Downregulated in adenoma and putative anion transporter are regulated by CFTR in cultured pancreatic duct cells

Metabolic alkalosis in cystic fibrosis: from vascular volume depletion to impaired bicarbonate excretion

Cystic fibrosis (CF) is the most common life-threatening genetic disease in the United States and among people of European descent. Despite the widespread distribution of the cystic fibrosis transmembrane conductance regulator (CFTR) along kidney tubules, specific renal phenotypes attributable to CF have not been well documented. Recent studies have demonstrated the downregulation of the apical Cl-/HCO3 - exchanger pendrin (Slc26a4) in kidney B-intercalated cells of CF mouse models. These studies have shown that kidneys of both mice and humans with CF have an impaired ability to excrete excess HCO3 -, thus developing metabolic alkalosis when subjected to excess HCO3 - intake. The purpose of this minireview is to discuss the latest advances on the role of pendrin as a molecule with dual critical roles in acid base regulation and systemic vascular volume homeostasis, specifically in CF. Given the immense prevalence of vascular volume depletion, which is primarily precipitated via enhanced chloride loss through perspiration, we suggest that the dominant presentation of metabolic alkalosis in CF is due to the impaired function of pendrin, which plays a critical role in systemic vascular volume and acid base homeostasis.

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.

Hyperglycinuria: diagnosis in middle age

Isolated hyperglycinuria is a rare disorder that is associated with osteoporosis and renal calculi. We report findings in a middle-aged, black woman who presented for renal function evaluation with a history of transient hypobicarbonataemia associated with topiramate therapy. She displayed the full triad of high urinary glycine, early-onset osteopenia despite normal reproductive hormones, and renal calculus with high urinary oxalate, phosphate and uric acid. Parathyroid hormone and fibroblast growth factor 23 were both normal. Formal genetic testing did not reveal mutations in SLC6A20, SLC6A18, SLC6A19, SLC36A2, the known genes associated with glycinuria; however, black individuals are poorly represented in the genetic databases. It may well be that otherwise unidentified mutations may be present or that topiramate may result in a lingering proximal tubule defect even after cessation of the drug.

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.

Activation of the PKA signaling pathway stimulates oxalate transport by human intestinal Caco2-BBE cells

Most kidney stones are composed of calcium oxalate, and small increases in urine oxalate enhance the stone risk. The mammalian intestine plays a crucial role in oxalate homeostasis, and we had recently reported that Oxalobacter-derived factors stimulate oxalate transport by human intestinal Caco2-BBE (C2) cells through PKA activation. We therefore evaluated whether intestinal oxalate transport is directly regulated by activation of the PKA signaling pathway. To this end, PKA was activated with forskolin and IBMX (F/I). F/I significantly stimulated (3.7-fold) [14C]oxalate transport by C2 cells [≥49% of which is mediated by the oxalate transporter SLC26A6 (A6)], an effect completely blocked by the PKA inhibitor H89, indicating that it is PKA dependent. PKA stimulation of intestinal oxalate transport is not cell line specific, since F/I similarly stimulated oxalate transport by the human intestinal T84 cells. F/I significantly increased (2.5-fold) A6 surface protein expression by use of immunocytochemistry. Assessing [14C]oxalate transport as a function of increasing [14C]oxalate concentration in the flux medium showed that the observed stimulation is due to a F/I-induced increase (1.8-fold) in Vmax and reduction (2-fold) in Km. siRNA knockdown studies showed that significant components of the observed stimulation are mediated by A6 and SLC26A2 (A2). Besides enhancing A6 surface protein expression, it is also possible that the observed stimulation is due to PKA-induced enhanced A6 and/or A2 transport activity in view of the reduced Km. We conclude that PKA activation positively regulates oxalate transport by intestinal epithelial cells and that PKA agonists might therapeutically impact hyperoxalemia, hyperoxaluria, and related kidney stones.

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.

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.

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

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

Oxalobacter formigenes-Derived Bioactive Factors Stimulate Oxalate Transport by Intestinal Epithelial Cells

Hyperoxaluria is a major risk factor for kidney stones and has no specific therapy, although Oxalobacter formigenes colonization is associated with reduced stone risk. O. formigenes interacts with colonic epithelium and induces colonic oxalate secretion, thereby reducing urinary oxalate excretion, via an unknown secretagogue. The difficulties in sustaining O. formigenes colonization underscore the need to identify the derived factors inducing colonic oxalate secretion. We therefore evaluated the effects of O. formigenes culture conditioned medium (CM) on apical ¹⁴C-oxalate uptake by human intestinal Caco-2-BBE cells. Compared with control medium, O. formigenes CM significantly stimulated oxalate uptake (>2.4-fold), whereas CM from Lactobacillus acidophilus did not. Treating the O. formigenes CM with heat or pepsin completely abolished this bioactivity, and selective ultrafiltration of the CM revealed that the O. formigenes-derived factors have molecular masses of 10-30 kDa. Treatment with the protein kinase A inhibitor H89 or the anion exchange inhibitor 4,4'-diisothiocyano-2,2'-stilbenedisulfonic acid completely blocked the CM-induced oxalate transport. Knockdown of the oxalate transporter SLC26A6 also significantly restricted the induction of oxalate transport by CM. In a mouse model of primary hyperoxaluria type 1, rectal administration of O. formigenes CM significantly reduced (>32.5%) urinary oxalate excretion and stimulated (>42%) distal colonic oxalate secretion. We conclude that O. formigenes-derived bioactive factors stimulate oxalate transport in intestinal cells through mechanisms including PKA activation. The reduction in urinary oxalate excretion in hyperoxaluric mice treated with O. formigenes CM reflects the in vivo retention of biologic activity and the therapeutic potential of these factors.

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.

