Effect of Slc26a6 deletion on apical Cl-/HCO3- exchanger activity and cAMP-stimulated bicarbonate secretion in pancreatic duct

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.

Identification of Hub Genes in Colorectal Adenocarcinoma by Integrated Bioinformatics

An improved understanding of the molecular mechanism of colorectal adenocarcinoma is necessary to predict the prognosis and develop new target gene therapy strategies. This study aims to identify hub genes associated with colorectal adenocarcinoma and further analyze their prognostic significance. In this study, The Cancer Genome Atlas (TCGA) COAD-READ database and the gene expression profiles of GSE25070 from the Gene Expression Omnibus were collected to explore the differentially expressed genes between colorectal adenocarcinoma and normal tissues. The weighted gene co-expression network analysis (WGCNA) and differential expression analysis identified 82 differentially co-expressed genes in the collected datasets. Enrichment analysis was applied to explore the regulated signaling pathway in colorectal adenocarcinoma. In addition, 10 hub genes were identified in the protein–protein interaction (PPI) network by using the cytoHubba plug-in of Cytoscape, where five genes were further proven to be significantly related to the survival rate. Compared with normal tissues, the expressions of the five genes were both downregulated in the GSE110224 dataset. Subsequently, the expression of the five hub genes was confirmed by the Human Protein Atlas database. Finally, we used Cox regression analysis to identify genes associated with prognosis, and a 3-gene signature (CLCA1–CLCA4–GUCA2A) was constructed to predict the prognosis of patients with colorectal cancer. In conclusion, our study revealed that the five hub genes and CLCA1–CLCA4–GUCA2A signature are highly correlated with the development of colorectal adenocarcinoma and can serve as promising prognosis factors to predict the overall survival rate of patients.

The Identification of Key Genes and Biological Pathways in Heart Failure by Integrated Bioinformatics Analysis

Purpose

Heart failure (HF) is a clinical syndrome caused by ventricular insufficiency. In order to further explore the biomarkers related to HF, we apply the high-throughput database.

Materials and Methods

The GSE21610 was applied for the differentially expressed gene (DEG) analysis. The Database for Annotation, Visualization, and Integrated Discovery (DAVID) was performed to assess Gene ontology (GO) and the Kyoto Encyclopedia of Genes and Genomes (KEGG) analyses. The Gene Set Enrichment Analysis (GSEA) was used for gene expression profile GSE21610. The Protein-Protein Interaction (PPI) network and modules were also constructed for research. These hub gene function pathways were estimated in HF progression.

Result

We have identified 434 DEGs in total, including 304 downregulated DEGs and 130 upregulated DEGs. GO and KEGG illustrated that DEGs in HF were significantly enriched in G protein-coupled receptor binding, peroxisome, and cAMP signaling pathway. GSEA results showed gene set GSE21610 was gathered in lipid digestion, defense response to fungus, and intestinal lipid absorption. Finally, through analyzing the PPI network, we screened hub genes CDH1, TFRC, CCL2, BUB1B, and CD19 by the Cytoscape software.

Conclusion

This study uses a series of bioinformatics technologies to obtain hug genes and key pathways related to HF. These analysis results provide us with new ideas for finding biomarkers and treatment methods for HF.

Slc26a3 (DRA) in the Gut: Expression, Function, Regulation, Role in Infectious Diarrhea and Inflammatory Bowel Disease

BACKGROUND

The transport of transepithelial Cl- and HCO3- is crucial for the function of the intestinal epithelium and maintains the acid-based homeostasis. Slc26a3 (DRA), as a key chloride-bicarbonate exchanger protein in the intestinal epithelial luminal membrane, participates in the electroneutral NaCl absorption of intestine, together with Na+/H+ exchangers. Increasing recent evidence supports the essential role of decreased DRA function or expression in infectious diarrhea and inflammatory bowel disease (IBD).

METHOD

In this review, we give an overview of the current knowledge of Slc26a3, including its cloning and expression, function, roles in infectious diarrhea and IBD, and mechanisms of actions. A better understanding of the physiological and pathophysiological relevance of Slc26a3 in infectious diarrhea and IBD may reveal novel targets for future therapy.

CONCLUSION

Understanding the physiological function, regulatory interactions, and the potential mechanisms of Slc26a3 in the pathophysiology of infectious diarrhea and IBD will define novel therapeutic approaches in future.

Physiological and Pathological Functions of SLC26A6

Solute Carrier Family 26 (SLC26) is a conserved anion transporter family with 10 members in human (SLC26A1-A11, A10 being a pseudogene). All SLC26 genes except for SLC26A5 (prestin) are versatile anion exchangers with notable ability to transport a variety of anions. SLC26A6 has the most extensive exchange functions in the SLC26 family and is widely expressed in various organs and tissues of mammals. SLC26A6 has some special properties that make it play a particularly important role in ion homeostasis and acid-base balance. In the past few years, the function of SLC26A6 in the diseases has received increasing attention. SLC26A6 not only participates in the development of intestinal and pancreatic diseases but also serves a significant role in mediating nephrolithiasis, fetal skeletal dysplasia and arrhythmia. This review aims to explore the role of SLC26A6 in physiology and pathophysiology of relative mammalian organs to guide in-depth studies about related diseases of human.

Slc26a6 is an apical membrane anion exchanger that drives HCO3--dependent fluid secretion in murine pancreatic acinar cells

The nonselective anion exchanger Slc26a6, also known as putative anion transporter 1 and chloride/formate exchanger, is thought to play a major role in HCO 3 - transport in exocrine glands. In this study, Slc26a6 null mice were used to explore the function of Slc26a6 in the exocrine pancreas. Slc26a6 primarily localized to the apical membrane of pancreatic exocrine acinar cells. The volume of stimulated juice secretion by the ex vivo pancreas was significantly reduced ~35% in Slc26a6-/- mice, but no changes occurred in the gross structure or gland weights of Slc26a6 null mice. The secretion of pancreatic juice by Slc26a6+/+ mice was dependent on HCO 3 - while, in contrast, fluid secretion by Slc26a6-/- mice was independent of HCO 3 - , suggesting that Slc26a6 mediates the HCO 3 - -dependent component of fluid secretion. Consistent with these observations, disruption of Slc26a6 also significantly reduced HCO 3 - secretion by the pancreas ~35%. Taken together, these results demonstrate that the apical Slc26a6 anion exchanger in acinar cells is involved in HCO 3 - -dependent fluid secretion but that another major HCO 3 - -independent pathway is the primary driver of the fluid secretion process in the mouse pancreas.

Importance of SLC26 Transmembrane Anion Exchangers in Sperm Post-testicular Maturation and Fertilization Potential

In mammals, sperm cells produced within the testis are structurally differentiated but remain immotile and are unable to fertilize the oocyte unless they undergo a series of maturation events during their transit in the male and female genital tracts. This post-testicular functional maturation is known to rely on the micro-environment of both male and female genital tracts, and is tightly controlled by the pH of their luminal milieus. In particular, within the epididymis, the establishment of a low bicarbonate (HCO₃^–) concentration contributes to luminal acidification, which is necessary for sperm maturation and subsequent storage in a quiescent state. Following ejaculation, sperm is exposed to the basic pH of the female genital tract and bicarbonate (HCO₃^–), calcium (Ca²⁺), and chloride (Cl^–) influxes induce biochemical and electrophysiological changes to the sperm cells (cytoplasmic alkalinization, increased cAMP concentration, and protein phosphorylation cascades), which are indispensable for the acquisition of fertilization potential, a process called capacitation. Solute carrier 26 (SLC26) members are conserved membranous proteins that mediate the transport of various anions across the plasma membrane of epithelial cells and constitute important regulators of pH and HCO₃^– concentration. Most SLC26 members were shown to physically interact and cooperate with the cystic fibrosis transmembrane conductance regulator channel (CFTR) in various epithelia, mainly by stimulating its Cl^– channel activity. Among SLC26 members, the function of SLC26A3, A6, and A8 were particularly investigated in the male genital tract and the sperm cells. In this review, we will focus on SLC26s contributions to ionic- and pH-dependent processes during sperm post-testicular maturation. We will specify the current knowledge regarding their functions, based on data from the literature generated by means of in vitro and in vivo studies in knock-out mouse models together with genetic studies of infertile patients. We will also discuss the limits of those studies, the current research gaps and identify some key points for potential developments in this field.

