people with CF. We speculate that CF airway disease, which begins soon after birth, is responsible for the adaptation.

FUNDING

Vertex Inc., the NIH (P30DK089507, 1K08HL135433, HL091842, HL136813, K24HL102246), the Cystic Fibrosis Foundation (SINGH17A0 and SINGH15R0), and the Burroughs Wellcome Fund.

SLC26A3 inhibitor identified in small molecule screen blocks colonic fluid absorption and reduces constipation

SLC26A3 (downregulated in adenoma; DRA) is a Cl-/anion exchanger expressed in the luminal membrane of intestinal epithelial cells, where it facilitates electroneutral NaCl absorption. SLC26A3 loss of function in humans or mice causes chloride-losing diarrhea. Here, we identified slc26a3 inhibitors in a screen of 50,000 synthetic small molecules done in Fischer rat thyroid (FRT) cells coexpressing slc26a3 and a genetically encoded halide sensor. Structure-activity relationship studies were done on the most potent inhibitor classes identified in the screen: 4,8-dimethylcoumarins and acetamide-thioimidazoles. The dimethylcoumarin DRAinh-A250 fully and reversibly inhibited slc26a3-mediated Cl- exchange with HCO3-, I-, and thiocyanate (SCN-), with an IC50 of ~0.2 μM. DRAinh-A250 did not inhibit the homologous anion exchangers slc26a4 (pendrin) or slc26a6 (PAT-1), nor did it alter activity of other related proteins or intestinal ion channels. In mice, intraluminal DRAinh-A250 blocked fluid absorption in closed colonic loops but not in jejunal loops, while the NHE3 (SLC9A3) inhibitor tenapanor blocked absorption only in the jejunum. Oral DRAinh-A250 and tenapanor comparably reduced signs of constipation in loperamide-treated mice, with additive effects found on coadministration. DRAinh-A250 was also effective in loperamide-treated cystic fibrosis mice. These studies support a major role of slc26a3 in colonic fluid absorption and suggest the therapeutic utility of SLC26A3 inhibition in constipation.

Most bicarbonate secretion by Calu-3 cells is mediated by CFTR and independent of pendrin

Bicarbonate plays an important role in airway host defense, however, its transport mechanisms remain uncertain. Here we examined the relative contributions of the anion channel CFTR (cystic fibrosis transmembrane conductance regulator, ABCC7) and the anion exchanger pendrin (SLC26A4) to HCO3- secretion by the human airway cell line Calu-3. Pendrin and CFTR were both detected in parental Calu-3 cells, although mRNA and protein expression appeared higher for CFTR than for pendrin. Targeting pendrin transcripts with lentiviral shRNA reduced pendrin detection by immunofluorescence staining but did not alter the rates of HCO3- or fluid secretion, HCO3- transport under pH-stat conditions, or net HCO3- flux across basolaterally permeabilized monolayers. Intracellular pH varied with step changes in apical Cl- and HCO3- concentrations in control and pendrin knockdown Calu-3 cells, but not in CFTR deficient cells. Exposure to the proinflammatory cytokine IL-4, which strongly upregulates pendrin expression in airway surface epithelia, had little effect on Calu-3 pendrin expression and did not alter fluid or HCO3- secretion. Similar results were obtained using air-liquid interface and submerged cultures, although CFTR and pendrin mRNA expression were both lower when cells were cultured under submerged conditions. While the conclusions cannot be extrapolated to other airway epithelia, the present results demonstrate that most HCO3- secretion by Calu-3 cells is mediated by CFTR.

ATP12A promotes mucus dysfunction during Type 2 airway inflammation

Allergic airway disease is known to cause significant morbidity due to impaired mucociliary clearance, however the mechanism that leads to the mucus dysfunction is not entirely understood. Interleukin 13 (IL-13), a key mediator of Type 2 (T2) inflammation, profoundly alters the ion transport properties of airway epithelium. However, these electrophysiological changes cannot explain the thick, tenacious airway mucus that characterizes the clinical phenotype. Here we report that IL-13 dramatically increases the airway surface liquid (ASL) viscosity in cultured primary human bronchial epithelial cells and thereby inhibits mucus clearance. These detrimental rheological changes require ATP12A, a non-gastric H⁺/K⁺-ATPase that secretes protons into the ASL. ATP12A knockdown or inhibition prevented the IL-13 dependent increase in ASL viscosity but did not alter the ASL pH. We propose that ATP12A promotes airway mucus dysfunction in individuals with T2 inflammatory airway diseases and that ATP12A may be a novel therapeutic target to improve mucus clearance.

Ion channels as targets to treat cystic fibrosis lung disease

Lung health relies on effective mucociliary clearance and innate immune defence mechanisms. In cystic fibrosis (CF), an imbalance in ion transport due to an absence of chloride ion secretion, caused by mutations in the cystic fibrosis transmembrane conductance regulator (CFTR) and a concomitant sodium hyperabsorption, caused by dyregulation of the epithelial sodium channel (ENaC), results in mucus stasis which predisposes the lungs to cycles of chronic infection and inflammation leading to lung function decline. An increased understanding of CFTR structure and function has provided opportunity for the development of a number of novel modulators targeting mutant CFTR however, it is important to also consider other ion channels and transporters present in the airways as putative targets for drug development. In this review, we discuss recent advances in CFTR biology which will contribute to further drug discovery in the field. We also examine developments to inhibit the epithelial sodium channel (ENaC) and potentially activate alternative chloride channels and transporters as a multi-tracked strategy to hydrate CF airways and restore normal mucociliary clearance mechanisms in a manner independent of CFTR mutation.

