Identification of transport abnormalities in duodenal mucosa and duodenal enterocytes from patients with cystic fibrosis

Improving growth in infants with CF

Cystic fibrosis (CF) results in chronic pulmonary infections, inflammation, pancreatic insufficiency, and multiple gastrointestinal manifestations. Malnutrition and poor growth are hallmarks of CF, and strongly associated with poor outcomes. Through newborn screening, many infants can be diagnosed within a few days of life, which allows for early initiation of nutritional counseling and close clinical follow-up. Obstacles to growth for infants with CF start in utero, as newborns with CF can have a lower birth weight than the general population. Improving infant growth has been linked to improved clinical outcomes and survival. It remains a top priority and challenge for caregivers and healthcare teams. An interdisciplinary approach, including registered dietitian and social work support, is essential to optimize health for infants with CF. Remaining barriers to normalcy include deficits in linear growth, lack of accurate nutrition biomarkers, persistence of inequities related to social determinant of health, particularly in the global CF community.

DRA involvement in linaclotide-stimulated bicarbonate secretion during loss of CFTR function

Duodenal bicarbonate secretion is critical to epithelial protection, as well as nutrient digestion and absorption, and is impaired in cystic fibrosis (CF). We examined if linaclotide, typically used to treat constipation, may also stimulate duodenal bicarbonate secretion. Bicarbonate secretion was measured in vivo and in vitro using mouse and human duodenum (biopsies and enteroids). Ion transporter localization was identified with confocal microscopy, and de novo analysis of human duodenal single-cell RNA sequencing (scRNA-Seq) data sets was performed. Linaclotide increased bicarbonate secretion in mouse and human duodenum in the absence of cystic fibrosis transmembrane conductance regulator (CFTR) expression (Cftr-knockout mice) or function (CFTRinh-172). Na+/H+ exchanger 3 inhibition contributed to a portion of this response. Linaclotide-stimulated bicarbonate secretion was eliminated by down-regulated in adenoma (DRA, SLC26A3) inhibition during loss of CFTR activity. ScRNA-Seq identified that 70% of villus cells expressed SLC26A3, but not CFTR, mRNA. Loss of CFTR activity and linaclotide increased apical brush border expression of DRA in non-CF and CF differentiated enteroids. These data provide further insights into the action of linaclotide and how DRA may compensate for loss of CFTR in regulating luminal pH. Linaclotide may be a useful therapy for CF individuals with impaired bicarbonate secretion.

Key role of down-regulated in adenoma (SLC26A3) chloride/bicarbonate exchanger in linaclotide-stimulated intestinal bicarbonate secretion upon loss of CFTR function

Duodenal bicarbonate secretion is critical to epithelial protection, nutrient digestion/absorption and is impaired in cystic fibrosis (CF). We examined if linaclotide, typically used to treat constipation, may also stimulate duodenal bicarbonate secretion. Bicarbonate secretion was measured in vivo and in vitro using mouse and human duodenum (biopsies and enteroids). Ion transporter localization was identified with confocal microscopy and de novo analysis of human duodenal single cell RNA sequencing (sc-RNAseq) datasets was performed. Linaclotide increased bicarbonate secretion in mouse and human duodenum in the absence of CFTR expression (Cftr knockout mice) or function (CFTRinh-172). NHE3 inhibition contributed to a portion of this response. Linaclotide-stimulated bicarbonate secretion was eliminated by down-regulated in adenoma (DRA, SLC26A3) inhibition during loss of CFTR activity. Sc-RNAseq identified that 70% of villus cells expressed SLC26A3, but not CFTR, mRNA. Loss of CFTR activity and linaclotide increased apical brush border expression of DRA in non-CF and CF differentiated enteroids. These data provide further insights into the action of linaclotide and how DRA may compensate for loss of CFTR in regulating luminal pH. Linaclotide may be a useful therapy for CF individuals with impaired bicarbonate secretion.

Nutritional Care in Children with Cystic Fibrosis

Patients with cystic fibrosis (CF) are prone to malnutrition and growth failure, mostly due to malabsorption caused by the derangement in the chloride transport across epithelial surfaces. Thus, optimal nutritional care and support should be an integral part of the management of the disease, with the aim of ameliorating clinical outcomes and life expectancy. In this report, we analyzed the nutrition support across the different ages, in patients with CF, with a focus on the relationships with growth, nutritional status, disease outcomes and the use of the CF transmembrane conductance regulator (CFTR) modulators. The nutrition support goal in CF care should begin as early as possible after diagnosis and include the achievement of an optimal nutritional status to support the growth stages and puberty development in children, that will further support the maintenance of an optimal nutritional status in adult life. The cornerstone of nutrition in patients with CF is a high calorie, high-fat diet, in conjunction with a better control of malabsorption due to pancreatic enzyme replacement therapy, and attention to the adequate supplementation of fat-soluble vitamins. When the oral caloric intake is not enough for reaching the anthropometric nutritional goals, supplemental enteral feeding should be initiated to improve growth and the nutritional status. In the last decade, the therapeutic possibilities towards CF have grown in a consistent way. The positive effects of CFTR modulators on nutritional status mainly consist in the improvement in weight gain and BMI, both in children and adults, and in an amelioration in terms of the pulmonary function and reduction of exacerbations. Several challenges need to be overcome with the development of new drugs, to transform CF from a fatal disease to a treatable chronic disease with specialized multidisciplinary care.

Pancreatic complications in children with cystic fibrosis

PURPOSE OF REVIEW

The pancreas is highly affected in cystic fibrosis, with complications occurring early in childhood. This review highlights recent research in exocrine pancreatic function in the era of cystic fibrosis transmembrane conductance regulator (CFTR) modulator therapies and discusses how these are affecting pancreatitis and exocrine pancreatic insufficiency (EPI) in children. Additionally, new research into exocrine--endocrine interactions sheds light on how CFTR dysfunction in ductal cells may affect beta cells.

RECENT FINDINGS

Ivacaftor has disproved the hypothesis that EPI in children with cystic fibrosis is irreversible. Improvements in pancreatic function have increased pancreatitis episodes in some children and reduced them in others. Imaging advances are providing complementary methods for exocrine pancreatic function testing. New research into the interplay between the exocrine and endocrine components of the pancreas are elucidating the intertwined and complex relationship between the exocrine and endocrine pancreas.

SUMMARY

Pancreatic complications contribute to the morbidity and mortality of children with cystic fibrosis. Increasing use of highly effective CFTR modulators will not only abrogate these but will also advance our understanding of pancreatic pathophysiology in cystic fibrosis. New frontiers into pancreatic gene therapy and exocrine--endocrine research will help provide new therapeutic opportunities for pancreatitis, EPI, and diabetes in cystic fibrosis.

Advances in Ca2+ modulation of gastrointestinal anion secretion and its dysregulation in digestive disorders (Review)

Intracellular calcium (Ca²⁺) is a critical cell signaling component in gastrointestinal (GI) physiology. Cytosolic calcium ([Ca²⁺]_cyt), as a secondary messenger, controls GI epithelial fluid and ion transport, mucus and neuropeptide secretion, as well as synaptic transmission and motility. The key roles of Ca²⁺ signaling in other types of secretory cell (including those in the airways and salivary glands) are well known. However, its action in GI epithelial secretion and the underlying molecular mechanisms have remained to be fully elucidated. The present review focused on the role of [Ca²⁺]_cyt in GI epithelial anion secretion. Ca²⁺ signaling regulates the activities of ion channels and transporters involved in GI epithelial ion and fluid transport, including Cl^- channels, Ca²⁺-activated K⁺ channels, cystic fibrosis (CF) transmembrane conductance regulator and anion/HCO₃^- exchangers. Previous studies by the current researchers have focused on this field over several years, providing solid evidence that Ca²⁺ signaling has an important role in the regulation of GI epithelial anion secretion and uncovering underlying molecular mechanisms. The present review is largely based on previous studies by the current researchers and provides an overview of the currently known molecular mechanisms of GI epithelial anion secretion with an emphasis on Ca²⁺-mediated ion secretion and its dysregulation in GI disorders. In addition, previous studies by the current researchers demonstrated that different regulatory mechanisms are in place for GI epithelial HCO₃^- and Cl^- secretion. An increased understanding of the roles of Ca²⁺ signaling and its targets in GI anion secretion may lead to the development of novel strategies to inhibit GI diseases, including the enhancement of fluid secretion in CF and protection of the GI mucosa in ulcer diseases.