Functional characteristics of L1156F-CFTR associated with alcoholic chronic pancreatitis in Japanese

Although cystic fibrosis is rare in Japanese, measurement of sweat Cl(-) has suggested mild dysfunction of cystic fibrosis transmembrane conductance regulator (CFTR) in some patients with chronic pancreatitis. In the present study, we have investigated the association of CFTR variants and chronic pancreatitis in Japanese and the functional characteristics of a Japanese- and pancreatitis-specific CFTR variant, L1156F. Seventy patients with alcoholic chronic pancreatitis, 18 patients with idiopathic chronic pancreatitis, and 180 normal subjects participated. All exons and their boundaries and promoter region of the CFTR gene were sequenced. Human embryonic kidney-293 cells were transfected with three CFTR variants (M470V, L1156F, and M470V+L1156F), and the protein expression was examined. Xenopus laevis oocytes were injected with the CFTR variants, and bicarbonate (HCO3 (-)) transport activity was examined. CFPAC-1 cells were transfected with the CFTR variants and Cl(-)/HCO3 (-) exchange activity was examined. Six variants (E217G, I556V, M470V, L1156F, Q1352H, and R1453W) were identified in the coding region of the CFTR gene. Cystic fibrosis-causing mutations were not found. The allele frequencies of L1156F and Q1352H in alcoholic chronic pancreatitis (5.0 and 7.9%) were significantly (P < 0.01) higher than those in normal subjects (0.6 and 1.9%). L1156F was linked with a worldwide CFTR variant, M470V. Combination of M470V and L1156F significantly reduced CFTR expression to ∼60%, impaired CFTR-mediated HCO3 (-)/Cl(-) transport activity to 50-60%, and impaired CFTR-coupled Cl(-)/HCO3 (-) exchange activity to 20-30%. The data suggest that the Japanese-specific CFTR variant L1156F causes mild dysfunction of CFTR and increases the risk of alcoholic chronic pancreatitis in Japanese.

Composition of mineralizing incisor enamel in cystic fibrosis transmembrane conductance regulator-deficient mice

Formation of crystals in the enamel space releases protons that need to be buffered to sustain mineral accretion. We hypothesized that apical cystic fibrosis transmembrane conductance regulator (CFTR) in maturation ameloblasts transduces chloride into forming enamel as a critical step to secrete bicarbonates. We tested this by determining the calcium, chloride, and fluoride levels in developing enamel of Cftr-null mice by quantitative electron probe microanalysis. Maturation-stage enamel from Cftr-null mice contained less chloride and calcium than did wild-type enamel, was more acidic when stained with pH dyes ex vivo, and formed no fluorescent modulation bands after in vivo injection of the mice with calcein. To acidify the enamel further we exposed Cftr-null mice to fluoride in drinking water to stimulate proton release during formation of hypermineralized lines. In Cftr-deficient mice, fluoride further lowered enamel calcium without further reducing chloride levels. The data support the view that apical CFTR in maturation ameloblasts tranduces chloride into developing enamel as part of the machinery to buffer protons released during mineral accretion.

Acid-base transport in pancreatic cancer: molecular mechanisms and clinical potential

Solid tumors are characterized by a microenvironment that is highly acidic, while intracellular pH (pHi) is normal or even elevated. This is the result of elevated metabolic rates in the highly proliferative cancer cells, in conjunction with often greatly increased rates of net cellular acid extrusion. Studies in various cancers have suggested that while the acid extrusion mechanisms employed are generally the same as those in healthy cells, the specific transporters upregulated vary with the cancer type. The main such transporters include Na(+)/H(+) exchangers, various HCO3(-) transporters, H(+) pumps, and lactate-H(+) cotransporters. The mechanisms leading to their dysregulation in cancer are incompletely understood but include changes in transporter expression levels, trafficking and membrane localization, and posttranslational modifications. In turn, accumulating evidence has revealed that in addition to supporting their elevated metabolic rate, their increased acid efflux capacity endows the cancer cells with increased capacity for invasiveness, proliferation, and chemotherapy resistance. The pancreatic duct exhibits an enormous capacity for acid-base transport, rendering pHi dysregulation a potentially very important topic in pancreatic ductal adenocarcinoma (PDAC). PDAC - accounting for about 90% of all pancreatic cancers - has one of the highest cancer mortality rates known, and new diagnostic and treatment options are highly needed. However, very little is known about whether pH regulation is altered in PDAC and, if so, the possible role of this in cancer development. Here, we review current models for pancreatic acid-base transport and pH homeostasis and summarize current views on acid-base dysregulation in cancer, focusing where possible on the few studies to date in PDAC. Finally, we present new data-mining analyses of acid-base transporter expression changes in PDAC and discuss essential directions for future work.

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.

Transcriptional networks driving enhancer function in the CFTR gene

A critical cis-regulatory element for the CFTR (cystic fibrosis transmembrane conductance regulator) gene is located in intron 11, 100 kb distal to the promoter, with which it interacts. This sequence contains an intestine-selective enhancer and associates with enhancer signature proteins, such as p300, in addition to tissue-specific TFs (transcription factors). In the present study we identify critical TFs that are recruited to this element and demonstrate their importance in regulating CFTR expression. In vitro DNase I footprinting and EMSAs (electrophoretic mobility-shift assays) identified four cell-type-selective regions that bound TFs in vitro. ChIP (chromatin immunoprecipitation) identified FOXA1/A2 (forkhead box A1/A2), HNF1 (hepatocyte nuclear factor 1) and CDX2 (caudal-type homeobox 2) as in vivo trans-interacting factors. Mutation of their binding sites in the intron 11 core compromised its enhancer activity when measured by reporter gene assay. Moreover, siRNA (small interfering RNA)-mediated knockdown of CDX2 caused a significant reduction in endogenous CFTR transcription in intestinal cells, suggesting that this factor is critical for the maintenance of high levels of CFTR expression in these cells. The ChIP data also demonstrate that these TFs interact with multiple cis-regulatory elements across the CFTR locus, implicating a more global role in intestinal expression of the gene.

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.

An intermediate-conductance Ca2+-activated K+ channel is important for secretion in pancreatic duct cells

Potassium channels play a vital role in maintaining the membrane potential and the driving force for anion secretion in epithelia. In pancreatic ducts, which secrete bicarbonate-rich fluid, the identity of K(+) channels has not been extensively investigated. In this study, we investigated the molecular basis of functional K(+) channels in rodent and human pancreatic ducts (Capan-1, PANC-1, and CFPAC-1) using molecular and electrophysiological techniques. RT-PCR analysis revealed mRNAs for KCNQ1, KCNH2, KCNH5, KCNT1, and KCNT2, as well as KCNN4 coding for the following channels: KVLQT1; HERG; EAG2; Slack; Slick; and an intermediate-conductance Ca(2+)-activated K(+) (IK) channel (K(Ca)3.1). The following functional studies were focused on the IK channel. 5,6-Dichloro-1-ethyl-1,3-dihydro-2H-benzimidazole-2-one (DC-EBIO), an activator of IK channel, increased equivalent short-circuit current (I(sc)) in Capan-1 monolayer, consistent with a secretory response. Clotrimazole, a blocker of IK channel, inhibited I(sc). IK channel blockers depolarized the membrane potential of cells in microperfused ducts dissected from rodent pancreas. Cell-attached patch-clamp single-channel recordings revealed IK channels with an average conductance of 80 pS in freshly isolated rodent duct cells. These results indicated that the IK channels may, at least in part, be involved in setting the resting membrane potential. Furthermore, the IK channels are involved in anion and potassium transport in stimulated pancreatic ducts.