Emerging Targets of Diuretic Therapy

Diuretics are commonly prescribed for treatment in patients with hypertension, edema, or heart failure. Studies on hypertensive and salt-losing disorders and on urea transporters have contributed to better understanding of mechanisms of renal salt and water reabsorption and their regulation. Proteins involved in the regulatory pathways are emerging targets for diuretic and aquaretic therapy. Integrative high-throughput screening, protein structure analysis, and chemical modification have identified promising agents for preclinical testing in animals. These include WNK-SPAK inhibitors, ClC-K channel antagonists, ROMK channel antagonists, and pendrin and urea transporter inhibitors. We discuss the potential advantages and side effects of these potential diuretics.

Interleukin-13 increases pendrin abundance to the cell surface in bronchial NCI-H292 cells via Rho/actin signaling

Interleukin-13 (IL13) is a major player in the development of airway hyperresponsiveness in several respiratory disorders. Emerging data suggest that an increased expression of pendrin in airway epithelia is associated with elevated airway hyperreactivity in asthma. Here, we investigate the effect of IL13 on pendrin localization and function using bronchiolar NCI-H292 cells. The data obtained revealed that IL13 increases the cell surface expression of pendrin. This effect was paralleled by a significant increase in the intracellular pH, possibly via indirect stimulation of NHE. IL13 effect on pendrin localization and intracellular pH was reversed by theophylline, a bronchodilator compound used to treat asthma. IL13 upregulated RhoA activity, a crucial protein controlling actin dynamics, via G-alpha-13. Specifically, IL13 stabilized actin cytoskeleton and promoted co-localization and a direct molecular interaction between pendrin and F-actin in the plasma membrane region. These effects were reversed following exposure of cells to theophylline. Selective inhibition of Rho kinase, a downstream effector of Rho, reduced the IL13-dependent cell surface expression of pendrin. Together, these data indicate that IL13 increases pendrin abundance to the cell surface via Rho/actin signaling, an effect reversed by theophylline.

Nanomolar-Potency Aminophenyl-1,3,5-triazine Activators of the Cystic Fibrosis Transmembrane Conductance Regulator (CFTR) Chloride Channel for Prosecretory Therapy of Dry Eye Diseases

Dry eye disorders are a significant health problem for which limited therapeutic options are available. CFTR is a major prosecretory chloride channel at the ocular surface. We previously identified, by high-throughput screening, aminophenyl-1,3,5-triazine CFTR_act-K089 (1) that activated CFTR with EC₅₀ ≈ 250 nM, which when delivered topically increased tear fluid secretion in mice and showed efficacy in an experimental dry eye model. Here, functional analysis of aminophenyl-1,3,5-triazine analogs elucidated structure-activity relationships for CFTR activation and identified substantially more potent analogs than 1. The most potent compound, 12, fully activated CFTR chloride conductance with EC₅₀ ≈ 30 nM, without causing cAMP or calcium elevation. 12 was rapidly metabolized by hepatic microsomes, which supports its topical use. Single topical administration of 25 pmol of 12 increased tear volume in wild-type mice with sustained action for 8 h and was without effect in CFTR-deficient mice. Topically delivered 12 may be efficacious in human dry eye diseases.

Small-Molecule Inhibitors of Pendrin Potentiate the Diuretic Action of Furosemide

Pendrin is a Cl^-/HCO₃^- exchanger expressed in type B and non-A, non-B intercalated cells in the distal nephron, where it facilitates Cl^- absorption and is involved in Na⁺ absorption and acid-base balance. Pendrin-knockout mice show no fluid-electrolyte abnormalities under baseline conditions, although mice with double knockout of pendrin and the Na⁺/Cl^- cotransporter (NCC) manifest profound salt wasting. Thus, pendrin may attenuate diuretic-induced salt loss, but this function remains unconfirmed. To clarify the physiologic role of pendrin under conditions not confounded by gene knockout, and to test the potential utility of pendrin inhibitors for diuretic therapy, we tested in mice a small-molecule pendrin inhibitor identified from a high-throughput screen. In vitro, a pyrazole-thiophenesulfonamide, PDS_inh-C01, inhibited Cl^-/anion exchange mediated by mouse pendrin with a 50% inhibitory concentration of 1-3 µM, without affecting other major kidney tubule transporters. Administration of PDS_inh-C01 to mice at predicted therapeutic doses, determined from serum and urine pharmacokinetics, did not affect urine output, osmolality, salt excretion, or acid-base balance. However, in mice treated acutely with furosemide, administration of PDS_inh-C01 produced a 30% increase in urine output, with increased Na⁺ and Cl^- excretion. In mice treated long term with furosemide, in which renal pendrin is upregulated, PDS_inh-C01 produced a 60% increase in urine output. Our findings clarify the role of pendrin in kidney function and suggest pendrin inhibition as a novel approach to potentiate the action of loop diuretics. Such combination therapy might enhance diuresis and salt excretion for treatment of hypertension and edema, perhaps including diuretic-resistant edema.