Choline in cystic fibrosis: relations to pancreas insufficiency, enterohepatic cycle, PEMT and intestinal microbiota

BACKGROUND

Cystic Fibrosis (CF) is an autosomal recessive disorder with life-threatening organ manifestations. 87% of CF patients develop exocrine pancreas insufficiency, frequently starting in utero and requiring lifelong pancreatic enzyme substitution. 99% develop progressive lung disease, and 20-60% CF-related liver disease, from mild steatosis to cirrhosis. Characteristically, pancreas, liver and lung are linked by choline metabolism, a critical nutrient in CF. Choline is a tightly regulated tissue component in the form of phosphatidylcholine (Ptd'Cho) and sphingomyelin (SPH) in all membranes and many secretions, particularly of liver (bile, lipoproteins) and lung (surfactant, lipoproteins). Via its downstream metabolites, betaine, dimethylglycine and sarcosine, choline is the major one-carbon donor for methionine regeneration from homocysteine. Methionine is primarily used for essential methylation processes via S-adenosyl-methionine.

CLINICAL IMPACT

CF patients with exocrine pancreas insufficiency frequently develop choline deficiency, due to loss of bile Ptd'Cho via feces. ~ 50% (11-12 g) of hepatic Ptd'Cho is daily secreted into the duodenum. Its re-uptake requires cleavage to lyso-Ptd'Cho by pancreatic and small intestinal phospholipases requiring alkaline environment. Impaired CFTR-dependent bicarbonate secretion, however, results in low duodenal pH, impaired phospholipase activity, fecal Ptd'Cho loss and choline deficiency. Low plasma choline causes decreased availability for parenchymal Ptd'Cho metabolism, impacting on organ functions. Choline deficiency results in hepatic choline/Ptd'Cho accretion from lung tissue via high density lipoproteins, explaining the link between choline deficiency and lung function. Hepatic Ptd'Cho synthesis from phosphatidylethanolamine by phosphatidylethanolamine-N-methyltransferase (PEMT) partly compensates for choline deficiency, but frequent single nucleotide polymorphisms enhance choline requirement. Additionally, small intestinal bacterial overgrowth (SIBO) frequently causes intraluminal choline degradation in CF patients prior to its absorption. As adequate choline supplementation was clinically effective and adult as well as pediatric CF patients suffer from choline deficiency, choline supplementation in CF patients of all ages should be evaluated.

Assessing the impact of cystic fibrosis on the antipyretic response of ibuprofen in children: Physiologically-based modeling as a candle in the dark

AIM

The goal of this study is to present the utility of quantitative modelling for extrapolation of drug safety and efficacy to underrepresented populations in controlled clinical trials. To illustrate this, the stepwise development of an integrated disease/pharmacokinetics/pharmacodynamics model of antipyretic efficacy of ibuprofen in children with cystic fibrosis (CF) is presented along with therapy optimization suggestions.

METHOD

Published clinical trials, in vitro data, and drug physiochemical properties were used to develop an ibuprofen-mediated antipyresis model for febrile children also having CF. Workflow included first developing a mechanistic absorption model using in vitro-in vivo extrapolation followed by physiologically-based pharmacokinetic (PBPK) modelling. The verified PBPK model was then scaled to paediatric patients with CF. Once verified, the PBPK model was linked to an indirect response model of antipyresis for simulation of the overall antipyretic efficacy of ibuprofen in CF children.

RESULTS

Model simulations showed therapeutic inequivalence between healthy children and paediatric patients with CF; C_max and AUC decreased by 39% (32-46%) and 44% (36-52%), respectively, in patients. Further, and in agreement with literature reports, predicted pharmacodynamics time courses suggest a slower onset and faster offset of action in patients compared to healthy children, 30 and 60 minutes, respectively. Exploratory simulations suggest an increase in dosing frequency for CF children as a better therapeutic strategy.

CONCLUSION

Model-informed approaches to leveraging knowledge obtained throughout the life cycle of drug development may play a key role in extrapolating drug efficacy and safety to underrepresented populations.

Physiology of Electrolyte Transport in the Gut: Implications for Disease

We now have an increased understanding of the genetics, cell biology, and physiology of electrolyte transport processes in the mammalian intestine, due to the availability of sophisticated methodologies ranging from genome wide association studies to CRISPR-CAS technology, stem cell-derived organoids, 3D microscopy, electron cryomicroscopy, single cell RNA sequencing, transgenic methodologies, and tools to manipulate cellular processes at a molecular level. This knowledge has simultaneously underscored the complexity of biological systems and the interdependence of multiple regulatory systems. In addition to the plethora of mammalian neurohumoral factors and their cross talk, advances in pyrosequencing and metagenomic analyses have highlighted the relevance of the microbiome to intestinal regulation. This article provides an overview of our current understanding of electrolyte transport processes in the small and large intestine, their regulation in health and how dysregulation at multiple levels can result in disease. Intestinal electrolyte transport is a balance of ion secretory and ion absorptive processes, all exquisitely dependent on the basolateral Na⁺ /K⁺ ATPase; when this balance goes awry, it can result in diarrhea or in constipation. The key transporters involved in secretion are the apical membrane Cl^- channels and the basolateral Na⁺ -K⁺ -2Cl^- cotransporter, NKCC1 and K⁺ channels. Absorption chiefly involves apical membrane Na⁺ /H⁺ exchangers and Cl^- /HCO₃ ^- exchangers in the small intestine and proximal colon and Na⁺ channels in the distal colon. Key examples of our current understanding of infectious, inflammatory, and genetic diarrheal diseases and of constipation are provided. © 2019 American Physiological Society. Compr Physiol 9:947-1023, 2019.

Bicarbonate in cystic fibrosis

BACKGROUND

Cystic fibrosis (CF, mucoviscidosis) is caused by mutations in the gene encoding CF transmembrane conductance regulator (CFTR), which is a chloride and bicarbonate channel necessary for fluid secretion and extracellular alkalization. For a long time, research concentrated on abnormal Cl^- and Na⁺ transport, but neglected bicarbonate as a crucial factor in CF.

METHODS

The present short review reports early findings as well as recent insights into the role of CFTR for bicarbonate transport and its defects in CF.

RESULTS

The available data indicate impaired bicarbonate transport not only in pancreas, intestine, airways, and reproductive organs, but also in salivary glands, sweat duct and renal tubular epithelial cells. Defective bicarbonate transport is closely related to the impaired mucus properties and mucus blocking in secretory organs of CF patients, causing the life threatening lung disease.

CONCLUSIONS

Apart from the devastating lung disease, abrogated bicarbonate transport also leads to many other organ dysfunctions, which are outlined in the present review.

AP2 α modulates cystic fibrosis transmembrane conductance regulator function in the human intestine

BACKGROUND

AP2 is a clathrin-based endocytic adaptor complex comprising α, β2, μ2 and σ2 subunits. μ2 regulates CFTR endocytosis. The α subunit interacts with CFTR in the intestine but its physiologic significance is unclear.

METHODS

CFTR short circuit current was measured in intestinal T84 cells following shRNA knock down of AP2α (AP2αKD). Clathrin-coated structures (CCS) were immunolabeled and quantified in AP2αKD intestinal Caco2BBe (C2BBe) cells. GST tagged human AP2α appendage domain was cloned and its interaction with CFTR determined by GST pull down assay.

RESULT

AP2αKD in T84 cells resulted in higher CFTR current (57%) compared to control, consistent with increased functional CFTR and delayed endocytosis. Depletion of AP2α reduced CCS in C2BBe cells. Pull down assays revealed an interaction between human AP2α appendage domain and CFTR.