Molecular mechanism of pancreatic and salivary gland fluid and HCO3 secretion

Fluid and HCO(3)(-) secretion is a vital function of all epithelia and is required for the survival of the tissue. Aberrant fluid and HCO(3)(-) secretion is associated with many epithelial diseases, such as cystic fibrosis, pancreatitis, Sjögren's syndrome, and other epithelial inflammatory and autoimmune diseases. Significant progress has been made over the last 20 years in our understanding of epithelial fluid and HCO(3)(-) secretion, in particular by secretory glands. Fluid and HCO(3)(-) secretion by secretory glands is a two-step process. Acinar cells secrete isotonic fluid in which the major salt is NaCl. Subsequently, the duct modifies the volume and electrolyte composition of the fluid to absorb the Cl(-) and secrete HCO(3)(-). The relative volume secreted by acinar and duct cells and modification of electrolyte composition of the secreted fluids varies among secretory glands to meet their physiological functions. In the pancreas, acinar cells secrete a small amount of NaCl-rich fluid, while the duct absorbs the Cl(-) and secretes HCO(3)(-) and the bulk of the fluid in the pancreatic juice. Fluid secretion appears to be driven by active HCO(3)(-) secretion. In the salivary glands, acinar cells secrete the bulk of the fluid in the saliva that is driven by active Cl(-) secretion and contains high concentrations of Na(+) and Cl(-). The salivary glands duct absorbs both the Na(+) and Cl(-) and secretes K(+) and HCO(3)(-). In this review, we focus on the molecular mechanism of fluid and HCO(3)(-) secretion by the pancreas and salivary glands, to highlight the similarities of the fundamental mechanisms of acinar and duct cell functions, and to point out the differences to meet gland-specific secretions.

The SLC36 family of proton-coupled amino acid transporters and their potential role in drug transport

Members of the solute carrier (SLC) 36 family are involved in transmembrane movement of amino acids and derivatives. SLC36 consists of four members. SLC36A1 and SLC36A2 both function as H(+) -coupled amino acid symporters. SLC36A1 is expressed at the luminal surface of the small intestine but is also commonly found in lysosomes in many cell types (including neurones), suggesting that it is a multipurpose carrier with distinct roles in different cells including absorption in the small intestine and as an efflux pathway following intralysosomal protein breakdown. SLC36A1 has a relatively low affinity (K(m) 1-10 mM) for its substrates, which include zwitterionic amino and imino acids, heterocyclic amino acids and amino acid-based drugs and derivatives used experimentally and/or clinically to treat epilepsy, schizophrenia, bacterial infections, hyperglycaemia and cancer. SLC36A2 is expressed at the apical surface of the human renal proximal tubule where it functions in the reabsorption of glycine, proline and hydroxyproline. SLC36A2 also transports amino acid derivatives but has a narrower substrate selectivity and higher affinity (K(m) 0.1-0.7 mM) than SLC36A1. Mutations in SLC36A2 lead to hyperglycinuria and iminoglycinuria. SLC36A3 is expressed only in testes and is an orphan transporter with no known function. SLC36A4 is widely distributed at the mRNA level and is a high-affinity (K(m) 2-3 µM) transporter for proline and tryptophan. We have much to learn about this family of transporters, but from current knowledge, it seems likely that their function will influence the pharmacokinetic profiles of amino acid-based drugs by mediating transport in both the small intestine and kidney.

Functional Cftr in crypt epithelium of organotypic enteroid cultures from murine small intestine

Physiological studies of intact crypt epithelium have been limited by problems of accessibility in vivo and dedifferentiation in standard primary culture. Investigations of murine intestinal stem cells have recently yielded a primary intestinal culture in three-dimensional gel suspension that recapitulates crypt structure and epithelial differentiation (Sato T, Vries RG, Snippert HJ, van de Wetering M, Barker N, Stange DE, Van Es JH, Abo A, Kujala P, Peters PJ, Clevers H. Nature 459: 262-265, 2009). We investigated the utility of murine intestinal crypt cultures (termed "enteroids") for physiological studies of crypt epithelium by focusing on the transport activity of the cystic fibrosis transmembrane conductance regulator Cftr. Enteroids had multiple crypts with well-differentiated goblet and Paneth cells that degranulated on exposure to the muscarinic agonist carbachol. Modified growth medium provided a crypt proliferation rate, as measured by 5-ethynyl-2'-deoxyuridine labeling, which was similar to proliferation in vivo. Immunoblots demonstrated equivalent Cftr expression in comparisons of freshly isolated crypts with primary and passage 1 enteroids. Apparent enteroid differences in mRNA expression of other transporters were primarily associated with villous epithelial contamination of freshly isolated crypts. Microelectrode analysis revealed cAMP-stimulated membrane depolarization in enteroid epithelium from wild-type (WT) but not Cftr knockout (KO) mice. Morphological and microfluorimetric studies, respectively, demonstrated Cftr-dependent cell shrinkage and lower intracellular pH in WT enteroid epithelium in contrast to Cftr KO epithelium or WT epithelium treated with Cftr inhibitor 172. We conclude that crypt epithelium of murine enteroids exhibit Cftr expression and activity that recapitulates crypt epithelium in vivo. Enteroids provide a primary culture model that is suitable for physiological studies of regenerating crypt epithelium.

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.

Basolateral ion transporters involved in colonic epithelial electrolyte absorption, anion secretion and cellular homeostasis

Electrolyte transporters located in the basolateral membrane of the colonic epithelium are increasingly appreciated as elaborately regulated components of specific transport functions and cellular homeostasis: During electrolyte absorption, Na(+) /K(+) ATPase, Cl⁻ conductance, Cl⁻/HCO₃⁻ exchange, K(+) /Cl⁻ cotransport and K(+) channels are candidates for basolateral Na(+) , Cl⁻ and K(+) extrusion. The process of colonic anion secretion involves basolateral Na(+) /K(+) /2Cl⁻ , and probably also Na(+) /HCO₃⁻ cotransport, as well as Na(+) /K(+) ATPase and K(+) channels to supply substrate, stabilize the membrane potential and generate driving force respectively. Together with a multitude of additional transport systems, Na(+) /H(+) exchange and Na(+) /HCO₃⁻ cotransport have been implicated in colonocyte pH(i) and volume homeostasis. The purpose of this article is to summarize recently gathered information on the molecular identity, function and regulation of the involved basolateral transport systems in native tissue. Furthermore, we discuss how these findings can help to integrate these systems into the transport function and the cellular homoeostasis of colonic epithelial cells. Finally, disturbances of basolateral electrolyte transport during disease states such as mucosal inflammation will be reviewed.