Sex-independent expression of chloride/formate exchanger Cfex (Slc26a6) in rat pancreas, small intestine, and liver, and male-dominant expression in kidneys

Chloride/formate exchanger (CFEX; SLC26A6) mediates oxalate transport in various mammalian organs. Studies in Cfex knockout mice indicated its possible role in development of male-dominant hyperoxaluria and oxalate urolithiasis. Rats provide an important model for studying this pathophysiological condition, but data on Cfex (rCfex) localisation and regulation in their organs are limited. Here we applied the RT-PCR and immunochemical methods to investigate rCfex mRNA and protein expression and regulation by sex hormones in the pancreas, small intestine, liver, and kidneys from intact prepubertal and adult as well as gonadectomised adult rats treated with sex hormones. rCfex cDNA-transfected HEK293 cells were used to confirm the specificity of the commercial anti-CFEX antibody. Various biochemical parameters were measured in 24-h urine collected in metabolic cages. rCfex mRNA and related protein expression varied in all tested organs. Sex-independent expression of the rCfex protein was detected in pancreatic intercalated ducts (apical domain), small intestinal enterocytes (brush-border membrane; duodenum > jejunum > ileum), and hepatocytes (canalicular membrane). In kidneys, the rCfex protein was immunolocalised to the proximal tubule brush-border with segment-specific pattern (S1=S2<S3), and both rCfex mRNA and protein expression exhibited male-dominant sex differences driven by stimulatory effects of androgens after puberty. However, urinary oxalate excretion was unrelated to renal rCfex protein expression. While the effect of male-dominant expression of rCfex in renal proximal tubules on urine oxalate excretion remains unknown, its expression in the hepatocyte canalicular membrane may be a pathway of oxalate elimination via bile.

Development of a polarized pancreatic ductular cell epithelium for physiological studies

Pancreatic ductular epithelial cells comprise the majority of duct cells in pancreas, control cystic fibrosis transmembrane conductance regulator (CFTR)-dependent bicarbonate ([Formula: see text]) secretion, but are difficult to grow as a polarized monolayer. Using NIH-3T3-J2 fibroblast feeder cells and a Rho-associated kinase inhibitor, we produced well-differentiated and polarized porcine pancreatic ductular epithelial cells. Cells grown on semipermeable filters at the air-liquid interface developed typical epithelial cell morphology and stable transepithelial resistance and expressed epithelial cell markers (zona occludens-1 and β-catenin), duct cell markers (SOX-9 and CFTR), but no acinar (amylase) or islet cell (chromogranin) markers. Polarized cells were studied in Ussing chambers bathed in Krebs-Ringer [Formula: see text] solution at 37°C gassed with 5% CO2 to measure short-circuit currents ( Isc). Ratiometric measurement of extracellular pH was performed with fluorescent SNARF-conjugated dextran at 5% CO2. Cells demonstrated a baseline Isc (12.2 ± 3.2 μA/cm2) that increased significantly in response to apical forskolin-IBMX (∆ Isc: 35.4 ± 3.8 μA/cm2, P < 0.001) or basolateral secretin (∆ Isc: 31.4 ± 2.5 μA/cm2, P < 0.001), both of which increase cellular levels of cAMP. Subsequent addition of apical GlyH-101, a CFTR inhibitor, decreased the current (∆ Isc: 20.4 ± 3.8 μA/cm2, P < 0.01). Extracellular pH and [Formula: see text] concentration increased significantly after forskolin-IBMX (pH: 7.18 ± 0.23 vs. 7.53 ± 0.19; [Formula: see text] concentration, 14.5 ± 5.9 vs. 31.8 ± 13.4 mM; P < 0.05 for both). We demonstrate the development of a polarized pancreatic ductular epithelial cell epithelium with CFTR-dependent [Formula: see text] secretion in response to secretin and cAMP. This model is highly relevant, as porcine pancreas physiology is very similar to humans and pancreatic damage in the cystic fibrosis pig model recapitulates that of humans. NEW & NOTEWORTHY Pancreas ductular epithelial cells control cystic fibrosis transmembrane conductance regulator (CFTR)-dependent bicarbonate secretion. Their function is critical because when CFTR is deficient in cystic fibrosis bicarbonate secretion is lost and the pancreas is damaged. Mechanisms that control pancreatic bicarbonate secretion are incompletely understood. We generated well-differentiated and polarized porcine pancreatic ductular epithelial cells and demonstrated feasibility of bicarbonate secretion. This novel method will advance our understanding of pancreas physiology and mechanisms of bicarbonate secretion.

The apical anion exchanger Slc26a6 promotes oxalate secretion by murine submandibular gland acinar cells

The solute carrier family 26 (SLC26) gene family encodes at least 10 different anion exchangers. SLC26 member 6 (SLC26A6 or CFEX/PAT-1) and the cystic fibrosis transmembrane conductance regulator (CFTR) co-localize to the apical membrane of pancreatic duct cells, where they act in concert to drive HCO3- and fluid secretion. In contrast, in the small intestine, SLC26A6 serves as the major pathway for oxalate secretion. However, little is known about the function of Slc26a6 in murine salivary glands. Here, RNA sequencing-based transcriptional profiling and Western blots revealed that Slc26a6 is highly expressed in mouse submandibular and sublingual salivary glands. Slc26a6 localized to the apical membrane of salivary gland acinar cells with no detectable immunostaining in the ducts. CHO-K1 cells transfected with mouse Slc26a6 exchanged Cl- for oxalate and HCO3-, whereas two other anion exchangers known to be expressed in salivary gland acinar cells, Slc4a4 and Slc4a9, mediated little, if any, Cl-/oxalate exchange. Of note, both Cl-/oxalate exchange and Cl-/HCO3- exchange were significantly reduced in acinar cells isolated from the submandibular glands of Slc26a6-/- mice. Oxalate secretion in submandibular saliva also decreased significantly in Slc26a6-/- mice, but HCO3- secretion was unaffected. Taken together, our findings indicate that Slc26a6 is located at the apical membrane of salivary gland acinar cells, where it mediates Cl-/oxalate exchange and plays a critical role in the secretion of oxalate into saliva.

Electrophysiological properties of anion exchangers in the luminal membrane of guinea pig pancreatic duct cells

The pancreatic duct epithelium secretes the HCO₃⁻-rich pancreatic juice. The HCO₃⁻ transport across the luminal membrane has been proposed to be mediated by SLC26A Cl⁻–HCO₃⁻ exchangers. To examine the electrophysiological properties of Cl⁻–HCO₃⁻ exchangers, we directly measured HCO₃⁻ conductance in the luminal membrane of the interlobular pancreatic duct cells from guinea pigs using an inside-out patch-clamp technique. Intracellular HCO₃⁻ increased the HCO₃⁻ conductance with a half-maximal effective concentration value of approximately 30 mM. The selectivity sequence based on permeability ratios was SCN⁻ (1.4) > Cl⁻ (1.2) = gluconate (1.1) = I⁻ (1.1) = HCO₃⁻ (1.0) > methanesulfonate (0.6). The sequence of the relative conductance was HCO₃⁻ (1.0) > SCN⁻ (0.7) = I⁻ (0.7) > Cl⁻ (0.5) = gluconate (0.4) > methanesulfonate (0.2). The current dependent on intracellular HCO₃⁻ was reduced by replacement of extracellular Cl⁻ with gluconate or by H₂DIDS, an inhibitor of Cl⁻–HCO₃⁻ exchangers. RT-PCR analysis revealed that the interlobular and main ducts expressed all SLC26A family members except Slc26a5 and Slc26a8. SLC26A1, SLC26A4, SLC26A6, and SLC26A10 were found to be localized to the luminal membrane of the guinea pig pancreatic duct by immunohistochemistry. These results demonstrate that these SLC26A Cl⁻–HCO₃⁻ exchangers may mediate the electrogenic HCO₃⁻ transport through the luminal membrane and may be involved in pancreatic secretion in guinea pig ducts.