CONCLUSION

AP2 α interacts with and modulates CFTR function in the intestine by participating in clathrin assembly and recruitment of CFTR to CCS.

Impact of CFTR Modulation on Intestinal pH, Motility, and Clinical Outcomes in Patients With Cystic Fibrosis and the G551D Mutation

Objectives:

A defect in bicarbonate secretion contributes to the pathophysiology of gastrointestinal complications in patients with cystic fibrosis (CF). We measured gastrointestinal pH, clinical outcomes, and intestinal transit profiles in patients with the G551D mutation before and after treatment with ivacaftor, a CF transmembrane regulator channel (CFTR) potentiator.

Methods:

Observational studies of ivacaftor effectiveness were conducted in the United States and Canada. A subset of subjects ingested a wireless motility capsule (n=10) that measures in vivo pH, both before therapy with ivacaftor and 1 month after treatment; values obtained were compared for mean pH and area under the pH curve, and regional intestinal motility. We also queried subjects about abdominal pain and recorded body weight before and after treatment.

Results:

One month after administering ivacaftor, a significant increase in mean pH was observed after gastric emptying (P<0.05). Area under the pH curve analyses indicate increased bicarbonate mass (P<0.05 for select 5 min intervals and all segments >30 min); mean weight gain was 1.1 kg (P=0.08). No difference in abdominal pain or regional transit times was seen.

Conclusions:

CFTR modulation improves the proximal small intestinal pH profile in patients with the G551D CFTR mutation and we observed clinically relevant, contemporaneous weight gain, although it did not reach statistical significance. These data provide in vivo evidence that CFTR is an important regulator of bicarbonate secretion, which may be a translational link between CFTR function and clinical improvement.

Defective goblet cell exocytosis contributes to murine cystic fibrosis-associated intestinal disease

Cystic fibrosis (CF) intestinal disease is associated with the pathological manifestation mucoviscidosis, which is the secretion of tenacious, viscid mucus that plugs ducts and glands of epithelial-lined organs. Goblet cells are the principal cell type involved in exocytosis of mucin granules; however, little is known about the exocytotic process of goblet cells in the CF intestine. Using intestinal organoids from a CF mouse model, we determined that CF goblet cells have altered exocytotic dynamics, which involved intrathecal granule swelling that was abruptly followed by incomplete release of partially decondensated mucus. Some CF goblet cells exhibited an ectopic granule location and distorted cellular morphology, a phenotype that is consistent with retrograde intracellular granule movement during exocytosis. Increasing the luminal concentration of bicarbonate, which mimics CF transmembrane conductance regulator-mediated anion secretion, increased spontaneous degranulation in WT goblet cells and improved exocytotic dynamics in CF goblet cells; however, there was still an apparent incoordination between granule decondensation and exocytosis in the CF goblet cells. Compared with those within WT goblet cells, mucin granules within CF goblet cells had an alkaline pH, which may adversely affect the polyionic composition of the mucins. Together, these findings indicate that goblet cell dysfunction is an epithelial-autonomous defect in the CF intestine that likely contributes to the pathology of mucoviscidosis and the intestinal manifestations of obstruction and inflammation.

Molecular mechanisms of calcium-sensing receptor-mediated calcium signaling in the modulation of epithelial ion transport and bicarbonate secretion

Epithelial ion transport is mainly under the control of intracellular cAMP and Ca(2+) signaling. Although the molecular mechanisms of cAMP-induced epithelial ion secretion are well defined, those induced by Ca(2+) signaling remain poorly understood. Because calcium-sensing receptor (CaSR) activation results in an increase in cytosolic Ca(2+) ([Ca(2+)]cyt) but a decrease in cAMP levels, it is a suitable receptor for elucidating the mechanisms of [Ca(2+)]cyt-mediated epithelial ion transport and duodenal bicarbonate secretion (DBS). CaSR proteins have been detected in mouse duodenal mucosae and human intestinal epithelial cells. Spermine and Gd(3+), two CaSR activators, markedly stimulated DBS without altering duodenal short circuit currents in wild-type mice but did not affect DBS and duodenal short circuit currents in cystic fibrosis transmembrane conductance regulator (CFTR) knockout mice. Clotrimazole, a selective blocker of intermediate conductance Ca(2+)-activated K(+) channels but not chromanol 293B, a selective blocker of cAMP-activated K(+) channels (KCNQ1), significantly inhibited CaSR activator-induced DBS, which was similar in wild-type and KCNQ1 knockout mice. HCO3 (-) fluxes across epithelial cells were activated by a CFTR activator, but blocked by a CFTR inhibitor. CaSR activators induced HCO3 (-) fluxes, which were inhibited by a receptor-operated channel (ROC) blocker. Moreover, CaSR activators dose-dependently raised cellular [Ca(2+)]cyt, which was abolished in Ca(2+)-free solutions and inhibited markedly by selective CaSR antagonist calhex 231, and ROC blocker in both animal and human intestinal epithelial cells. Taken together, CaSR activation triggers Ca(2+)-dependent DBS, likely through the ROC, intermediate conductance Ca(2+)-activated K(+) channels, and CFTR channels. This study not only reveals that [Ca(2+)]cyt signaling is critical to modulate DBS but also provides novel insights into the molecular mechanisms of CaSR-mediated Ca(2+)-induced DBS.

Digestive system dysfunction in cystic fibrosis: challenges for nutrition therapy

Cystic fibrosis can affect food digestion and nutrient absorption. The underlying mutation of the cystic fibrosis trans-membrane regulator gene depletes functional cystic fibrosis trans-membrane regulator on the surface of epithelial cells lining the digestive tract and associated organs, where Cl(-) secretion and subsequently secretion of water and other ions are impaired. This alters pH and dehydrates secretions that precipitate and obstruct the lumen, causing inflammation and the eventual degradation of the pancreas, liver, gallbladder and intestine. Associated conditions include exocrine pancreatic insufficiency, impaired bicarbonate and bile acid secretion and aberrant mucus formation, commonly leading to maldigestion and malabsorption, particularly of fat and fat-soluble vitamins. Pancreatic enzyme replacement therapy is used to address this insufficiency. The susceptibility of pancreatic lipase to acidic and enzymatic inactivation and decreased bile availability often impedes its efficacy. Brush border digestive enzyme activity and intestinal uptake of certain disaccharides and amino acids await clarification. Other complications that may contribute to maldigestion/malabsorption include small intestine bacterial overgrowth, enteric circular muscle dysfunction, abnormal intestinal mucus, and intestinal inflammation. However, there is some evidence that gastric digestive enzymes, colonic microflora, correction of fatty acid abnormalities using dietary n-3 polyunsaturated fatty acid supplementation and emerging intestinal biomarkers can complement nutrition management in cystic fibrosis.

Functional vacuolar ATPase (V-ATPase) proton pumps traffic to the enterocyte brush border membrane and require CFTR

Vacuolar ATPases (V-ATPases) are highly conserved proton pumps that regulate organelle pH. Epithelial luminal pH is also regulated by cAMP-dependent traffic of specific subunits of the V-ATPase complex from endosomes into the apical membrane. In the intestine, cAMP-dependent traffic of cystic fibrosis transmembrane conductance regulator (CFTR) channels and the sodium hydrogen exchanger (NHE3) in the brush border regulate luminal pH. V-ATPase was found to colocalize with CFTR in intestinal CFTR high expresser (CHE) cells recently. Moreover, apical traffic of V-ATPase and CFTR in rat Brunner's glands was shown to be dependent on cAMP/PKA. These observations support a functional relationship between V-ATPase and CFTR in the intestine. The current study examined V-ATPase and CFTR distribution in intestines from wild-type, CFTR(-/-) mice and polarized intestinal CaCo-2BBe cells following cAMP stimulation and inhibition of CFTR/V-ATPase function. Coimmunoprecipitation studies examined V-ATPase interaction with CFTR. The pH-sensitive dye BCECF determined proton efflux and its dependence on V-ATPase/CFTR in intestinal cells. cAMP increased V-ATPase/CFTR colocalization in the apical domain of intestinal cells and redistributed the V-ATPase Voa1 and Voa2 trafficking subunits from the basolateral membrane to the brush border membrane. Voa1 and Voa2 subunits were localized to endosomes beneath the terminal web in untreated CFTR(-/-) intestine but redistributed to the subapical cytoplasm following cAMP treatment. Inhibition of CFTR or V-ATPase significantly decreased pHi in cells, confirming their functional interdependence. These data establish that V-ATPase traffics into the brush border membrane to regulate proton efflux and this activity is dependent on CFTR in the intestine.