Reciprocal regulation of the primary sodium absorptive pathways in rat intestinal epithelial cells

Sodium absorption in the mammalian small intestine occurs predominantly by two primary pathways that include Na/H exchange (NHE3) and Na-glucose cotransport (SGLT1) on the brush border membrane (BBM) of villus cells. However, whether NHE3 and SGLT1 function together to regulate intestinal sodium absorption is unknown. Nontransformed small intestinal epithelial cells (IEC-18) were transfected with either NHE3 or SGLT1 small interfering RNAs (siRNAs) and were grown in confluent monolayers on transwell plates to measure the effects on Na absorption. Uptake studies were performed as well as molecular studies to determine the effects on NHE3 and SGLT1 activity. When IEC-18 monolayers were transfected with silencing NHE3 RNA, the cells demonstrated decreased NHE3 activity as well as decreased NHE3 mRNA and protein. However, in NHE3 siRNA-transected cells, SGLT1 activity, mRNA, and protein in the BBM were significantly increased. Thus, inhibition of NHE3 expression regulates the expression and function of SGLT1 in the BBM of intestinal epithelial cells. In addition, IEC-18 cells transected with silencing SGLT1 RNA demonstrated an inhibition of Na-dependent glucose uptake and a decrease in SGLT1 activity, mRNA, and protein levels. However, in these cells, Na/H exchange activity was significantly increased. Furthermore, NHE3 mRNA and protein levels were also increased. Therefore, the inhibition of SGLT1 expression stimulates the transcription and function of NHE3 and vice versa in the BBM of intestinal epithelial cells. Thus this study demonstrates that the major sodium absorptive pathways together function to regulate sodium absorption in epithelial cells.

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.

The switch of intestinal Slc26 exchangers from anion absorptive to HCOFormula secretory mode is dependent on CFTR anion channel function

CFTR has been recognized to function as both an anion channel and a key regulator of Slc26 anion transporters in heterologous expression systems. Whether this regulatory relationship between CFTR and Slc26 transporters is seen in native intestine, and whether this effect is coupled to CFTR transport function or other features of this protein, has not been studied. The duodena of anesthetized CFTR-, NHE3-, Slc26a6-, and Scl26a3-deficient mice and wild-type (WT) littermates were perfused, and duodenal bicarbonate (HCO(3)(-)) secretion (DBS) and fluid absorptive or secretory rates were measured. The selective NHE3 inhibitor S1611 or genetic ablation of NHE3 significantly reduced fluid absorptive rates and increased DBS. Slc26a6 (PAT1) or Slc26a3 (DRA) ablation reduced the S1611-induced DBS increase and reduced fluid absorptive rates, suggesting that the effect of S1611 or NHE3 ablation on HCO(3)(-) secretion may be an unmasking of Slc26a6- and Slc26a3-mediated Cl(-)/HCO(3)(-) exchange activity. In the absence of CFTR expression or after application of the CFTR(inh)-172, fluid absorptive rates were similar to those of WT, but S1611 induced virtually no increase in DBS, demonstrating that CFTR transport activity, and not just its presence, is required for Slc26-mediated duodenal HCO(3)(-) secretion. A functionally active CFTR is an absolute requirement for Slc26-mediated duodenal HCO(3)(-) secretion, but not for Slc26-mediated fluid absorption, in which these transporters operate in conjunction with the Na(+)/H(+) exchanger NHE3. This suggests that Slc26a6 and Slc26a3 need proton recycling via NHE3 to operate in the Cl(-) absorptive mode and Cl(-) exit via CFTR to operate in the HCO(3)(-) secretory mode.

Review article: the clinical management of congenital chloride diarrhoea

BACKGROUND

Congenital chloride diarrhoea in a newborn is a medical emergency, requiring early diagnostics and treatment to prevent severe dehydration and infant mortality. While most of the 250 cases reported arise from Finland, Poland and Arab countries, single cases with this autosomal recessive disorder appear worldwide. Such congenital chloride diarrhoea rarity makes diagnosis difficult. Life-long salt substitution with NaCl and KCl stabilizes fluid, electrolyte and acid-base balance diagnosis. When properly treated, the long-term outcome is favourable.

AIM

To summarize data on congenital chloride diarrhoea diagnosis, pathophysiology and treatment, and to provide guidelines for both acute and long-term management of congenital chloride diarrhoea.

METHODS

Data are based on MEDLINE search for 'chloride diarrhoea', in addition to clinical experience in the treatment of the largest known series of patients.

RESULTS

Treatment of congenital chloride diarrhoea involves (i) life-long salt substitution; (ii) management of acute dehydration and hypokalaemia during gastroenteritis or other infections; and (iii) recognition and treatment of other manifestations of the disease, such as intestinal inflammation, renal impairment and male subfertility.

CONCLUSIONS

This review summarizes data on congenital chloride diarrhoea and provides guidelines for treatment. After being a mostly paediatric problem, adult patients constitute a rare challenge for gastroenterologists worldwide.

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.

Slc26a9--anion exchanger, channel and Na+ transporter

The SLC26 gene family encodes anion transporters with diverse functional attributes: (a) anion exchanger, (b) anion sensor, and (c) anion conductance (likely channel). We have cloned and studied Slc26a9, a paralogue expressed mostly in lung and stomach. Immunohistochemistry shows that Slc26a9 is present at apical and intracellular membranes of lung and stomach epithelia. Using expression in Xenopus laevis oocytes and ion-sensitive microelectrodes, we discovered that Slc26a9 has a novel function not found in any other Slc26 proteins: cation coupling. Intracellular pH and voltage measurements show that Slc26a9 is a nCl(-)-HCO(3)(-) exchanger, suggesting roles in gastric HCl secretion or pulmonary HCO(3)(-) secretion; Na(+) electrodes and uptakes reveal that Slc26a9 has a cation dependence. Single-channel measurements indicate that Slc26a9 displays discrete open and closed states. These experiments show that Slc26a9 has three discrete physiological modes: nCl(-)-HCO(3)(-) exchanger, Cl(-) channel, and Na(+)-anion cotransporter. Thus, the Slc26a9 transporter channel is uniquely suited for dynamic and tissue-specific physiology or regulation in epithelial tissues.