An Exploration of Charge Compensating Ion Channels across the Phagocytic Vacuole of Neutrophils

Neutrophils phagocytosing bacteria and fungi exhibit a burst of non-mitochondrial respiration that is required to kill and digest the engulfed microbes. This respiration is accomplished by the movement of electrons across the wall of the phagocytic vacuole by the neutrophil NADPH oxidase, NOX2. In this study, we have attempted to identify the non-proton ion channels or transporters involved in charge compensation by examining the effect of inhibitors on vacuolar pH and cross-sectional area, and on oxygen consumption. The chloride channel inhibitors 4-[(2-Butyl-6,7-dichloro-2-cyclopentyl-2,3-dihydro-1-oxo-1H-inden-5-yl)oxy]butanoic acid (DCPIB) and flufenamic acid (FFA) were the most effective inhibitors of alkalinisation in human neutrophil vacuoles, suggesting an efflux of chloride from the vacuole. The proton channel inhibitor, zinc (Zn²⁺), combined with DCPIB caused more vacuolar swelling than either compound alone, suggesting the conductance of osmotically active cations into the vacuole. Support for cation influx was provided by the broad-spectrum cation transport inhibitors anandamide and quinidine which inhibited vacuolar alkalinisation and swelling when applied with zinc. Oxygen consumption was generally unaffected by these anion or cation inhibitors alone, but when combined with Zn²⁺ it was dramatically reduced, suggesting that multiple channels in combination can compensate the charge. In an attempt to identify specific channels, we tested neutrophils from knock-out mouse models including CLIC1, ClC3, ClC4, ClC7, KCC3, KCNQ1, KCNE3, KCNJ15, TRPC1/3/5/6, TRPA1/TRPV1, TRPM2, and TRPV2, and double knockouts of CLIC1, ClC3, KCC3, TRPM2, and KCNQ1 with HVCN1, and humans with channelopathies involving BEST1, ClC7, CFTR, and MCOLN1. No gross abnormalities in vacuolar pH or area were found in any of these cells suggesting that we had not tested the correct channel, or that there is redundancy in the system. The respiratory burst was suppressed in the KCC3^-/- and enhanced in the CLIC1^-/- cells, but was normal in all others, including ClC3^-/-. These results suggest charge compensation by a chloride conductance out of the vacuole and by cation/s into it. The identity of these channels remains to be established.

The Contributions of Human Mini-Intestines to the Study of Intestinal Physiology and Pathophysiology

The lack of accessibility to normal and diseased human intestine and the inability to separate the different functional compartments of the intestine even when tissue could be obtained have held back the understanding of human intestinal physiology. Clevers and his associates identified intestinal stem cells and established conditions to grow "mini-intestines" ex vivo in differentiated and undifferentiated conditions. This pioneering work has made a new model of the human intestine available and has begun making contributions to the understanding of human intestinal transport in normal physiologic conditions and the pathophysiology of intestinal diseases. However, this model is reductionist and lacks many of the complexities of normal intestine. Consequently, it is not yet possible to predict how great the advances using this model will be for understanding human physiology and pathophysiology, nor how the model will be modified to include multiple other intestinal cell types and physical forces necessary to more closely approximate normal intestine. This review describes recent studies using mini-intestines, which have readdressed previously established models of normal intestinal transport physiology and newly examined intestinal pathophysiology. The emphasis is on studies with human enteroids grown either as three-dimensional spheroids or two-dimensional monolayers. In addition, comments are provided on mouse studies in cases when human studies have not yet been described.

Bicarbonate-rich fluid secretion predicted by a computational model of guinea-pig pancreatic duct epithelium

KEY POINTS

The ductal system of the pancreas secretes large volumes of alkaline fluid containing HCO₃^- concentrations as high as 140 mm during hormonal stimulation. A computational model has been constructed to explore the underlying ion transport mechanisms. Parameters were estimated by fitting the model to experimental data from guinea-pig pancreatic ducts. The model was readily able to secrete 140 mm HCO₃^- . Its capacity to do so was not dependent upon special properties of the cystic fibrosis transmembrane conductance regulator (CFTR) anion channels and solute carrier family 26 member A6 (SLC26A6) anion exchangers. We conclude that the main requirement for secreting high HCO₃^- concentrations is to minimize the secretion of Cl^- ions. These findings help to clarify the mechanism responsible for pancreatic HCO₃^- secretion, a vital process that prevents the formation of protein plugs and viscous mucus in the ducts, which could otherwise lead to pancreatic disease.

ABSTRACT

A computational model of guinea-pig pancreatic duct epithelium was developed to determine the transport mechanism by which HCO₃^- ions are secreted at concentrations in excess of 140 mm. Parameters defining the contributions of the individual ion channels and transporters were estimated by least-squares fitting of the model predictions to experimental data obtained from isolated ducts and intact pancreas under a range of experimental conditions. The effects of cAMP-stimulated secretion were well replicated by increasing the activities of the basolateral Na⁺ -HCO₃^- cotransporter (NBC1) and apical Cl^- /HCO₃^- exchanger (solute carrier family 26 member A6; SLC26A6), increasing the basolateral K⁺ permeability and apical Cl^- and HCO₃^- permeabilities (CFTR), and reducing the activity of the basolateral Cl^- /HCO₃^- exchanger (anion exchanger 2; AE2). Under these conditions, the model secreted ∼140 mm HCO₃^- at a rate of ∼3 nl min^-1  mm^-2 , which is consistent with experimental observations. Alternative 1:2 and 1:1 stoichiometries for Cl^- /HCO₃^- exchange via SLC26A6 at the apical membrane were able to support a HCO₃^- -rich secretion. Raising the HCO₃^- /Cl^- permeability ratio of CFTR from 0.4 to 1.0 had little impact upon either the secreted HCO₃^- concentration or the volume flow. However, modelling showed that a reduction in basolateral AE2 activity by ∼80% was essential in minimizing the intracellular Cl^- concentration following cAMP stimulation and thereby maximizing the secreted HCO₃^- concentration. The addition of a basolateral Na⁺ -K⁺ -2Cl^- cotransporter (NKCC1), assumed to be present in rat and mouse ducts, raised intracellular Cl^- and resulted in a lower secreted HCO₃^- concentration, as is characteristic of those species. We conclude therefore that minimizing the driving force for Cl^- secretion is the main requirement for secreting 140 mm HCO₃^- .

Role of CFTR in epithelial physiology

Salt and fluid absorption and secretion are two processes that are fundamental to epithelial function and whole body fluid homeostasis, and as such are tightly regulated in epithelial tissues. The CFTR anion channel plays a major role in regulating both secretion and absorption in a diverse range of epithelial tissues, including the airways, the GI and reproductive tracts, sweat and salivary glands. It is not surprising then that defects in CFTR function are linked to disease, including life-threatening secretory diarrhoeas, such as cholera, as well as the inherited disease, cystic fibrosis (CF), one of the most common life-limiting genetic diseases in Caucasian populations. More recently, CFTR dysfunction has also been implicated in the pathogenesis of acute pancreatitis, chronic obstructive pulmonary disease (COPD), and the hyper-responsiveness in asthma, underscoring its fundamental role in whole body health and disease. CFTR regulates many mechanisms in epithelial physiology, such as maintaining epithelial surface hydration and regulating luminal pH. Indeed, recent studies have identified luminal pH as an important arbiter of epithelial barrier function and innate defence, particularly in the airways and GI tract. In this chapter, we will illustrate the different operational roles of CFTR in epithelial function by describing its characteristics in three different tissues: the airways, the pancreas, and the sweat gland.

A Novel Mechanism of pH Buffering in C. elegans Glia: Bicarbonate Transport via the Voltage-Gated ClC Cl- Channel CLH-1

An important function of glia is the maintenance of the ionic composition and pH of the synaptic microenvironment. In terms of pH regulation, HCO3 (-) buffering has been shown to be important in both glia and neurons. Here, we used in vivo fluorescent pH imaging and RNA sequencing of the amphid sheath glia of Caenorhabditis elegans to reveal a novel mechanism of cellular HCO3 (-) uptake. While the classical mechanism of HCO3 (-) uptake involves Na(+)/HCO3 (-) cotransporters, here we demonstrate that the C. elegans ClC Cl(-) channel CLH-1 is highly permeable to HCO3 (-) and mediates HCO3 (-) uptake into amphid sheath glia. CLH-1 has homology and electrophysiological properties similar to the mammalian ClC-2 Cl(-) channel. Our data suggest that, in addition to maintaining synaptic Cl(-) concentration, these channels may also be involved in maintenance of synaptic pH via HCO3 (-) flux. These findings provide an exciting new facet of study regarding how pH is regulated in the brain.