Intestinal pH and gastrointestinal transit profiles in cystic fibrosis patients measured by wireless motility capsule

BACKGROUND AND AIMS

The effect of the cystic fibrosis transmembrane conductance regulator protein (CFTR) defect in pancreatic insufficient (PI) patients with cystic fibrosis (CF) on the gastrointestinal pH profile is poorly defined. Adequate and efficient neutralization of the gastric acidity in the duodenum is important for nutrient absorption and timely release of pancreatic enzyme replacement therapy (PERT). We utilized a wireless motility capsule (WMC) to study intestinal pH profile and gastrointestinal transit profile in CF subjects.

METHODS

WMC studies were done on ten adult CF patients with PI while off acid suppression medication and ten age, gender and BMI matched healthy controls. Mean pH over 1 min increments and area under the pH curve over 5 min increments was calculated for the first hour post gastric emptying. Paired t-test was used to compare means of the pH recordings, transit profiles and analysis of time interval required to reach and maintain pH >5.5 and 6.0.

RESULTS

A statistically significant difference was observed between mean pH values during the first 23 min of small bowel transit (p < 0.05). In CF subjects, there was a significant delay in time interval required to reach and sustain pH 5.5 and pH 6.0 (p < 0.001), which is required for PERT dissolution. Only small bowel transit in CF subjects was noted to be significantly delayed (p = 0.004) without a compensatory increase in whole gut transit time.

CONCLUSIONS

We have demonstrated a significant delay in the small intestinal transit and a deficient buffering capacity required to neutralize gastric acid in the proximal small bowel of patients with CF.

Regulated traffic of anion transporters in mammalian Brunner's glands: a role for water and fluid transport

The Brunner's glands of the proximal duodenum exert barrier functions through secretion of glycoproteins and antimicrobial peptides. However, ion transporter localization, function, and regulation in the glands are less clear. Mapping the subcellular distribution of transporters is an important step toward elucidating trafficking mechanisms of fluid transport in the gland. The present study examined 1) changes in the distribution of intestinal anion transporters and the aquaporin 5 (AQP5) water channel in rat Brunner's glands following second messenger activation and 2) anion transporter distribution in Brunner's glands from healthy and disease-affected human tissues. Cystic fibrosis transmembrane conductance regulator (CFTR), AQP5, sodium-potassium-coupled chloride cotransporter 1 (NKCC1), sodium-bicarbonate cotransporter (NBCe1), and the proton pump vacuolar ATPase (V-ATPase) were localized to distinct membrane domains and in endosomes at steady state. Carbachol and cAMP redistributed CFTR to the apical membrane. cAMP-dependent recruitment of CFTR to the apical membrane was accompanied by recruitment of AQP5 that was reversed by a PKA inhibitor. cAMP also induced apical trafficking of V-ATPase and redistribution of NKCC1 and NBCe1 to the basolateral membranes. The steady-state distribution of AQP5, CFTR, NBCe1, NKCC1, and V-ATPase in human Brunner's glands from healthy controls, cystic fibrosis, and celiac disease resembled that of rat; however, the distribution profiles were markedly attenuated in the disease-affected duodenum. These data support functional transport of chloride, bicarbonate, water, and protons by second messenger-regulated traffic in mammalian Brunner's glands under physiological and pathophysiological conditions.

Efficacy and safety of a unique enteric-coated bicarbonate-buffered pancreatic enzyme replacement therapy in children and adults with cystic fibrosis

BACKGROUND

Pancreatic enzyme replacement therapy (PERT) is used to treat exocrine pancreatic insufficiency in cystic fibrosis.

RESULTS/METHODS

Efficacy and safety of a unique enteric-coated (EC) bicarbonate-buffered PERT product (PERTZYE^®/PANCRECARB^®; Digestive Care, Inc., Bethlehem, PA, USA) was studied in a randomized, double-blind, placebo-controlled cross-over design. Subjects were stabilized on EC-bicarbonate-buffered PERT and a high-fat diet. During two treatment periods, subjects were randomized to EC-bicarbonate-buffered PERT or placebo, followed by a 72-h stool collection employing an ingested stool dye marker. Mean coefficient of fat absorption with EC-bicarbonate-buffered PERT was 82.5% compared with 46.3% with the placebo (absolute difference 36.2%; p < 0.001), a 78.2% improvement for active over placebo. Similar improvements in nitrogen absorption were observed. Overall stool frequency and stool weight decreased (p < 0.001). No safety concerns were identified.

SUMMARY

EC-bicarbonate-buffered PERT is effective in treating cystic fibrosis-associated exocrine pancreatic insufficiency.

Dehydrocostuslactone, a sesquiterpene lactone activates wild-type and ΔF508 mutant CFTR chloride channel

Cystic fibrosis transmembrane conductance regulator (CFTR) represents the main cAMP-activated Cl⁻ channel expressed in the apical membrane of serous epithelial cells. Both deficiency and overactivation of CFTR may cause fluid and salt secretion related diseases. The aim of this study was to identify natural compounds that are able to stimulate wild-type (wt) and ΔF508 mutant CFTR channel activities in CFTR-expressing Fischer rat thyroid (FRT) cells. We found that dehydrocostuslactone [DHC, (3aS, 6aR, 9aR, 9bS)-decahydro-3,6,9-tris (methylene) azuleno [4,5-b] furan-2(3H)-one)] dose dependently potentiates both wt and ΔF508 mutant CFTR-mediated iodide influx in cell-based fluorescent assays and CFTR-mediated Cl⁻ currents in short-circuit current studies, and the activations could be reversed by the CFTR inhibitor CFTRinh-172. Maximal CFTR-mediated apical Cl⁻ current secretion in CFTR-expressing FRT cells was stimulated by 100 μM DHC. Determination of intracellular cAMP content showed that DHC modestly but significantly increased cAMP level in FRT cells, but cAMP elevation effects contributed little to DHC-stimulated iodide influx. DHC also stimulated CFTR-mediated apical Cl⁻ current secretion in FRT cells expressing ΔF508-CFTR. Subsequent studies demonstrated that activation of CFTR by DHC is forskolin dependent. DHC represents a new class of CFTR potentiators that may have therapeutic potential in CFTR-related diseases.

Molecular motors and apical CFTR traffic in epithelia

Intracellular protein traffic plays an important role in the regulation of Cystic Fibrosis Transmembrane Conductance Regulator (CFTR) chloride channels. Microtubule and actin-based motor proteins direct CFTR movement along trafficking pathways. As shown for other regulatory proteins such as adaptors, the involvement of protein motors in CFTR traffic is cell-type specific. Understanding motor specificity provides insight into the biology of the channel and opens opportunity for discovery of organ-specific drug targets for treating CFTR-mediated diseases.