Bestrophin expression and function in the human pancreatic duct cell line, CFPAC-1

Pancreatic duct epithelial cells (PDECs) have been shown to express calcium activated chloride channels (CaCCs) and there is evidence for their involvement in fluid secretion from these cells. The molecular identity of the CaCC in PDECs remains unknown. Recently, the bestrophin family of proteins have been proposed as a potential molecular candidate for CaCCs. Expression of bestrophins is strongly correlated with the function of CaCCs in a variety of tissues. In the present study, the expression of bestrophins has been investigated in the cystic fibrosis pancreatic duct cell line, CFPAC-1. Iodide efflux analysis was used to characterise native CaCCs in CFPAC-1 cell monolayers. Efflux was induced with the addition of UTP (100 microM, 10.2 +/- 1.5 nmol min(-1)), which was blocked by the chloride channel blockers niflumic acid (81%) and DIDS (90%). The UTP-stimulated iodide efflux was shown to be Ca(2+) dependent and cAMP independent. RT-PCR analysis of RNA isolated from CFPAC-1 cells demonstrated positive identification of all four human bestrophin mRNAs. Western blot of CFPAC-1 cell protein isolates with antibodies specific to human bestrophin 1 (hBest1) showed that hBest1 protein was expressed in this cell line. HBest1 was present on the cell surface, demonstrated using biotinylation and confocal imaging, as well as in the cytoplasm. SiRNA-mediated silencing of hBest1 in CFPAC-1 cells reduced the UTP-stimulated iodide efflux by around 40%. This study provides evidence that the bestrophins are expressed in pancreatic duct cells and, more specifically, that hBest1 plays a role in the CaCCs found in these cells.

Diarrhea as a cause of mortality in a mouse model of infectious colitis

Analysis of gene expression in the colons of Citrobacter rodentium-infected susceptible and resistant mice suggests that mortality is associated with impaired intestinal ion transport.

Background

Comparative characterization of genome-wide transcriptional changes during infection can help elucidate the mechanisms underlying host susceptibility. In this study, transcriptional profiling of the mouse colon was carried out in two cognate lines of mice that differ in their response to Citrobacter rodentium infection; susceptible inbred FVB/N and resistant outbred Swiss Webster mice. Gene expression in the distal colon was determined prior to infection, and at four and nine days post-inoculation using a whole mouse genome Affymetrix array.

Results

Computational analysis identified 462 probe sets more than 2-fold differentially expressed between uninoculated resistant and susceptible mice. In response to C. rodentium infection, 5,123 probe sets were differentially expressed in one or both lines of mice. Microarray data were validated by quantitative real-time RT-PCR for 35 selected genes and were found to have a 94% concordance rate. Transcripts represented by 1,547 probe sets were differentially expressed between susceptible and resistant mice regardless of infection status, a host effect. Genes associated with transport were over-represented to a greater extent than even immune response-related genes. Electrolyte analysis revealed reduction in serum levels of chloride and sodium in susceptible animals.

Conclusion

The results support the hypothesis that mortality in C. rodentium-infected susceptible mice is associated with impaired intestinal ion transport and development of fatal fluid loss and dehydration. These studies contribute to our understanding of the pathogenesis of C. rodentium and suggest novel strategies for the prevention and treatment of diarrhea associated with intestinal bacterial infections.

The roles and mechanisms of intestinal oxalate transport in oxalate homeostasis

The mammalian intestine has an important role in the dynamics of oxalate exchange and thereby is significant in the etiology of calcium oxalate nephrolithiasis. Here we review some of the phenomenologic observations that have led to the conclusion that anion exchangers (antiporters) are important mediators of secondarily active, net oxalate transport along the intestine (both absorptive and secretory). Understanding the mechanisms of transepithelial oxalate transport has been advanced radically in recent years by the identification of the solute-linked carrier (SLC)26 family of anion transporters, which has facilitated the identification of specific proteins mediating individual apical or basolateral oxalate transport pathways. Moreover, identification of specific exchangers has underscored their relative importance to oxalate homeostasis as revealed by using knockout mouse models and has facilitated studies of oxalate transport regulation in heterologous expression systems. Finally, the significance of oxalate degrading bacteria to oxalate homeostasis is considered from basic and applied perspectives.

Mechanisms of neuronal chloride accumulation in intact mouse olfactory epithelium

When olfactory receptor neurons respond to odours, a depolarizing Cl(-) efflux is a substantial part of the response. This requires that the resting neuron accumulate Cl(-) against an electrochemical gradient. In isolated olfactory receptor neurons, the Na(+)-K(+)-2Cl(-) cotransporter NKCC1 is essential for Cl(-) accumulation. However, in intact epithelium, a robust electrical olfactory response persists in mice lacking NKCC1. This response is largely due to a neuronal Cl(-) efflux. It thus appears that NKCC1 is an important part of a more complex system of Cl(-) accumulation. To identify the remaining transport proteins, we first screened by RT-PCR for 21 Cl(-) transporters in mouse nasal tissue containing olfactory mucosa. For most of the Cl(-) transporters, the presence of mRNA was demonstrated. We also investigated the effects of pharmacological block or genetic ablation of Cl(-) transporters on the olfactory field potential, the electroolfactogram (EOG). Mice lacking the common Cl(-)/HCO(3)(-) exchanger AE2 had normal EOGs. Block of NKCC cotransport with bumetanide reduced the EOG in epithelia from wild-type mice but had no effect in mice lacking NKCC1. Hydrochlorothiazide, a blocker of the Na(+)-Cl(-) cotransporter, had only a small effect. DIDS, a blocker of some KCC cotransporters and Cl(-)/HCO(3)(-) exchangers, reduced the EOG in epithelia from both wild-type and NKCC1 knockout mice. A combination of bumetanide and DIDS decreased the response more than either drug alone. However, no combination of drugs completely abolished the Cl(-) component of the response. These results support the involvement of both NKCC1 and one or more DIDS-sensitive transporters in Cl(-) accumulation in olfactory receptor neurons.

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

The role of Slc26a6 (PAT1) on apical Cl-/HCO3- exchange and bicarbonate secretion in pancreatic duct cells was investigated using Slc26a6 null and wild-type (WT) mice. Apical Cl-/HCO3- exchange activity was measured with the pH-sensitive dye BCECF in microperfused interlobular ducts. The HCO3(-)-influx mode of apical [Cl-]i/[HCO3-]o exchange (where brackets denote concentration and subscripts i and o denote intra- and extracellular, respectively) was dramatically upregulated in Slc26a6 null mice (P < 0.01 vs. WT), whereas the HCO3(-)-efflux mode of apical [Cl-]o/[HCO3-]i exchange was decreased in Slc26a6 null mice (P < 0.05 vs. WT), suggesting the unidirectionality of the Slc26a6-mediated HCO3- transport. Fluid secretory rate in interlobular ducts were comparable in WT and Slc26a6 null mice (P > 0.05). In addition, when pancreatic juice was collected from whole animal in basal and secretin-stimulated conditions, neither juice volume nor its pH showed differences between WT and Slc26a6 null mice. Semiquantitative RT-PCR demonstrated more than fivefold upregulation in Slc26a3 (DRA) expression in Slc26a6 knockout pancreas. In conclusion, these results point to the role of Slc26a6 in HCO3- efflux at the apical membrane and also suggest the presence of a robust Slc26a3 compensatory upregulation, which can replace the function of Slc26a6 in pancreatic ducts.