SIGNIFICANCE STATEMENT

Maintenance of pH is essential for the physiological function of the nervous system. HCO3 (-) is crucial for pH regulation and is transported into the cell via ion transporters, including ion channels, the molecular identity of which remains unclear. In this manuscript, we describe our discovery that the C. elegans amphid sheath glia regulate intracellular pH via HCO3 (-) flux through the voltage-gated ClC channel CLH-1. This represents a novel function for ClC channels, which has implications for their possible role in mammalian glial pH regulation. This discovery may also provide a novel therapeutic target for pathologic conditions, such as ischemic stroke where acidosis leads to widespread death of glia and subsequently neurons.

Effect of SLC26 anion transporter disease-causing mutations on the stability of the homologous STAS domain of E. coli DauA (YchM)

The human solute carrier 26 (SLC26) family of anion transporters consists of ten members that are found in various organs in the body including the stomach, intestine, kidney, thyroid and ear where they transport anions including bicarbonate, chloride and sulfate, typically in an exchange mode. Mutations in these genes cause a plethora of diseases such as diastrophic dysplasia affecting sulfate uptake into chondrocytes (SLC26A2), congenital chloride-losing diarrhoea (SLC26A3) affecting chloride secretion in the intestine and Pendred's syndrome (SLC26A4) resulting in hearing loss. To understand how these mutations affect the structures of the SLC26 membrane proteins and their ability to function properly, 12 human disease-causing mutants from SLC26A2, SLC26A3 and SLC26A4 were introduced into the equivalent sites of the sulfate transporter anti-sigma factor antagonist (STAS) domain of a bacterial homologue SLC26 protein DauA (YchM). Biophysical analyses including size-exclusion chromatography, circular dichroism (CD), differential scanning fluorimetry (DSF) and tryptophan fluorescence revealed that most mutations caused protein instability and aggregation. The mutation A463K, equivalent to N558K in human SLC26A4, which is located within α-helix 1 of the DauA STAS domain, stabilized the protein. CD measurements showed that most disease-related mutants had a mildly reduced helix content, but were more sensitive to thermal denaturation. Fluorescence spectroscopy showed that the mutants had more open structures and were more readily denatured by urea, whereas DSF indicated more labile folds. Overall, we conclude that the disease-associated mutations destabilized the STAS domain resulting in an increased propensity to misfold and aggregate.

The Physiology and Pathophysiology of Pancreatic Ductal Secretion: The Background for Clinicians

The human exocrine pancreas consists of 2 main cell types: acinar and ductal cells. These exocrine cells interact closely to contribute to the secretion of pancreatic juice. The most important ion in terms of the pancreatic ductal secretion is HCO3. In fact, duct cells produce an alkaline fluid that may contain up to 140 mM NaHCO3, which is essential for normal digestion. This article provides an overview of the basics of pancreatic ductal physiology and pathophysiology. In the first part of the article, we discuss the ductal electrolyte and fluid transporters and their regulation. The central role of cystic fibrosis transmembrane conductance regulator (CFTR) is highlighted, which is much more than just a Cl channel. We also review the role of pancreatic ducts in severe debilitating diseases such as cystic fibrosis (caused by various genetic defects of cftr), pancreatitis, and diabetes mellitus. Stimulation of ductal secretion in cystic fibrosis and pancreatitis may have beneficial effects in their treatment.

Modulation of sodium-bicarbonate co-transporter (SLC4A4/NBCe1) protein and mRNA expression in rat's uteri by sex-steroids and at different phases of the oestrous cycle

Oestrogen-induced uterine fluid sodium (Na(+)) and bicarbonate (HCO3(-)) secretion may involve SLC4A4. We hypothesized that uterine SLC4A4 expression changes under different sex-steroid influence, therefore may account for the fluctuation in uterine fluid Na(+) and HCO3(-) content throughout the oestrous cycle. The aim of this study is to investigate the differential effects of sex-steroids and oestrous cycle phases on uterine SLC4A4 expression.

METHODS

Adult female WKY rats were ovariectomised and treated with different doses of 17β-oestradiol (E2) (0.2, 2, 20 and 50 μg/ml/day) or progesterone (P4) (4 mg/ml/day) for three consecutive days and 3 days treatment with 0.2 μg/ml/day E2 followed by another 3 days with P4 to mimic the hormonal changes in early pregnancy. Oestrous cycle phases in intact, non-ovariectomised rats were determined by vaginal smear. The animals were then sacrificed and uteri were removed for protein and mRNA expression analyses by Western blotting and Real Time PCR, respectively. SLC4A4 distribution was observed by immunohistochemistry.

RESULTS

Treatment with increasing E2 doses resulted in a dose-dependent increase in SLC4A4 protein expression. High SLC4A4 protein and mRNA expression can be seen at estrus. SLC4A4 is distributed mainly at the apical as well as basolateral membranes of the luminal and glandular epithelia following E2 treatment and at Es. Meanwhile, SLC4A4 expression was reduced following P4 treatment and was low at diestrus.

CONCLUSION

High SLC4A4 expression under estrogen dominance may contribute to the increase in uterine fluid Na(+) and HCO3(-) content, while its low expression under P4 dominance may result in vice versa.

SLC26 anion exchangers in uterine epithelial cells and spermatozoa: clues from the past and hints to the future

The solute carrier 26 (SLC26) family emerges as a distinct class of anion transporters with its members SLC26A3 (Slc26a3) and SLC26A6 (Slc26a6) reported to be electrogenic Cl(-)/HCO3(-) exchangers. While it is known that uterine fluid has high HCO3(-) content and that HCO3(-) is essential for sperm capacitation, the molecular mechanisms underlying the transport of HCO3(-) across uterine epithelial cells and sperm have not been fully investigated. The present review re-examines the results from early reports studying anion transport, finding clues for the involvement of Cl(-)/HCO3(-) anion exchangers in electrogenic HCO3(-) transport across endometrial epithelium. We also summarise recent work on Slc26a3 and Slc26a6 in uterine epithelial cells and sperm, revealing their functional role in working closely with the cystic fibrosis transmembrane conductance regulator (CFTR) for HCO3(-) transport in these cells. The possible involvement of these anion exchangers in other HCO3(-) dependent reproductive processes and their implications for infertility are also discussed.

Transcellular oxalate and Cl- absorption in mouse intestine is mediated by the DRA anion exchanger Slc26a3, and DRA deletion decreases urinary oxalate

Active transcellular oxalate transport in the mammalian intestine contributes to the homeostasis of this important lithogenic anion. Several members of the Slc26a gene family of anion exchangers have a measurable oxalate affinity and are expressed along the gut, apically and basolaterally. Mouse Slc26a6 (PAT1) targets to the apical membrane of enterocytes in the small intestine, and its deletion results in net oxalate absorption and hyperoxaluria. Apical exchangers of the Slc26a family that mediate oxalate absorption have not been established, yet the Slc26a3 [downregulated in adenoma (DRA)] protein is a candidate mediator of oxalate uptake. We evaluated the role of DRA in intestinal oxalate and Cl(-) transport by comparing unidirectional and net ion fluxes across short-circuited segments of small (ileum) and large (cecum and distal colon) intestine from wild-type (WT) and DRA knockout (KO) mice. In WT mice, all segments demonstrated net oxalate and Cl(-) absorption to varying degrees. In KO mice, however, all segments exhibited net anion secretion, which was consistently, and solely, due to a significant reduction in the absorptive unidirectional fluxes. In KO mice, daily urinary oxalate excretion was reduced 66% compared with that in WT mice, while urinary creatinine excretion was unchanged. We conclude that DRA mediates a predominance of the apical uptake of oxalate and Cl(-) absorbed in the small and large intestine of mice under short-circuit conditions. The large reductions in urinary oxalate excretion underscore the importance of transcellular intestinal oxalate absorption, in general, and, more specifically, the importance of the DRA exchanger in oxalate homeostasis.

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.