Essential role of the electroneutral Na+-HCO3- cotransporter NBCn1 in murine duodenal acid-base balance and colonic mucus layer build-up in vivo

Duodenal epithelial cells need efficient defence strategies during gastric acidification of the lumen, while colonic mucosa counteracts damage by pathogens by building up a bacteria-free adherent mucus layer. Transport of HCO3(-) is considered crucial for duodenal defence against acid as well as for mucus release and expansion, but the transport pathways involved are incompletely understood. This study investigated the significance of the electroneutral Na(+)-HCO3(-) cotransporter NBCn1 for duodenal defence against acid and colonic mucus release. NBCn1 was localized to the basolateral membrane of duodenal villous enterocytes and of colonic crypt cells, with predominant expression in goblet cells. Duodenal villous enterocyte intracellular pH was studied before and during a luminal acid load by two-photon microscopy in exteriorized, vascularly perfused, indicator (SNARF-1 AM)-loaded duodenum of isoflurane-anaesthetized, systemic acid-base-controlled mice. Acid-induced HCO3(-) secretion was measured in vivo by single-pass perfusion and pH-stat titration. After a luminal acid load, NBCn1-deficient duodenocytes were unable to recover rapidly from intracellular acidification and could not respond adequately with protective HCO3(-) secretion. In the colon, build-up of the mucus layer was delayed, and a decreased thickness of the adherent mucus layer was observed, suggesting that basolateral HCO3(-) uptake is essential for optimal release of mucus. The electroneutral Na(+)-HCO3(-) cotransporter NBCn1 displays a differential cellular distribution in the murine intestine and is essential for HCO3(-)-dependent mucosal protective functions, such as recovery of intracellular pH and HCO3(-) secretion in the duodenum and secretion of mucus in the colon.

Cystic fibrosis, gastroduodenal inflammation, duodenal ulcer, and H. pylori infection: the "cystic fibrosis paradox" revisited

BACKGROUND

In cystic fibrosis (CF) patients a duodenal impaired bicarbonate secretion and unbuffered gastric acid are always described and the development of duodenal ulceration is uncommon (CF paradox). Helicobacter pylori (HP) infection is the main cause for duodenal ulceration and its prevalence in CF patients is controversial.

AIM

The objective of this study is to evaluate HP prevalence, gastric histology, and duodenal ulceration in adult FC patients.

METHODS

32 adult CF patients were submitted to (13)C-urea breath test and serum immunoblotting test for HP diagnosis. Among them, 20 patients were submitted to endoscopy.

RESULTS

19/32 (68%) patients showed positive serology. Endoscopy showed erosive duodenitis (15%), and duodenal ulcer scar in 10%. On duodenal histology, 94.5%, showed active inflammation and 66.7% gastric metaplasia.

CONCLUSION

HP infection prevalence in adult CF patients was similar to that of general Brazilian population. CF patients have all the duodenal spectrum of alterations, including duodenal ulcer. CF paradox may not exist.

Measurements of CFTR-mediated Cl- secretion in human rectal biopsies constitute a robust biomarker for Cystic Fibrosis diagnosis and prognosis

Background

Cystic Fibrosis (CF) is caused by ∼1,900 mutations in the CF transmembrane conductance regulator (CFTR) gene encoding for a cAMP-regulated chloride (Cl⁻) channel expressed in several epithelia. Clinical features are dominated by respiratory symptoms, but there is variable organ involvement thus causing diagnostic dilemmas, especially for non-classic cases.

Methodology/Principal Findings

To further establish measurement of CFTR function as a sensitive and robust biomarker for diagnosis and prognosis of CF, we herein assessed cholinergic and cAMP-CFTR-mediated Cl⁻ secretion in 524 freshly excised rectal biopsies from 118 individuals, including patients with confirmed CF clinical diagnosis (n = 51), individuals with clinical CF suspicion (n = 49) and age-matched non-CF controls (n = 18). Conclusive measurements were obtained for 96% of cases. Patients with “Classic CF”, presenting earlier onset of symptoms, pancreatic insufficiency, severe lung disease and low Shwachman-Kulczycki scores were found to lack CFTR-mediated Cl⁻ secretion (<5%). Individuals with milder CF disease presented residual CFTR-mediated Cl⁻ secretion (10–57%) and non-CF controls show CFTR-mediated Cl⁻ secretion ≥30–35% and data evidenced good correlations with various clinical parameters. Finally, comparison of these values with those in “CF suspicion” individuals allowed to confirm CF in 16/49 individuals (33%) and exclude it in 28/49 (57%). Statistical discriminant analyses showed that colonic measurements of CFTR-mediated Cl⁻ secretion are the best discriminator among Classic/Non-Classic CF and non-CF groups.

Conclusions/Significance

Determination of CFTR-mediated Cl⁻ secretion in rectal biopsies is demonstrated here to be a sensitive, reproducible and robust predictive biomarker for the diagnosis and prognosis of CF. The method also has very high potential for (pre-)clinical trials of CFTR-modulator therapies.

Myosin Ia is required for CFTR brush border membrane trafficking and ion transport in the mouse small intestine

In enterocytes of the small intestine, endocytic trafficking of CFTR channels from the brush border membrane (BBM) to the subapical endosomes requires the minus-end motor, myosin VI (Myo6). The subapical localization of Myo6 is dependent on myosin Ia (Myo1a) the major plus-end motor associated with the BBM, suggestive of functional synergy between these two motors. In villus enterocytes of the Myo1a KO mouse small intestine, CFTR accumulated in syntaxin-3 positive subapical endosomes, redistributed to the basolateral domain and was absent from the BBM. In colon, where villi are absent and Myo1a expression is low, CFTR exhibited normal localization to the BBM in the Myo1a KO similar to WT. cAMP-stimulated CFTR anion transport in the small intestine was reduced by 58% in the KO, while anion transport in the colon was comparable to WT. Co-immunoprecipitation confirmed the association of CFTR with Myo1a. These data indicate that Myo1a is an important regulator of CFTR traffic and anion transport in the BBM of villus enterocytes and suggest that Myo1a may power apical CFTR movement into the BBM from subapical endosomes. Alternatively, it may anchor CFTR channels in the BBM of villus enterocytes as was proposed for Myo1a's role in BBM localization of sucrase-isomaltase.

Stimulation of Airway and Intestinal Mucosal Secretion by Natural Coumarin CFTR Activators

Mutations of cystic fibrosis (CF) transmembrane conductance regulator (CFTR) cause lethal hereditary disease CF that involves extensive destruction and dysfunction of serous epithelium. Possible pharmacological therapy includes correction of defective intracellular processing and abnormal channel gating. In a previous study, we identified five natural coumarin potentiators of ΔF508-CFTR including osthole, imperatorin, isopsoralen, praeruptorin A, and scoparone. The present study was designed to determine the activity of these coumarine compounds on CFTR activity in animal tissues as a primary evaluation of their therapeutic potential. In the present study, we analyzed the affinity of these coumarin potentiators in activating wild-type CFTR and found that they are all potent activators. Osthole showed the highest affinity with K_d values <50 nmol/L as determined by Ussing chamber short-circuit current assay. Stimulation of rat colonic mucosal secretion by osthole was tested by the Ussing chamber short-circuit current assay. Osthole reached maximal activation of colonic Cl⁻ secretion at 5 μmol/L. Stimulation of mouse tracheal mucosal secretion was analyzed by optical measurement of single gland secretion. Fluid secretion rate of tracheal single submucosal gland stimulated by osthole at 10 μmol/L was three-fold more rapid than that in negative control. In both cases the stimulated secretions were fully abolished by CFTR_inh-172. In conclusion, the effective stimulation of Cl^– and fluid secretion in colonic and tracheal mucosa by osthole suggested the therapeutic potential of natural coumarin compounds for the treatment of CF and other CFTR-related diseases.

A new role for bicarbonate secretion in cervico-uterine mucus release

Cervical mucus thinning and release during the female reproductive cycle is thought to rely mainly on fluid secretion. However, we now find that mucus released from the murine reproductive tract critically depends upon concurrent bicarbonate (HCO(3)(-)) secretion. Prostaglandin E(2) (PGE(2))- and carbachol-stimulated mucus release was severely inhibited in the absence of serosal HCO(3)(-), HCO(3)(-) transport, or functional cystic fibrosis transmembrane conductance regulator (CFTR). In contrast to mucus release, PGE(2)- and carbachol-stimulated fluid secretion was not dependent on bicarbonate or on CFTR, but was completely blocked by niflumic acid. We found stimulated mucus release was severely impaired in the cystic fibrosis F508 reproductive tract, even though stimulated fluid secretion was preserved. Thus, CFTR mutations and/or poor bicarbonate secretion may be associated with reduced female fertility associated with abnormal mucus and specifically, may account for the increased viscosity and lack of cyclical changes in cervical mucus long noted in women with cystic fibrosis.