Mechanisms of acid and base secretion by the airway epithelium

One of the main functions of the airway epithelium is to inactivate and remove infectious particles from inhaled air and thereby prevent infection of the distal lung. This function is achieved by mucociliary and cough clearance and by antimicrobial factors present in the airway surface liquid (ASL). There are indications that airway defenses are affected by the pH of the ASL and historically, acidification of the airway surfaces has been suggested as a measure of airway disease. However, even in health, the ASL is slightly acidic, and this acidity might be part of normal airway defense. Only recently research has focused on the mechanisms responsible for acid and base secretion into the ASL. Advances resulted from research into the airway disease associated with cystic fibrosis (CF) after it was found that the CFTR Cl(-) channel conducts HCO (3) (-) and, therefore, may contribute to ASL pH. However, the acidity of the ASL indicated parallel mechanisms for H(+) secretion. Recent investigations identified several H(+) transporters in the apical membrane of the airway epithelium. These include H(+) channels and ATP-driven H(+) pumps, including a non-gastric isoform of the H(+)-K(+) ATPase and a vacuolar-type H(+) ATPase. Current knowledge of acid and base transporters and their potential roles in airway mucosal pH regulation is reviewed here.

The emerging role of PDZ adapter proteins for regulation of intestinal ion transport

In the gastrointestinal tract, CFTR, in conjunction with one or several members of the SLC26 anion exchanger family, mediates electrogenic Cl- and HCO3- secretion. Na+/H+ exchanger isoform NHE3, on the other hand, coupled to one or several of the SLC26 isoforms, mediates electroneutral NaCl absorption. The agonist-induced activation of anion secretion and inhibition of salt absorption causes secretory diarrhea. Current dogma sees the formation of a multiprotein complex of transport proteins, postsynaptic density-95/discs large/zonula occludens-1 (PDZ) adapter proteins, anchoring proteins, the cytoskeleton, and the involved protein kinases as one crucial step in the regulation of these transport processes. Data obtained in heterologous expression studies suggest an important role of these PDZ adapter proteins in trafficking, endocytic recycling, and membrane retention of the respective transmembrane proteins. This article reviews recent advances in our understanding of the role of the PDZ adapter proteins NHERF, E3KARP, PDZK1, IKEPP (NHERF-1 to NHERF-4), CAL, and Shank-2 that bind to CFTR, NHE3, and the intestinal SLC26 members in the regulation of intestinal fluid transport. Current concepts are mostly derived from heterologous expression studies and studies on their role in organ physiology are still in infancy. Recently, however, PDZ adapter protein-deficient mice and organ-specific cell lines have become available, and the first results suggest a more cell-type and possibly signal-specific role of these adapter proteins. This opens the potential for drug development targeted to PDZ domain interactions, which is, in theory, one of the most efficient antidiarrheal strategies.

Melatonin modulates acid/base transport in human pancreatic carcinoma cells

Melatonin was found to improve pancreatic organ function in diseased animals. To study whether pancreatic bicarbonate secretion is stimulated by melatonin, investigations were done in two human ductal pancreatic adenocarcinoma cell lines MIA PaCa-2 (MIA) and PANC-1 (PANC). Using the fluorescence pH-sensor BCECF-AM, we monitored melatonin effects on basal intracellular pH (pH(i)), and on pH(i) recovery after intracellular alkalinization produced by the removal of extracellular HCO(3) (-)/CO(2). Exposure to 1 microM melatonin for 24 hrs and presence of the indoleamine during the experiment increases the basal pH(i). Moreover, pHi recovery and HCO(3) (-) secretion are facilitated after the alkaline load. These findings are in line with the observed increase in mRNA expression of the Na(+)/HCO(3) (-)-cotransporter SLC4A4b for the uptake and the Cl(-)/HCO(3) (-)-exchanger SLC26A6 for the secretion of HCO(3) (-). The reduction in Na(+)/H(+)- exchanger SLC9A1 mRNA would favor pH(i) recovery after alkalinization, but it does not explain the initial increase in pHi. This controversial effect and the requirement for continuous presence of melatonin throughout the experiment suggest that nontranscriptional signalling may contribute to the effects of melatonin on acid/base movements. In summary, we show a stimulatory effect of melatonin on bicarbonate secretion in the pancreatic cancer cell lines which may help to prevent duodenal damage.

Characterization of H+ and HCO3- transporters in CFPAC-1 human pancreatic duct cells

AIM: To characterize H⁺ and HCO₃^- transporters in polarized CFPAC-1 human pancreatic duct cells, which were derived from a cystic fibrosis patient with the ΔF508 CFTR mutation.

METHODS: CFPAC-1 cells were seeded at high density onto permeable supports and grown to confluence. The cells were loaded with the pH-sensitive fluorescent dye BCECF, and mounted into a perfusion chamber, which allowed the simultaneous perfusion of the basolateral and apical membranes. Transmembrane base flux was calculated from the changes in intracellular pH and the buffering capacity of the cells.

RESULTS: Our results showed differential permeability to HCO₃^-/CO₂ at the apical and basolateral membranes of CFPAC-1 cells. Na⁺/HCO₃^- co-transporters (NBCs) and Cl^-/HCO₃^- exchangers (AEs) were present on the basolateral membrane, and Na⁺/H⁺ exchangers (NHEs) on both the apical and basolateral membranes of the cells. Basolateral HCO₃^- uptake was sensitive to variations of extracellular K⁺ concentration, the membrane permeable carbonic anhydrase (CA) inhibitors acetazolamide (100 µmol/L) and ethoxyzolamide (100 µmol/L), and was partially inhibited by H₂-DIDS (600 µmol/L). The membrane-impermeable CA inhibitor 1-N-(4-sulfamoylphenylethyl)-2,4,6-trimethylpyridine perchlorate did not have any effect on HCO₃^- uptake. The basolateral AE had a much higher activity than that in the apical membrane, whereas there was no such difference with the NHE under resting conditions. Also, 10 µmol/L forskolin did not significantly influence Cl^-/HCO₃^- exchange on the apical and basolateral membranes. The administration of 250 µmol/L H₂-DIDS significantly inhibited the basolateral AE. Amiloride (300 µmol/L) completely inhibited NHEs on both membranes of the cells. RT-PCR revealed the expression of pNBC1, AE2, and NHE1 mRNA.