The SLC4 family of bicarbonate (HCO₃⁻) transporters

The SLC4 family consists of 10 genes (SLC4A1-5; SLC4A7-11). All encode integral membrane proteins with very similar hydropathy plots-consistent with 10-14 transmembrane segments. Nine SLC4 members encode proteins that transport HCO3(-) (or a related species, such as CO3(2-)) across the plasma membrane. Functionally, eight of these proteins fall into two major groups: three Cl-HCO3 exchangers (AE1-3) and five Na(+)-coupled HCO3(-) transporters (NBCe1, NBCe2, NBCn1, NBCn2, NDCBE). Two of the Na(+)-coupled transporters (NBCe1, NBCe2) are electrogenic; the other three Na(+)-coupled HCO3(-) transporters and all three AEs are electroneutral. In addition, two other SLC4 members (AE4, SLC4A9 and BTR1, SLC4A11) do not yet have a firmly established function. Most, though not all, SLC4 members are functionally inhibited by 4,4'-diisothiocyanatostilbene-2,2'-disulfonate (DIDS). SLC4 proteins play important roles many modes of acid-base homeostasis: the carriage of CO2 by erythrocytes, the transport of H(+) or HCO3(-) by several epithelia, as well as the regulation of cell volume and intracellular pH.

Physiology of epithelial chloride and fluid secretion

Epithelial salt and water secretion serves a variety of functions in different organ systems, such as the airways, intestines, pancreas, and salivary glands. In cystic fibrosis (CF), the volume and/or composition of secreted luminal fluids are compromised owing to mutations in the gene encoding CFTR, the apical membrane anion channel that is responsible for salt secretion in response to cAMP/PKA stimulation. This article examines CFTR and related cellular transport processes that underlie epithelial anion and fluid secretion, their regulation, and how these processes are altered in CF disease to account for organ-specific secretory phenotypes.

The cystic fibrosis of exocrine pancreas

The cystic fibrosis transmembrane conductance regulator (CFTR) protein is highly expressed in the pancreatic duct epithelia and permits anions and water to enter the ductal lumen. This results in an increased volume of alkaline fluid allowing the highly concentrated proteins secreted by the acinar cells to remain in a soluble state. This work will expound on the pathophysiology and pathology caused by the malfunctioning CFTR protein with special reference to ion transport and acid-base abnormalities both in humans and animal models. We will also discuss the relationship between cystic fibrosis (CF) and pancreatitis, and outline present and potential therapeutic approaches in CF treatment relevant to the pancreas.

The divergence, actions, roles, and relatives of sodium-coupled bicarbonate transporters

The mammalian Slc4 (Solute carrier 4) family of transporters is a functionally diverse group of 10 multi-spanning membrane proteins that includes three Cl-HCO3 exchangers (AE1-3), five Na(+)-coupled HCO3(-) transporters (NCBTs), and two other unusual members (AE4, BTR1). In this review, we mainly focus on the five mammalian NCBTs-NBCe1, NBCe2, NBCn1, NDCBE, and NBCn2. Each plays a specialized role in maintaining intracellular pH and, by contributing to the movement of HCO3(-) across epithelia, in maintaining whole-body pH and otherwise contributing to epithelial transport. Disruptions involving NCBT genes are linked to blindness, deafness, proximal renal tubular acidosis, mental retardation, and epilepsy. We also review AE1-3, AE4, and BTR1, addressing their relevance to the study of NCBTs. This review draws together recent advances in our understanding of the phylogenetic origins and physiological relevance of NCBTs and their progenitors. Underlying these advances is progress in such diverse disciplines as physiology, molecular biology, genetics, immunocytochemistry, proteomics, and structural biology. This review highlights the key similarities and differences between individual NCBTs and the genes that encode them and also clarifies the sometimes confusing NCBT nomenclature.

In-vivo functional study on the involvement of CFTR, SLC26A6, NHE-1 and CA isoenzymes II and XII in uterine fluid pH, volume and electrolyte regulation in rats under different sex-steroid influence

Precise control of uterine fluid pH, volume and electrolytes is important for the reproductive processes. In this study, we examined the functional involvement of multiple proteins including Cystic Fibrosis Transmembrane Regulator (CFTR), Cl(-)/HCO3 (-) exchanger (SLC26A6), sodium-hydrogen exchanger-1 (NHE-1) and carbonic anhydrase (CA) in the regulation of these uterine fluid parameters.

METHODS

Adult female WKY rats were divided into intact, non-ovariectomised at different oestrous cycle phases and ovariectomised treated with sex-steroids. Following oestrous phase identification or sex-steroid treatment, in-vivo uterine perfusion was performed with and without the presence of these inhibitors: glibenclamide, DIDS, ACTZ and EIPA. The pH, volume, Cl(-), HCO3 (-) and Na(+) concentrations of the perfusate from different groups were then analyzed. Meanwhile, the expression of CFTR, SLC26A6, NHE-1, CAII and CAXII was visualized by immunohistochemistry (IHC).

RESULTS

Parallel increase in the pH, volume, Cl(-), HCO3 (-) and Na(+) concentrations was observed at estrus (Es), proestrus (Ps) and following 17β-oestradiol (E) treatment, which was inhibited by glibenclamide, DIDS and ACTZ while parallel reduction in these parameters was observed at diestrus (Ds) and following progesterone (P) treatment which was inhibited by ACTZ and EIPA. CFTR and SLC26A6 expression were up-regulated under E dominance, while NHE-1 expression was up-regulated under P dominance. Meanwhile, CA isoenzymes were expressed under both E and P influence.

CONCLUSION

CFTR, SLC26A6 and CA were involved in mediating parallel increase in the uterine fluid volume, pH and electrolyte concentration under E while NHE and CA were involved in mediating the reduction of these parameters under P.

Deletion of Slc26a6 alters the stoichiometry of apical Cl-/HCO-3 exchange in mouse pancreatic duct

To define the stoichiometry and molecular identity of the Cl(-)/HCO(3)(-) exchanger in the apical membrane of pancreatic duct cells, changes in luminal pH and volume were measured simultaneously in interlobular pancreatic ducts isolated from wild-type and Slc26a6-null mice. Transepithelial fluxes of HCO(3)(-) and Cl(-) were measured in the presence of anion gradients favoring rapid exchange of intracellular HCO(3)(-) with luminal Cl(-) in cAMP-stimulated ducts. The flux ratio of Cl(-) absorption/HCO(3)(-) secretion was ∼0.7 in wild-type ducts and ∼1.4 in Slc26a6(-/-) ducts where a different Cl(-)/HCO(3)(-) exchanger, most likely SLC26A3, was found to be active. Interactions between Cl(-)/HCO(3)(-) exchange and cystic fibrosis transmembrane conductance regulator (CFTR) in cAMP-stimulated ducts were examined by measuring the recovery of intracellular pH after alkali-loading by acetate prepulse. Hyperpolarization induced by luminal application of CFTRinh-172 enhanced HCO(3)(-) efflux across the apical membrane via SLC26A6 in wild-type ducts but significantly reduced HCO(3)(-) efflux in Slc26a6(-/-) ducts. In microperfused wild-type ducts, removal of luminal Cl(-), or luminal application of dihydro-4,4'-diisothiocyanatostilbene-2,2'-disulphonic acid to inhibit SLC26A6, caused membrane hyperpolarization, which was abolished in Slc26a6(-/-) ducts. In conclusion, we have demonstrated that deletion of Slc26a6 alters the apparent stoichiometry of apical Cl(-)/HCO(3)(-) exchange in native pancreatic duct. Our results are consistent with SLC26A6 mediating 1:2 Cl(-)/HCO(3)(-) exchange, and the exchanger upregulated in its absence, most probably SLC26A3, mediating 2:1 exchange.

Participation of the Cl-/HCO(3)- exchangers SLC26A3 and SLC26A6, the Cl- channel CFTR, and the regulatory factor SLC9A3R1 in mouse sperm capacitation

Sperm capacitation is required for fertilization and involves several ion permeability changes. Although Cl(-) and HCO(3)(-) are essential for capacitation, the molecular entities responsible for their transport are not fully known. During mouse sperm capacitation, the intracellular concentration of Cl(-) ([Cl(-)](i)) increases and membrane potential (Em) hyperpolarizes. As in noncapacitated sperm, the Cl(-) equilibrium potential appears to be close to the cell resting Em, opening of Cl(-) channels could not support the [Cl(-)](i) increase observed during capacitation. Alternatively, the [Cl(-)](i) increase might be mediated by anion exchangers. Among them, SLC26A3 and SLC26A6 are good candidates, since, in several cell types, they increase [Cl(-)](i) and interact with cystic fibrosis transmembrane conductance regulator (CFTR), a Cl(-) channel present in mouse and human sperm. This interaction is known to be mediated and probably regulated by the Na(+)/H(+) regulatory factor-1 (official symbol, SLC9A3R1). Our RT-PCR, immunocytochemistry, Western blot, and immunoprecipitation data indicate that SLC26A3, SLC26A6, and SLC9A3R1 are expressed in mouse sperm, localize to the midpiece, and interact between each other and with CFTR. Moreover, we present evidence indicating that CFTR and SLC26A3 are involved in the [Cl(-)](i) increase induced by db-cAMP in noncapacitated sperm. Furthermore, we found that inhibitors of SLC26A3 (Tenidap and 5099) interfere with the Em changes that accompany capacitation. Together, these findings indicate that a CFTR/SLC26A3 functional interaction is important for mouse sperm capacitation.