Small intestinal glucose absorption in cystic fibrosis: a study in human and transgenic ΔF508 cystic fibrosis mouse tissues

Intestinal transport is disturbed in cystic fibrosis (CF), with both defective Cl− secretion and changes in absorption being reported. We have examined the effects of the disease on Na+-dependent glucose absorption by the small intestine. Active glucose absorption was monitored as changes in short-circuit current (SCC) in intact and stripped intestinal sheets from normal (Swiss) and transgenic CF (Cftrtm 1 Eur and Cftrtm 2 Cam) mice with the ΔF508 mutation, and in jejunal biopsies from children with CF and normal controls. Na+-dependent glucose uptake at the luminal membrane was measured in brush-border membrane vesicles (BBMVs). Intact and stripped sheets of jejunum and midintestine from Swiss mice exhibited a concentration-dependent increase in SCC with glucose. Apparent Km values were similar in the two preparations, but the apparent Vmax was greater in stripped sheets. This difference was not due to a loss of neural activity in stripped sheets as tetrodotoxin did not influence the glucose-induced SCC in intact sheets. Similar results were observed in stripped sheets of jejunum and mid-intestine from wild-type Cftrtm 1 Eur mice, but in tissues from CF mice the apparent Vmax value was reduced significantly. A lower Vmax was also obtained in intact sheets of mid-intestine from CF (Cftrtm 2 Cam) mice. Jejunal biopsies from CF patients however, exhibited an enhanced glucose-dependent rise in SCC. Na+-dependent uptake by BBMVs from CF (Cftrtm 1 Eur) mice was not reduced compared with wild-type and Swiss BBMVs. It was concluded that, in contrast to human intestine, intestinal glucose absorption was reduced in transgenic mouse models of CF with the ΔF508 mutation, but that this could not be detected in an isolated preparation of brush-border membranes. Transgenic mouse models of CF may not accurately reflect all aspects of intestinal dysfunction in the human disease.

Infectious diarrhea: Cellular and molecular mechanisms

Diarrhea caused by enteric infections is a major factor in morbidity and mortality worldwide. An estimated 2-4 billion episodes of infectious diarrhea occur each year and are especially prevalent in infants. This review highlights the cellular and molecular mechanisms underlying diarrhea associated with the three classes of infectious agents, i.e., bacteria, viruses and parasites. Several bacterial pathogens have been chosen as model organisms, including Vibrio cholerae as a classical example of secretory diarrhea, Clostridium difficile and Shigella species as agents of inflammatory diarrhea and selected strains of pathogenic Escherichia coli (E. coli) to discuss the recent advances in alteration of epithelial ion absorption. Many of the recent studies addressing epithelial ion transport and barrier function have been carried out using viruses and parasites. Here, we focus on the rapidly developing field of viral diarrhea including rotavirus, norovirus and astrovirus infections. Finally we discuss Giardia lamblia and Entamoeba histolytica as examples of parasitic diarrhea. Parasites have a greater complexity than the other pathogens and are capable of creating molecules similar to those produced by the host, such as serotonin and PGE(2). The underlying mechanisms of infectious diarrhea discussed include alterations in ion transport and tight junctions as well as the virulence factors, which alter these processes either through direct effects or indirectly through inflammation and neurotransmitters.

Normal mouse intestinal mucus release requires cystic fibrosis transmembrane regulator-dependent bicarbonate secretion

The mechanisms underlying mucus-associated pathologies in cystic fibrosis (CF) remain obscure. However, recent studies indicate that CF transmembrane conductance regulator (CFTR) is required for bicarbonate (HCO3-) transport and that HCO3- is critical for normal mucus formation. We therefore investigated the role of HCO3- in mucus secretion using mouse small intestine segments ex vivo. Basal rates of mucus release in the presence or absence of HCO3- were similar. However, in the absence of HCO3-, mucus release stimulated by either PGE2 or 5-hydroxytryptamine (5-HT) was approximately half that stimulated by these molecules in the presence of HCO3-. Inhibition of HCO3- and fluid transport markedly reduced stimulated mucus release. However, neither absence of HCO3- nor inhibition of HCO3- transport affected fluid secretion rates, indicating that the effect of HCO3- removal on mucus release was not due to decreased fluid secretion. In a mouse model of CF (mice homozygous for the most common human CFTR mutation), intestinal mucus release was minimal when stimulated with either PGE2 or 5-HT in the presence or absence of HCO3-. These data suggest that normal mucus release requires concurrent HCO3- secretion and that the characteristically aggregated mucus observed in mucin-secreting organs in individuals with CF may be a consequence of defective HCO3- transport.

Role of down-regulated in adenoma anion exchanger in HCO3- secretion across murine duodenum

BACKGROUND & AIMS

The current model of duodenal HCO(3)(-) secretion proposes that basal secretion results from Cl(-)/HCO(3)(-) exchange, whereas cyclic adenosine monophosphate (cAMP)-stimulated secretion depends on a cystic fibrosis transmembrane conductance regulator channel (Cftr)-mediated HCO(3)(-) conductance. However, discrepancies in applying the model suggest that Cl(-)/HCO(3)(-) exchange also contributes to cAMP-stimulated secretion. Of 2 candidate Cl(-)/HCO(3)(-) exchangers, studies of putative anion transporter-1 knockout (KO) mice find little contribution of putative anion transporter-1 to basal or cAMP-stimulated secretion. Therefore, the role of down-regulated in adenoma (Dra) in duodenal HCO(3)(-) secretion was investigated using DraKO mice.

METHODS

Duodenal HCO(3)(-) secretion was measured by pH stat in Ussing chambers. Apical membrane Cl(-)/HCO(3)(-) exchange was measured by microfluorometry of intracellular pH in intact villous epithelium. Dra expression was assessed by immunofluorescence.

RESULTS

Basal HCO(3)(-) secretion was reduced approximately 55%-60% in the DraKO duodenum. cAMP-stimulated HCO(3)(-) secretion was reduced approximately 50%, but short-circuit current was unchanged, indicating normal Cftr activity. Microfluorimetry of villi demonstrated that Dra is the dominant Cl(-)/HCO(3)(-) exchanger in the lower villous epithelium. Dra expression increased from villous tip to crypt. DraKO and wild-type villi also demonstrated regulation of apical Na(+)/H(+) exchange by Cftr-dependent cell shrinkage during luminal Cl(-) substitution.

CONCLUSIONS

In murine duodenum, Dra Cl(-)/HCO(3)(-) exchange is concentrated in the lower crypt-villus axis where it is subject to Cftr regulation. Dra activity contributes most basal HCO(3)(-) secretion and approximately 50% of cAMP-stimulated HCO(3)(-) secretion. Dra Cl(-)/HCO(3)(-) exchange should be considered in efforts to normalize HCO(3)(-) secretion in duodenal disorders such as ulcer disease and cystic fibrosis.

Gender-specific protection of estrogen against gastric acid-induced duodenal injury: stimulation of duodenal mucosal bicarbonate secretion

Because human duodenal mucosal bicarbonate secretion (DMBS) protects duodenum against acid-peptic injury, we hypothesize that estrogen stimulates DMBS, thereby attributing to the clinically observed lower incidence of duodenal ulcer in premenopausal women than the age-matched men. We found that basal and acid-stimulated DMBS responses were 1.5 and 2.4-fold higher in female than male mice in vivo, respectively. Acid-stimulated DMBS in both genders was abolished by ICI 182,780 and tamoxifen. Estradiol-17beta (E2) and the selective estrogen receptor (ER) agonists of ERalpha [1,3,5-Tris(4-hydroxyphenyl)-4-propyl-1H-pyrazole] and ERbeta [2,3-bis(4-hydroxyphenyl) propionitrile], but not progesterone, rapidly stimulated ER-dependent murine DMBS in vivo. E2 dose dependently stimulated murine DMBS, which was attenuated by a Cl(-)/HCO3(-) anion exchanger inhibitor 4,4'-didsothio- cyanostilbene-2, 2'-disulfonic acid, removal of extracellular Cl(-), and in cystic fibrosis transmembrane conductance regulator knockout female mice. E2 stimulated murine DMBS in vitro in both genders with significantly greater response in female than male mice (female to male ratio = 4.3). ERalpha and ERbeta mRNAs and proteins were detected in murine duodenal epithelium of both genders; however, neither ERalpha nor ERbeta mRNA and protein expression levels differed according to gender. E2 rapidly mobilized intracellular calcium in a duodenal epithelial SCBN cell line that expresses ERalpha and ERbeta, whereas BAPTA-AM abolished E2-stimulated murine DMBS. Thus, our data show that E2 stimulates DMBS via ER dependent mechanisms linked to intracellular calcium, cystic fibrosis transmembrane conductance regulator, and Cl(-)/HCO3(-) anion exchanger. Gender-associated differences in basal, acid- and E2-stimulated DMBS may have offered a reasonable explanation for the clinically observed lower incidence of duodenal ulcer in premenopausal women than age-matched men.