CONCLUSION: These data suggest that apart from the lack of CFTR and apical Cl^-/HCO₃^- exchanger activity, CFPAC-1 cells express similar H⁺ and HCO₃^- transporters to those observed in native animal tissue.

Expression of SLC26A3, CFTR and NHE3 in the human male reproductive tract: role in male subfertility caused by congenital chloride diarrhoea

Congenital chloride diarrhoea (CLD) is a rare inherited disease caused by mutations in the solute carrier family 26 member 3 (SLC26A3) gene. Disruption of intestinal Cl(-)/HCO(3)(-) exchange causes watery Cl(-) rich diarrhoea from birth, and recently male subfertility was observed as a novel manifestation. Expression of SLC26A3, together with interacting proteins cystic fibrosis transmembrane conductance regulator (CFTR) and Na(+)/H(+) exchanger 3 (NHE3), was studied using immunohistochemistry in the testis (n = 2) and efferent ducts (ED) (n = 1) of patients with CLD (V317del genotype) and in the testis and epididymis (n = 11), seminal vesicle (n = 9) and prostate (n = 4) of the controls. SLC26A3 was immunolocalized in the head of the elongating spermatids (stages III-VI) and CFTR in the elongating spermatids (stages III and IV) and pachytene (stages III-V) and diplotene spermatocytes. In the non-ciliated cells of the ED, apical expression of all three proteins was observed, but only SLC26A3 and CFTR were detected on the luminal border of the apical mitochondria-rich cells (AMRC) of the ductus epididymis and in the epithelium of the seminal vesicle. Only CFTR was present in the epithelium of the prostatic duct. In the patient with CLD, the expression of both SLC26A3 and CFTR was absent in the ED, but testicular expression was identical to that of the controls. These results suggest a primary role for SLC26A3 in male reproduction. Tissue-specific co-expression with CFTR and NHE3 supports diverse functions of SLC26A3 and may have an impact on pathophysiology of male subfertility both in CLD and in cystic fibrosis (CF), as well as spermatoceles.

Disruption of the SLC26A3-mediated anion transport is associated with male subfertility

Male subfertility in congenital chloride diarrhea (CLD) was possible after identification of expression of an epithelial Cl-/HCO3- exchanger SLC26A3 in the male reproductive tract and by the observation that adult men with CLD had very few children. A prospective clinical and laboratory study among eight adult Finnish men with CLD revealed constant oligoasthenoteratozoospermia but normal spermatogenesis, high chloride and low pH in seminal plasma, and three spermatoceles, suggesting that male subfertility is a clinical manifestation of CLD and could be caused by an analogous defect in the epithelial Cl-/HCO3- and water transport, as described for the CLD intestine.

Apical ammonium inhibition of cAMP-stimulated secretion in T84 cells is bicarbonate dependent

Normal human colonic luminal (NH(4)(+)) concentration ([NH(4)(+)]) ranges from approximately 10 to 100 mM. However, the nature of the effects of NH(4)(+) on transport, as well as NH(4)(+) transport itself, in colonic epithelium is poorly understood. We elucidate here the effects of apical NH(4)(+) on cAMP-stimulated Cl(-) secretion in colonic T84 cells. In HEPES-buffered solutions, 10 mM apical NH(4)(+) had no significant effect on cAMP-stimulated current. In contrast, 10 mM apical NH(4)(+) reduced current within 5 min to 61 +/- 4% in the presence of 25 mM HCO(3)(-). Current inhibition was not simply due to an increase in extracellular K(+)-like cations, in that the current magnitude was 95 +/- 5% with 10 mM apical K(+) and 46 +/- 3% with 10 mM apical NH(4)(+) relative to that with 5 mM apical K(+). We previously demonstrated that inhibition of Cl(-) secretion by basolateral NH(4)(+) occurs in HCO(3)(-)-free conditions and exhibits anomalous mole fraction behavior. In contrast, apical NH(4)(+) inhibition of current in HCO(3)(-) buffer did not show anomalous mole fraction behavior and followed the absolute [NH(4)(+)] in K(+)-NH(4)(+) mixtures, where K(+) concentration + [NH(4)(+)] = 10 mM. The apical NH(4)(+) inhibitory effect was not prevented by 100 microM methazolamide, suggesting no role for apical carbonic anhydrase. However, apical NH(4)(+) inhibition of current was prevented by 10 min of pretreatment of the apical surface with 500 microM DIDS, 100 microM 4,4'-dinitrostilbene-2,2'-disulfonic acid (DNDS), or 25 microM niflumic acid, suggesting a role for NH(4)(+) action through an apical anion exchanger. mRNA and protein for the apical anion exchangers SLC26A3 [downregulated in adenoma (DRA)] and SLC26A6 [putative anion transporter (PAT1)] were detected in T84 cells by RT-PCR and Northern and Western blots. DRA and PAT1 appear to associate with CFTR in the apical membrane. We conclude that the HCO(3)(-) dependence of apical NH(4)(+) inhibition of secretion is due to the action of NH(4)(+) on an apical anion exchanger.

Expression of a wild-type CFTR maintains the integrity of the biosynthetic/secretory pathway in human cystic fibrosis pancreatic duct cells

The structural integrity of the Golgi complex is essential to its functions in the maturation, sorting, and transport of plasma membrane proteins. Previously, we demonstrated that in pancreatic duct CFPAC-1 cells, which express DeltaF508 CFTR (cystic fibrosis transmembrane conductance regulator), the intracellular trafficking of carbonic anhydrase IV (CA IV), a membrane protein involved in HCO(3)(-) secretion, was impaired. To determine whether these abnormalities were related to changes in the Golgi complex, we examined the ultrastructure and distribution of Golgi compartments with regard to the microtubule cytoskeleton in CFPAC-1 cells transfected or not with the wild-type CFTR. Ultrastructural and immunocytochemical analysis showed that in polarized CFPAC-1 cells, Golgi stacks were disconnected from one another and scattered throughout the cytoplasm. The colocalization of CA IV with markers of Golgi compartments indicated the ability of stacks to transfer this enzyme. This Golgi dispersal was associated with abnormal microtubule distribution and multiplicity of the microtubule-organizing centers (MTOCs). In reverted cells, the normalization of Golgi structure, microtubule distribution, and MTOC number was observed. These observations suggest that the entire biosynthetic/secretory pathway is disrupted in CFPAC-1 cells, which might explain the abnormal intracellular transport of CA IV. Taken together, these results point to the fact that the expression of DeltaF508 CFTR affects the integrity of the secretory pathway.