SLC26 anion exchangers of guinea pig pancreatic duct: molecular cloning and functional characterization

The secretin-stimulated human pancreatic duct secretes HCO(3)(-)-rich fluid essential for normal digestion. Optimal stimulation of pancreatic HCO(3)(-) secretion likely requires coupled activities of the cystic fibrosis transmembrane regulator (CFTR) anion channel and apical SLC26 Cl(-)/HCO(3)(-) exchangers. However, whereas stimulated human and guinea pig pancreatic ducts secrete ∼140 mM HCO(3)(-) or more, mouse and rat ducts secrete ∼40-70 mM HCO(3)(-). Moreover, the axial distribution and physiological roles of SLC26 anion exchangers in pancreatic duct secretory processes remain controversial and may vary among mammalian species. Thus the property of high HCO(3)(-) secretion shared by human and guinea pig pancreatic ducts prompted us to clone from guinea pig pancreatic duct cDNAs encoding Slc26a3, Slc26a6, and Slc26a11 polypeptides. We then functionally characterized these anion transporters in Xenopus oocytes and human embryonic kidney (HEK) 293 cells. In Xenopus oocytes, gpSlc26a3 mediated only Cl(-)/Cl(-) exchange and electroneutral Cl(-)/HCO(3)(-) exchange. gpSlc26a6 in Xenopus oocytes mediated Cl(-)/Cl(-) exchange and bidirectional exchange of Cl(-) for oxalate and sulfate, but Cl(-)/HCO(3)(-) exchange was detected only in HEK 293 cells. gpSlc26a11 in Xenopus oocytes exhibited pH-dependent Cl(-), oxalate, and sulfate transport but no detectable Cl(-)/HCO(3)(-) exchange. The three gpSlc26 anion transporters exhibited distinct pharmacological profiles of (36)Cl(-) influx, including partial sensitivity to CFTR inhibitors Inh-172 and GlyH101, but only Slc26a11 was inhibited by PPQ-102. This first molecular and functional assessment of recombinant SLC26 anion transporters from guinea pig pancreatic duct enhances our understanding of pancreatic HCO(3)(-) secretion in species that share a high HCO(3)(-) secretory output.

Regulation of electroneutral NaCl absorption by the small intestine

Na(+) and Cl(-) movement across the intestinal epithelium occurs by several interconnected mechanisms: (a) nutrient-coupled Na(+) absorption, (b) electroneutral NaCl absorption, (c) electrogenic Cl(-) secretion by CFTR, and (d) electrogenic Na(+) absorption by ENaC. All these transport modes require a favorable electrochemical gradient maintained by the basolateral Na(+)/K(+)-ATPase, a Cl(-) channel, and K(+) channels. Electroneutral NaCl absorption is observed from the small intestine to the distal colon. This transport is mediated by apical Na(+)/H(+) (NHE2/3) and Cl(-)/HCO(3)(-) (Slc26a3/a6 and others) exchangers that provide the major route of NaCl absorption. Electroneutral NaCl absorption and Cl(-) secretion by CFTR are oppositely regulated by the autonomic nerve system, the immune system, and the endocrine system via PKAα, PKCα, cGKII, and/or SGK1. This integrated regulation requires the formation of macromolecular complexes, which are mediated by the NHERF family of scaffold proteins and involve internalization of NHE3. Through use of knockout mice and human mutations, a more detailed understanding of the integrated as well as subtle regulation of electroneutral NaCl absorption by the mammalian intestine has emerged.

Effects of CFTR gene silencing by siRNA or the luminal application of a CFTR activator on fluid secretion from guinea-pig pancreatic duct cells

AIMS

The cystic fibrosis transmembrane conductance regulator (CFTR) is a cyclic AMP regulated chloride channel expressed in the apical plasma membrane of pancreatic duct cells where it plays an important role in fluid secretion. The purpose of this study was to elucidate the role of the CFTR chloride channel on ion and fluid secretion from the guinea-pig pancreas by manipulating the expression of CFTR by RNA interference or by luminal application of a CFTR selective activator, MPB91, in isolated cultured pancreatic ducts.

MATERIALS AND METHODS

Using cDNA isolated from the guinea-pig small intestine, fragments of the CFTR gene were generated by polymerase chain reaction and directly sequenced. Two different RNA duplexes for small interference RNA (siRNA) were designed from the sequence obtained. Fluid secretion from the isolated guinea-pig pancreatic ducts was measured using video-microscopy. The amount of CFTR chloride channel or AQP1 water channel expressed in pancreatic ducts was examined by immunoblotting with antibodies against CFTR or AQP1, respectively.

RESULTS

Guinea-pig CFTR consists of 1481 amino acid residues. An additional glutamine residue was found to be inserted between amino acid residues 403 and 404 of human CFTR. Forskolin-stimulated fluid secretion from intact pancreatic ducts was significantly higher in the presence of MPB91 compared to fluid secretion in the absence of MPB91. Both basal and forskolin-stimulated fluid secretion in pancreatic ducts transfected with CFTR specific siRNAs were reduced by ∼50% compared to fluid secretion from ducts transfected with scrambled negative control dsRNAs. The amount of CFTR and AQP1 proteins was reduced to 34% and 45% of control, respectively.

CONCLUSIONS

The activity of the CFTR chloride channel or the amount of CFTR protein expressed determines the rate of fluid secretion from the isolated guinea-pig pancreatic ducts.

Solution structure of the guanine nucleotide-binding STAS domain of SLC26-related SulP protein Rv1739c from Mycobacterium tuberculosis

The structure and intrinsic activities of conserved STAS domains of the ubiquitous SulP/SLC26 anion transporter superfamily have until recently remained unknown. Here we report the heteronuclear, multidimensional NMR spectroscopy solution structure of the STAS domain from the SulP/SLC26 putative anion transporter Rv1739c of Mycobacterium tuberculosis. The 0.87-Å root mean square deviation structure revealed a four-stranded β-sheet with five interspersed α-helices, resembling the anti-σ factor antagonist fold. Rv1739c STAS was shown to be a guanine nucleotide-binding protein, as revealed by nucleotide-dependent quench of intrinsic STAS fluorescence and photoaffinity labeling. NMR chemical shift perturbation analysis partnered with in silico docking calculations identified solvent-exposed STAS residues involved in nucleotide binding. Rv1739c STAS was not an in vitro substrate of mycobacterial kinases or anti-σ factors. These results demonstrate that Rv1739c STAS binds guanine nucleotides at physiological concentrations and undergoes a ligand-induced conformational change but, unlike anti-σ factor antagonists, may not mediate signals via phosphorylation.

The role of CFTR in bicarbonate secretion by pancreatic duct and airway epithelia

The secretory epithelia of the pancreatic duct and airway share the ability to generate HCO(3)(-)-rich fluids. They both express CFTR (cystic fibrosis transmembrane conductance regulator) at the apical membrane and both are adversely affected by cystic fibrosis. CFTR is predominantly a Cl(-) channel, and it is widely believed that HCO(3)(-) secretion in the pancreatic duct is mediated mainly by a Cl(-)/HCO(3)(-) exchanger at the apical membrane. Studies on airway epithelia, however, have suggested that CFTR, despite its low permeability to HCO(3)(-), may nonetheless be directly responsible for HCO(3)(-) secretion across the apical membrane. This article reviews recent work that has re-examined both of these hypotheses.