CFTR and its key role in in vivo resting and luminal acid-induced duodenal HCO3- secretion

BACKGROUND AND AIMS

We investigated the role of the recently discovered, villous-expressed anion exchanger Slc26a6 (PAT1) and the predominantly crypt-expressed cystic fibrosis transmembrane regulator (CFTR) in basal and acid-stimulated murine duodenal HCO(3)(-) secretion in vivo, and the influence of blood HCO(3)(-) concentration on both.

METHODS

The proximal duodenum of anaesthetized mice was perfused in situ, and HCO(3)(-) secretion was determined by back-titration. Duodenal mucosal permeability was assessed by determining (51)Cr-EDTA leakage from blood to lumen.

RESULTS

Compared with wild type (WT) littermates basal duodenal HCO(3)(-) secretory rates were slightly reduced in Slc26-deficient mice at low ( approximately 21 mm), and markedly reduced at high blood HCO(3)(-) concentration ( approximately 29 mm). In contrast, basal HCO(3)(-) secretion was markedly reduced in CFTR-deficient mice compared with WT littermates both at high and low blood HCO(3)(-) concentration. A short-term application of luminal acid increased duodenal HCO(3)(-) secretory rate in Slc26a6-deficient and WT mice to the same degree, but had no stimulatory effect in the absence of CFTR. Luminal acidification to pH 2.5 did not alter duodenal permeability.

CONCLUSIONS

The involvement of Slc26a6 in basal HCO(3)(-) secretion in murine duodenum in vivo is critically dependent on the systemic acid/base status, and this transporter is not involved in acid-stimulated HCO(3)(-) secretion. The presence of CFTR is essential for basal and acid-induced HCO(3)(-) secretion irrespective of acid/base status. This suggests a coupled action of Slc26a6 with CFTR for murine basal duodenal HCO(3)(-) secretion, but not acid-stimulated secretion, in vivo.

Heat-stable enterotoxin of Escherichia coli (STa) can stimulate duodenal HCO3(-) secretion via a novel GC-C- and CFTR-independent pathway

The heat-stable enterotoxin of Escherichia coli (STa) is a potent stimulant of intestinal chloride and bicarbonate secretion. Guanylyl cyclase C (GC-C) has been shown to be the primary receptor involved in mediating this response. However, numerous studies have suggested the existence of an alternative STa-binding receptor. The aims of this study were to determine whether a non-GC-C receptor exists for STa and what is the functional relevance of this for intestinal bicarbonate secretion in mice. (125)I-STa-binding experiments were performed with intestinal mucosae from GC-C knockout (KO) and wild type (WT) mice. Subsequently, the functional relevance of an alternative STa-binding receptor was explored by examining STa-, uroguanylin-, and guanylin-stimulated duodenal bicarbonate secretion (DBS) in GC-C KO mice in vitro and in vivo. Significant (125)I-STa-binding occurred in the proximal small intestines of GC-C KO and WT mice. Analysis of binding coefficients and pH dependence showed that (125)I-STa-binding in GC-C KO mice involved a receptor distinct from that of WT mice. Functionally, STa, uroguanylin, and guanylin all stimulated a significant increase in DBS in GC-C KO mice. Uroguanylin- and guanylin-stimulated DBS were significantly inhibited by glibenclamide, but not by 4,4'-diisothiocyanato-stilbene-2,2'-disulfonic acid (DIDS). However, STa-stimulated DBS was unaffected by glibenclamide but inhibited by DIDS. Taken together, our results suggest that alternative, non-GC-C, receptors likely exist for STa, uroguanylin, and guanylin in the intestines of mice. While uroguanylin- and guanylin-stimulated DBS are cystic fibrosis transmembrane conductance regulator (CFTR) dependent, STa-stimulated DBS is CFTR independent. Further understanding of this alternative receptor and its signaling pathway may provide important insights into rectification of intestinal bicarbonate secretion in cystic fibrosis.

Electrical parameters and ion species for active transport in human esophageal stratified squamous epithelium and Barrett's specialized columnar epithelium

The human esophagus is lined by stratified squamous epithelium (ESSE), and in some subjects with reflux disease the distal esophagus becomes lined by Barrett's specialized columnar epithelium (BSCE). ESSE and BSCE differ both histologically and functionally, the latter evident by differences in their in vivo transmural electrical potential difference (PD), ESSE averaging -15 mV and BSCE being greater than -25 mV. In this report we examine the basis for this difference in PD. This is done by mounting endoscopic biopsies of ESSE from 25 subjects without esophageal disease and BSCE from 19 with Barrett's esophagus in mini-Ussing chambers for electrical recordings basally and after bathing solution ion replacement. The results show that the PD of human ESSE reflects a low level of active ion transport (5.1 +/- 0.8 muA/cm(2)) combined with a high level of tissue (electrical) resistance (344 +/- 34 Omega.cm(2)) and that of BSCE reflects a high level of active transport (43.6 +/- 11.6 muA/cm(2)) combined with a low level of resistance (69 +/- 8 Omega.cm(2)). Furthermore, active transport in ESSE was principally due to sodium absorption whereas in BSCE it was equally divided between sodium absorption and anion (chloride/bicarbonate) secretion, the latter through an apical membrane, 4-acetamido4'-isothiocyano-2,2'-stilbenedisulfonic acid-sensitive anion channel. As an anion-secreting tissue with bicarbonate secretory capacity more than fivefold greater than ESSE, BSCE is better suited than ESSE for defense of the esophagus against reflux disease.

PAT-1 (Slc26a6) is the predominant apical membrane Cl-/HCO3- exchanger in the upper villous epithelium of the murine duodenum

Basal HCO(3)(-) secretion across the duodenum has been shown in several species to principally involve the activity of apical membrane Cl(-)/HCO(3)(-) exchanger(s). To investigate the identity of relevant anion exchanger(s), experiments were performed using wild-type (WT) mice and mice with gene-targeted deletion of the following Cl(-)/HCO(3)(-) exchangers localized to the apical membrane of murine duodenal villi: Slc26a3 [down-regulated in adenoma (DRA)], Slc26a6 [putative anion transporter 1 (PAT-1)], and Slc4a9 [anion exchanger 4 (AE4)]. RT-PCR of the isolated villous epithelium demonstrated PAT-1, DRA, and AE4 mRNA expression. Using the pH-sensitive dye BCECF, anion exchange rates were measured across the apical membrane of epithelial cells in the upper villus of the intact duodenal mucosa. Under basal conditions, Cl(-)/HCO(3)(-) exchange activity was reduced by 65-80% in the PAT-1(-) duodenum, 30-40% in the DRA(-) duodenum, and <5% in the AE4(-) duodenum compared with the WT duodenum. SO(4)(2-)/HCO(3)(-) exchange was eliminated in the PAT-1(-) duodenum but was not affected in the DRA(-) and AE4(-) duodenum relative to the WT duodenum. Intracellular pH (pH(i)) was reduced in the PAT-1(-) villous epithelium but increased to WT levels in the absence of CO(2)/HCO(3)(-) or during methazolamide treatment. Further experiments under physiological conditions indicated active pH(i) compensation in the PAT-1(-) villous epithelium by combined activities of Na(+)/H(+) exchanger 1 and Cl(-)-dependent transport processes at the basolateral membrane. We conclude that 1) PAT-1 is the major contributor to basal Cl(-)/HCO(3)(-) and SO(4)(2-)/HCO(3)(-) exchange across the apical membrane and 2) PAT-1 plays a role in pH(i) regulation in the upper villous epithelium of the murine duodenum.