Chloride conductance of CFTR facilitates basal Cl-/HCO3- exchange in the villous epithelium of intact murine duodenum

Villi of the proximal duodenum are situated for direct exposure to gastric acid chyme. However, little is known about active bicarbonate secretion across villi that maintains the protective alkaline mucus barrier, a process that may be compromised in cystic fibrosis (CF), i.e., in the absence of a functional CF transmembrane conductance regulator (CFTR) anion channel. We investigated Cl(-)/HCO(3)(-) exchange activity across the apical membrane of epithelial cells located at the midregion of villi in intact duodenal mucosa from wild-type (WT) and CF mice using the pH-sensitive dye BCECF. Under basal conditions, the Cl(-)/HCO(3)(-) exchange rate was reduced by approximately 35% in CF compared with WT villous epithelium. Cl(-)/HCO(3)(-) exchange in WT and CF villi responded similarly to inhibitors of anion exchange, and membrane depolarization enhanced rates of Cl(-)(out)/HCO(3)(-)(in) exchange in both epithelia. In anion substitution studies, anion(in)/HCO(3)(-)(out) exchange rates were greater in WT epithelium using Cl(-) or NO(3)(-), but decreased to the level of the CF epithelium using the CFTR-impermeant anion, SO(4)(2-). Similarly, treatment of WT epithelium with the CFTR-selective blocker glybenclamide decreased the Cl(-)/HCO(3)(-) exchange rate to the level of CF epithelium. The mRNA expression of Slc26a3 (downregulated in adenoma) and Slc26a6 (putative anion exchanger-1) was similar between WT and CF duodena. From these studies of murine duodenum, we conclude 1) characteristics of Cl(-)/HCO(3)(-) exchange in the villous epithelium are most consistent with Slc26a6 activity, and 2) Cl(-) channel activity of CFTR facilitates apical membrane Cl(-)(in)/HCO(3)(-)(out) exchange by providing a Cl(-) "leak" under basal conditions.

Mechanisms of bicarbonate secretion in the pancreatic duct

In many species the pancreatic duct epithelium secretes HCO3- ions at a concentration of around 140 mM by a mechanism that is only partially understood. We know that HCO3- uptake at the basolateral membrane is achieved by Na+-HCO3- cotransport and also by a H+-ATPase and Na+/H+ exchanger operating together with carbonic anhydrase. At the apical membrane, the secretion of moderate concentrations of HCO3- can be explained by the parallel activity of a Cl-/HCO3- exchanger and a Cl- conductance, either the cystic fibrosis transmembrane conductance regulator (CFTR) or a Ca2+-activated Cl- channel (CaCC). However, the sustained secretion of HCO3- into a HCO- -rich luminal fluid cannot be explained by conventional Cl-/HCO3- exchange. HCO3- efflux across the apical membrane is an electrogenic process that is facilitated by the depletion of intracellular Cl-, but it remains to be seen whether it is mediated predominantly by CFTR or by an electrogenic SLC26 anion exchanger.

SLC26A9 is expressed in gastric surface epithelial cells, mediates Cl-/HCO3- exchange, and is inhibited by NH4+

HCO3- secretion by gastric mucous cells is essential for protection against acidic injury and peptic ulcer. Herein we report the identification of an apical HCO3- transporter in gastric surface epithelial cells. Northern hybridization and RT-PCR demonstrate the expression of this transporter, also known as SLC26A9, in mouse and rat stomach and trachea (but not kidney). In situ hybridization in mouse stomach showed abundant expression of SLC26A9 in surface epithelial cells with apical localization on immunofluorescence labeling. Functional studies in HEK-293 cells demonstrated that SLC26A9 mediates Cl-/HCO3- exchange and is also capable of Cl--independent HCO3- extrusion. Unlike other anion exchangers or transport proteins reported to date, SLC26A9 activity is inhibited by ammonium (NH4+). The inhibitory effect of NH4+ on gastric HCO3- secretion was also indicated by reduced gastric juxtamucosal pH (pHjm) in rat stomach in vivo. This report is the first to describe the inhibition of HCO3- transport in vitro and the reduction of pHjm in stomach in vivo by NH4+. Given its critical localization on the apical membrane of surface epithelial cells, its ability to transport HCO3-, and its inhibition by NH4+, we propose that SLC26A9 mediates HCO3- secretion in surface epithelial cells and is essential for protection against acidic injury in the stomach. Disease states that are associated with increased ammonia (NH3)/NH4+ generation (e.g., Helicobacter pylori) may impair gastric HCO3- secretion and therefore predispose patients to peptic ulcer by inhibiting SLC26A9.

Electroneutral sodium absorption and electrogenic anion secretion across murine small intestine are regulated in parallel

Electrolyte transport processes of small intestinal epithelia maintain a balance between hydration of the luminal contents and systemic fluid homeostasis. Under basal conditions, electroneutral Na(+) absorption mediated by Na(+)/H(+) exchanger 3 (NHE3) predominates; under stimulated conditions, increased anion secretion mediated by CFTR occurs concurrently with inhibition of Na(+) absorption. Homeostatic adjustments to diseases that chronically affect the activity of one transporter (e.g., cystic fibrosis) may include adaptations in the opposing transport process to prevent enterosystemic fluid imbalance. To test this hypothesis, we measured electrogenic anion secretion (indexed by the short-circuit current) across NHE3-null [NHE3(-)] murine small intestine and electroneutral Na(+) absorption (by radioisotopic flux analysis) across small intestine of mice with gene-targeted disruptions of the anion secretory pathway, i.e., CFTR-null [CFTR(-)] or Na(+)-K(+)-2Cl(-) cotransporter-null [NKCC1(-)]. Protein expression of NHE3 and CFTR in the intestinal epithelia was measured by immunoblotting. In NHE3(-), compared with wild-type small intestine, maximal and bumetanide-sensitive anion secretion following cAMP stimulation was significantly reduced, and there was a corresponding decrease in CFTR protein expression. In CFTR(-) and NKCC1(-) intestine, Na(+) absorption was significantly reduced compared with wild-type. NHE3 protein expression was decreased in the CFTR(-) intestine but was unchanged in the NKCC1(-) intestine, indicating that factors independent of expression also downregulate NHE3 activity. Together, these data support the concept that absorptive and secretory processes determining NaCl and water movement across the intestinal epithelium are regulated in parallel to maintain balance between the systemic fluid volume and hydration of the luminal contents.

Intestinal transport of an obdurate anion: oxalate

In this review, we focus on the role of gastrointestinal transport of oxalate primarily from a contemporary physiological standpoint with an emphasis on those aspects that we believe may be most important in efforts to mitigate the untoward effects of oxalate. Included in this review is a general discussion of intestinal solute transport as it relates to oxalate, considering cellular and paracellular avenues, the transport mechanisms, and the molecular identities of oxalate transporters. In addition, we review the role of the intestine in oxalate disease states and various factors affecting oxalate absorption.