Effects of Slc26a6 deletion and CFTR inhibition on HCO3- secretion by mouse pancreatic duct

Pancreatic duct epithelium secretes HCO(3)(-)-rich fluid, which is dependent on cystic fibrosis transmembrane conductance regulator (CFTR). HCO(3)(-) transport across the apical membrane is thought to be mediated by both SLC26A6 Cl(-)-HCO(3)(-) exchange and CFTR HCO(3)(-) conductance. In this study we examined the relative contribution and interaction of SLC26A6 and CFTR in apical HCO(3)(-) transport. Interlobular pancreatic ducts were isolated from slc26a6 null mice. Intracellular pH (pH(i)) was measured by BCECF microfluorometry. Duct cells were stimulated with forskolin and alkalinized by acetate pre-pulse in the presence of HCO(3)(-)-CO(2). Apical HCO(3)(-) secretion was estimated from the recovery rate of pH(i) from alkaline load. When the lumen was perfused with high-Cl(-) solution, the rate of apical HCO(3)(-) secretion was increased by luminal application of CFTRinh-172 in ducts from wild-type mice but it was decreased in ducts from slc26a6 -/- mice. This suggests that slc26a6 and CFTR compensate/compete with each other for apical HCO(3)(-) secretion with high Cl(-) in the lumen. With high HCO(3)(-) in the lumen, luminal CFTRinh-172 reduced the rate of apical HCO(3)(-) secretion in both wild-type and slc26a6 -/- ducts. This suggests that HCO(3)(-) conductance of CFTR mediates a significant portion of apical HCO(3)(-) secretion with high HCO(3)(-) in the lumen.

Slc26a9 is inhibited by the R-region of the cystic fibrosis transmembrane conductance regulator via the STAS domain

SLC26 proteins function as anion exchangers, channels, and sensors. Previous cellular studies have shown that Slc26a3 and Slc26a6 interact with the R-region of the cystic fibrosis transmembrane conductance regulator (CFTR), (R)CFTR, via the Slc26-STAS (sulfate transporter anti-sigma) domain, resulting in mutual transport activation. We recently showed that Slc26a9 has both nCl(-)-HCO(3)(-) exchanger and Cl(-) channel function. In this study, we show that the purified STAS domain of Slc26a9 (a9STAS) binds purified (R)CFTR. When Slc26a9 and (R)CFTR fragments are co-expressed in Xenopus oocytes, both Slc26a9-mediated nCl(-)-HCO(3)(-) exchange and Cl(-) currents are almost fully inhibited. Deletion of the Slc26a9 STAS domain (a9-DeltaSTAS) virtually eliminated the Cl(-) currents with only a modest affect on nCl(-)-HCO(3)(-) exchange activity. Co-expression of a9-DeltaSTAS and the (R)CFTR fragment did not alter the residual a9-DeltaSTAS function. Replacing the Slc26a9 STAS domain with the Slc26a6 STAS domain (a6-a9-a6) does not change Slc26a9 function and is no longer inhibited by (R)CFTR. These data indicate that the Slc26a9-STAS domain, like other Slc26-STAS domains, binds CFTR in the R-region. However, unlike previously reported data, this binding interaction inhibits Slc26a9 ion transport activity. These results imply that Slc26-STAS domains may all interact with (R)CFTR but that the physiological outcome is specific to differing Slc26 proteins, allowing for dynamic and acute fine tuning of ion transport for various epithelia.

Functional coupling of apical Cl-/HCO3- exchange with CFTR in stimulated HCO3- secretion by guinea pig interlobular pancreatic duct

Pancreatic ductal epithelium produces a HCO(3)(-)-rich fluid. HCO(3)(-) transport across ductal apical membranes has been proposed to be mediated by both SLC26-mediated Cl(-)/HCO(3)(-) exchange and CFTR-mediated HCO(3)(-) conductance, with proportional contributions determined in part by axial changes in gene expression and luminal anion composition. In this study we investigated the characteristics of apical Cl(-)/HCO(3)(-) exchange and its functional interaction with Cftr activity in isolated interlobular ducts of guinea pig pancreas. BCECF-loaded epithelial cells of luminally microperfused ducts were alkalinized by acetate prepulse or by luminal Cl(-) removal in the presence of HCO(3)(-)-CO(2). Intracellular pH recovery upon luminal Cl(-) restoration (nominal Cl(-)/HCO(3)(-) exchange) in cAMP-stimulated ducts was largely inhibited by luminal dihydro-DIDS (H(2)DIDS), accelerated by luminal CFTR inhibitor inh-172 (CFTRinh-172), and was insensitive to elevated bath K(+) concentration. Luminal introduction of CFTRinh-172 into sealed duct lumens containing BCECF-dextran in HCO(3)(-)-free, Cl(-)-rich solution enhanced cAMP-stimulated HCO(3)(-) secretion, as calculated from changes in luminal pH and volume. Luminal Cl(-) removal produced, after a transient small depolarization, sustained cell hyperpolarization of approximately 15 mV consistent with electrogenic Cl(-)/HCO(3)(-) exchange. The hyperpolarization was inhibited by H(2)DIDS and potentiated by CFTRinh-172. Interlobular ducts expressed mRNAs encoding CFTR, Slc26a6, and Slc26a3, as detected by RT-PCR. Thus Cl(-)-dependent apical HCO(3)(-) secretion in pancreatic duct is mediated predominantly by an Slc26a6-like Cl(-)/HCO(3)(-) exchanger and is accelerated by inhibition of CFTR. This study demonstrates functional coupling between Cftr and Slc26a6-like Cl(-)/HCO(3)(-) exchange activity in apical membrane of guinea pig pancreatic interlobular duct.

CFTR functions as a bicarbonate channel in pancreatic duct cells

Pancreatic duct epithelium secretes a HCO(3)(-)-rich fluid by a mechanism dependent on cystic fibrosis transmembrane conductance regulator (CFTR) in the apical membrane. However, the exact role of CFTR remains unclear. One possibility is that the HCO(3)(-) permeability of CFTR provides a pathway for apical HCO(3)(-) efflux during maximal secretion. We have therefore attempted to measure electrodiffusive fluxes of HCO(3)(-) induced by changes in membrane potential across the apical membrane of interlobular ducts isolated from the guinea pig pancreas. This was done by recording the changes in intracellular pH (pH(i)) that occurred in luminally perfused ducts when membrane potential was altered by manipulation of bath K(+) concentration. Apical HCO(3)(-) fluxes activated by cyclic AMP were independent of Cl(-) and luminal Na(+), and substantially inhibited by the CFTR blocker, CFTR(inh)-172. Furthermore, comparable HCO(3)(-) fluxes observed in ducts isolated from wild-type mice were absent in ducts from cystic fibrosis (Delta F) mice. To estimate the HCO(3)(-) permeability of the apical membrane under physiological conditions, guinea pig ducts were luminally perfused with a solution containing 125 mM HCO(3)(-) and 24 mM Cl(-) in the presence of 5% CO(2). From the changes in pH(i), membrane potential, and buffering capacity, the flux and electrochemical gradient of HCO(3)(-) across the apical membrane were determined and used to calculate the HCO(3)(-) permeability. Our estimate of approximately 0.1 microm sec(-1) for the apical HCO(3)(-) permeability of guinea pig duct cells under these conditions is close to the value required to account for observed rates of HCO(3)(-) secretion. This suggests that CFTR functions as a HCO(3)(-) channel in pancreatic duct cells, and that it provides a significant pathway for HCO(3)(-) transport across the apical membrane.

The solute carrier 26 family of proteins in epithelial ion transport

Transepithelial Cl(-) and HCO(3)(-) transport is critically important for the function of all epithelia and, when altered or ablated, leads to a number of diseases, including cystic fibrosis, congenital chloride diarrhea, deafness, and hypotension (78, 111, 119, 126). HCO(3)(-) is the biological buffer that maintains acid-base balance, thereby preventing metabolic and respiratory acidosis (48). HCO(3)(-) also buffers the pH of the mucosal layers that line all epithelia, protecting them from injury (2). Being a chaotropic ion, HCO(3)(-) is essential for solubilization of ions and macromolecules such as mucins and digestive enzymes in secreted fluids. Most epithelia have a Cl(-)/HCO(3) exchange activity in the luminal membrane. The molecular nature of this activity remained a mystery for many years until the discovery of SLC26A3 and the realization that it is a member of a new family of Cl(-) and HCO(3)(-) transporters, the SLC26 family (73, 78). This review will highlight structural features, the functional diversity, and several regulatory aspects of the SLC26 transporters.