ΔF508 Mutation Results in Impaired Gastric Acid Secretion

The cystic fibrosis transmembrane conductance regulator (CFTR) is recognized as a multifunctional protein that is involved in Cl(-) secretion, as well as acting as a regulatory protein. In order for acid secretion to take place a complex interaction of transport proteins and channels must occur at the apical pole of the parietal cell. Included in this process is at least one K(+) and Cl(-) channel, allowing for both recycling of K(+) for the H,K-ATPase, and Cl(-) secretion, necessary for the generation of concentrated HCl in the gastric gland lumen. We have previously shown that an ATP-sensitive potassium channel (K(ATP)) is expressed in parietal cells. In the present study we measured secretagogue-induced acid secretion from wild-type and DeltaF508-deficient mice in isolated gastric glands and whole stomach preparations. Secretagogue-induced acid secretion in wild-type mouse gastric glands could be significantly reduced with either glibenclamide or the specific inhibitor CFTR-inh172. In DeltaF508-deficient mice, however, histamine-induced acid secretion was significantly less than in wild-type mice. Furthermore, immunofluorescent localization of sulfonylurea 1 and 2 failed to show expression of a sulfonylurea receptor in the parietal cell, thus further implicating CFTR as the ATP-binding cassette transporter associated with the K(ATP) channels. These results demonstrate a regulatory role for the CFTR protein in normal gastric acid secretion.

Review article: duodenal bicarbonate - mucosal protection, luminal chemosensing and acid-base balance

The duodenum serves as a buffer zone between the stomach and the jejunum. Over a length of only 25 cm, large volumes of strong acid secreted by the stomach must be converted to the neutral-alkaline chyme of the hindgut lumen, generating large volumes of CO(2). The duodenal mucosa consists of epithelial cells connected by low-resistance tight junctions, forming a leaky epithelial barrier. Despite this permeability, the epithelial cells, under intense stress from luminal mineral acid and highly elevated Pco(2), maintain normal functioning. Bicarbonate ion uniquely protects the duodenal epithelial cells from acid-related injury. The specific protective mechanisms likely involve luminal bicarbonate secretion, intracellular pH buffering and interstitial buffering. Furthermore, the duodenum plays an active role in foregut acid-base homeostasis, absorbing large amounts of H(+) and CO(2). We have studied mucosal protection and acid-base balance using live-animal fluorescence ratio microimaging and by performing H(+) and CO(2) balance studies on duodenal perfusates. On the basis of these data, we have formulated novel hypotheses with regard to mucosal protection.

A novel small molecule CFTR inhibitor attenuates HCO3- secretion and duodenal ulcer formation in rats

The cystic fibrosis (CF) transmembrane conductance regulator (CFTR) plays a crucial role in mediating duodenal bicarbonate (HCO(3)(-)) secretion (DBS). Although impaired DBS is observed in CF mutant mice and in CF patients, which would predict increased ulcer susceptibility, duodenal injury is rarely observed in CF patients and is reduced in CF mutant mice. To explain this apparent paradox, we hypothesized that CFTR dysfunction increases cellular [HCO(3)(-)] and buffering power. To further test this hypothesis, we examined the effect of a novel, potent, and highly selective CFTR inhibitor, CFTR(inh)-172, on DBS and duodenal ulceration in rats. DBS was measured in situ using a standard loop perfusion model with a pH stat under isoflurane anesthesia. Duodenal ulcers were induced in rats by cysteamine with or without CFTR(inh)-172 pretreatment 1 h before cysteamine. Superfusion of CFTR(inh)-172 (0.1-10 microM) over the duodenal mucosa had no effect on basal DBS but at 10 microM inhibited acid-induced DBS, suggesting that its effect was limited to CFTR activation. Acid-induced DBS was abolished at 1 and 3 h and was reduced 24 h after treatment with CFTR(inh)-172, although basal DBS was increased at 24 h. CFTR(inh)-172 treatment had no effect on gastric acid or HCO(3)(-) secretion. Duodenal ulcers were observed 24 h after cysteamine treatment but were reduced in CFTR(inh)-172-pretreated rats. CFTR(inh)-172 acutely produces CFTR dysfunction in rodents for up to 24 h. CFTR inhibition reduces acid-induced DBS but also prevents duodenal ulcer formation, supporting our hypothesis that intracellular HCO(3)(-) may be an important protective mechanism for duodenal epithelial cells.

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.

Fat absorption in cystic fibrosis mice is impeded by defective lipolysis and post-lipolytic events

Cystic fibrosis (CF) is frequently associated with progressive loss of exocrine pancreas function, leading to incomplete digestion and absorption of dietary fat. Supplementing patients with pancreatic lipase reduces fat excretion, but it does not completely correct fat malabsorption, indicating that additional pathological processes affect lipolysis and/or uptake of lipolytic products. To delineate the role of such (post) lipolytic processes in CF-related fat malabsorption, we assessed fat absorption, lipolysis, and fatty acid uptake in two murine CF models by measuring fecal fat excretion and uptake of oleate- and triolein-derived lipid. Pancreatic and biliary function was investigated by determining lipase secretion and biliary bile salt (BS) secretion, respectively. A marked increase in fecal fat excretion was observed in cftr null mice but not in homozygous DeltaF508 mice. Fecal BS loss was enhanced in both CF models, but biliary BS secretion rates were similar. Uptake of free fatty acid was delayed in both CF models, but only in null mice was a specific reduction in lipolytic activity apparent, characterized by strongly reduced triglyceride absorption. Impaired lipolysis was not due to reduced pancreatic lipase secretion. Suppression of gastric acid secretion partially restored lipolytic activity and lipid uptake, indicating that incomplete neutralization of gastric acid impedes fat absorption. We conclude that fat malabsorption in cftr null mice is caused by impairment of lipolysis, which may result from aberrant duodenal pH regulation.

Heat-stable enterotoxin of Escherichia coli stimulates a non-CFTR-mediated duodenal bicarbonate secretory pathway

The cystic fibrosis (CF) transmembrane conductance regulator (CFTR) is an important pathway for duodenal mucosal bicarbonate secretion. Duodenal biopsies from CF patients secrete bicarbonate in response to heat-stable enterotoxin from Escherichia coli (STa) but not cAMP. To explore the mechanism of STa-induced bicarbonate secretion in CF more fully, we examined the role of CFTR in STa-stimulated duodenal bicarbonate secretion in mice. In vivo, the duodenum of CFTR (-/-) or control mice was perfused with forskolin (10(-4) M), STa (10(-7) M), uroguanylin (10(-7) M), 8-bromoguanosine 3',5'-cGMP (8-Br-cGMP) (10(-3) M), genistein (10(-6) M) plus STa, or herbimycin A (10(-6) M) plus STa. In vitro, duodenal mucosae were voltage-clamped in Ussing chambers, and bicarbonate secretion was measured by pH-stat. The effect of genistein, DIDS (10(-4) M), and chloride removal was also studied in vitro. Control, but not CF, mice produced a significant increase in duodenal bicarbonate secretion after perfusion with forskolin, uroguanylin, or 8-Br-cGMP. However, both control and CF animals responded to STa with significant increases in bicarbonate output. Genistein and herbimycin A abolished this response in CF mice but not in controls. In vitro, STa-stimulated bicarbonate secretion in CF tissues was inhibited by genistein, DIDS, and chloride-free conditions, whereas bicarbonate secretion persisted in control mice. In the CF duodenum, STa can stimulate bicarbonate secretion via tyrosine kinase activity resulting in apical Cl(-)/HCO(3)(-) exchange. Further studies elucidating the intracellular mechanisms responsible for such non-CFTR mediated bicarbonate secretion may lead to important therapies for CF.