Accretion of visceral fat and hepatic insulin resistance in pregnant rats

Insulin resistance (IR) is a hallmark of pregnancy. Because increased visceral fat (VF) is associated with IR in nonpregnant states, we reasoned that fat accretion might be important in the development of IR during pregnancy. To determine whether VF depots increase in pregnancy and whether VF contributes to IR, we studied three groups of 6-mo-old female Sprague-Dawley rats: 1) nonpregnant sham-operated rats (Nonpreg; n = 6), 2) pregnant sham-operated rats (Preg; n = 6), and 3) pregnant rats in which VF was surgically removed 1 mo before mating (PVF-; n = 6). VF doubled by day 19 of pregnancy (Nonpreg 5.1 +/- 0.3, Preg 10.0 +/- 1.0 g, P < 0.01), and PVF- had similar amounts of VF compared with Nonpreg (PVF- 4.6 +/- 0.8 g). Insulin sensitivity was measured by hyperinsulinemic-euglycemic clamp in late gestation in chronically catheterized unstressed rats. Glucose IR (mg.kg(-1).min(-1)) was highest in Nonpreg (19.4 +/- 2.0), lowest in Preg (11.1 +/- 1.4), and intermediate in PVF- (14.7 +/- 0.6; P < 0.001 between all groups). During the clamp, Nonpreg had greater hepatic insulin sensitivity than Preg [hepatic glucose production (HGP): Nonpreg 4.5 +/- 1.3, Preg 9.3 +/- 0.5 mg.kg(-1).min(-1); P < 0.001]. With decreased VF, hepatic insulin sensitivity was similar to nonpregnant levels in PVF- (HGP 4.9 +/- 0.8 mg.kg(-1).min(-1)). Both pregnant groups had lower peripheral glucose uptake compared with Nonpreg. In parallel with hepatic insulin sensitivity, hepatic triglyceride content was increased in pregnancy (Nonpreg 1.9 +/- 0.4 vs. Preg 3.2 +/- 0.3 mg/g) and decreased with removal of VF (PVF- 1.3 +/- 0.4 mg/g; P < 0.05). Accretion of visceral fat is an important component in the development of hepatic IR in pregnancy, and accumulation of hepatic triglycerides is a mechanism by which visceral fat may modulate insulin action in pregnancy.

Expression and localization of cystic fibrosis transmembrane conductance regulator in the rat endocrine pancreas

Impaired glucose tolerance and overt diabetes mellitus are becoming increasingly common complications of cystic fibrosis (CF), most probably merely as a result of increased life expectancy. In order to understand the pathophysiology of cystic fibrosis-related diabetes (CFRD), knowledge on the possible expression and cell distribution of the cystic fibrosis transmembrane conductance regulator (CFTR) protein within the endocrine pancreas is required. In this report, we establish the first evidence for expression of CFTR protein in rat pancreatic islets by using independent techniques. First reverse transcriptase-polymerase chain reaction (RT-PCR) amplification showed that CFTR mRNA is present in isolated islets of Langerhans. Furthermore, the analysis of flow cytometry-separated islet cells indicated that the level of CFTR transcripts is significantly higher in the non-beta than in beta-cell populations. The expression of CFTR protein in rat islet cells was also demonstrated by Western blotting and the level of expression was also found significantly higher in the non-beta than in beta-cell populations. Last, in situ immunocytochemistry studies with two monoclonal antibodies recognizing different CFTR epitopes indicated that CFTR expression occurs mainly in glucagon-secreting alpha-cells.

Sodium and chloride absorptive defects in the small intestine in Slc26a6 null mice

PAT1 (Slc26a6) is located on the apical membrane of the small intestinal villi, but its role for salt absorption has not been studied. To ascertain the role of Slc26a6 in jejunal sodium and chloride absorption, and its interplay with NHE3, muscle-stripped jejuna from Slc26a6+/+ and -/- and NHE3 +/+ and -/- mice were mounted in Ussing chambers and electrical parameters, and (36)Cl(-) and (22)Na(+) fluxes were measured. In parallel studies, expression of the apical Na(+)/H(+) exchanger (NHE3) was examined by immunofluorescence labeling and immunoblot analysis in brush border membrane (BBM). In the basal state, net Cl(-) and Na(+) fluxes were absorptive in Slc26a6-/- and +/+ jejuni, but significantly decreased in -/- animals. Upon forskolin addition, net Na(+) absorption decreased, Isc strongly increased, and net Cl(-) flux became secretory in Slc26a6-/- and +/+ jejuni. When luminal glucose was added to activate Na(+)/glucose cotransport, concomitant Cl(-) absorption was significantly reduced in Slc26a6 -/- jejuni, while Na(+) absorption increased to the same degree in Slc26a6 -/- and +/+ jejuni. Identical experiments in NHE3-deficient jejuni also showed reduced Na(+) and Cl(-) absorption. Results further demonstrated that the lack of NHE3 rendered Na(+) and Cl(-) absorption unresponsive to inhibition by cAMP, but did not affect glucose-driven Na(+) and Cl(-) absorption. Immunoblotting revealed comparable NHE3 abundance and distribution in apical membranes in Slc26a6-/- and +/+ mice. The data strongly suggests that Slc26a6 acts in concert with NHE3 in electroneutral salt absorption in the small intestine. Slc26a6 also serves to absorb Cl(-) during glucose-driven salt absorption.

In a hypergravity environment neonatal survival is adversely affected by alterations in dam tissue metabolism rather than reduced food intake

Exposure of rat dams to hypergravity during pregnancy is associated with increased pup mortality, reduced food intake, and decreased rates of glucose oxidation and lipogenesis in mammary tissue. We hypothesized that increased pup mortality is due to changes in maternal metabolism and not to reduced food intake of dams. Effects of hypergravity on rate of glucose oxidation and lipogenesis in mammary, liver, and adipose tissue were measured in rat dams centrifuged at 2.0 G [hypergravity (HG)], kept at 1.0 G (control), or fed to match the intake of HG rats (pair fed) from gestation day 11 (G11) until G21 or postpartum day 3 (P3). Body weight, percent body fat, metabolizable energy, and nitrogen balance were significantly less in HG dams compared with controls ( P < 0.05); however, these factors were not different between HG and pair-fed dams. By P3, 100% of control and pair-fed pups survived, while only 49% of HG pups survived. At G21, rates of glucose oxidation and lipogenesis in mammary and adipose tissue were less in HG than in control and pair-fed dams ( P < 0.1 and P < 0.05). In liver, at G21, the rate of lipogenesis was greater in HG than control and pair-fed dams ( P < 0.01); at P3, lipogenesis was greater in control than HG and pair-fed dams ( P < 0.05). Gene expression of ATP citrate lyase, acetyl-CoA carboxylase, and fatty acid synthase increased in liver from pregnancy to lactation in control and pair-fed dams but not HG dams. Thus reduced food intake and body mass due to hypergravity exposure cannot explain the dramatic decrease in HG pup survival.

Slc26a6 regulates CFTR activity in vivo to determine pancreatic duct HCO3- secretion: relevance to cystic fibrosis

Fluid and HCO(3)(-) secretion are vital functions of the pancreatic duct and other secretory epithelia. CFTR and Cl(-)/HCO(3)(-) exchange activity at the luminal membrane are required for these functions. The molecular identity of the Cl(-)/HCO(3)(-) exchangers and their relationship with CFTR in determining fluid and HCO(3)(-) secretion are not known. We show here that the Cl(-)/HCO(3)(-) exchanger slc26a6 controls CFTR activity and ductal fluid and HCO(3)(-) secretion. Unexpectedly, deletion of slc26a6 in mice and measurement of fluid and HCO(3)(-) secretion into sealed intralobular pancreatic ducts revealed that deletion of slc26a6 enhanced spontaneous and decreased stimulated secretion. Remarkably, inhibition of CFTR activity with CFTR(inh)-172, knock-down of CFTR by siRNA and measurement of CFTR current in WT and slc26a6(-/-) duct cells revealed that deletion of slc26a6 resulted in dis-regulation of CFTR activity by removal of tonic inhibition of CFTR by slc26a6. These findings reveal the intricate regulation of CFTR activity by slc26a6 in both the resting and stimulated states and the essential role of slc26a6 in pancreatic HCO(3)(-) secretion in vivo.