Expression of delta F508 cystic fibrosis transmembrane conductance regulator protein and related chloride transport properties in the gallbladder epithelium from cystic fibrosis patients

Acquired dysfunction of CFTR underlies cystic fibrosis-like disease of the canine gallbladder

Mucocele formation in dogs is a unique and enigmatic muco-obstructive disease of the gallbladder caused by the amassment of abnormal mucus that bears striking pathological similarity to cystic fibrosis. We investigated the role of cystic fibrosis transmembrane conductance regulatory protein (CFTR) in the pathogenesis of this disease. The location and frequency of disease-associated variants in the coding region of CFTR were compared using whole genome sequence data from 2,642 dogs representing breeds at low-risk, high-risk, or with confirmed disease. Expression, localization, and ion transport activity of CFTR were quantified in control and mucocele gallbladders by NanoString, Western blotting, immunofluorescence imaging, and studies in Ussing chambers. Our results establish a significant loss of CFTR-dependent anion secretion by mucocele gallbladder mucosa. A significantly lower quantity of CFTR protein was demonstrated relative to E-cadherin in mucocele compared with control gallbladder mucosa. Immunofluorescence identified CFTR along the apical membrane of epithelial cells in control gallbladders but not in mucocele gallbladder epithelium. Decreases in mRNA copy number for CFTR were accompanied by decreases in mRNA for the Cl-/[Formula: see text] exchanger SLC26A3, K+ channels (KCNQ1, KCNN4), and vasoactive intestinal polypeptide receptor (VIPR1), which suggest a driving force for change in secretory function of gallbladder epithelial cells in the pathogenesis of mucocele formation. There were no significant differences in CFTR gene variant frequency, type, or predicted impact comparing low-risk, high-risk, and definitively diagnosed groups of dogs. This study describes a unique, naturally occurring muco-obstructive disease of the canine gallbladder, with uncanny similarity to cystic fibrosis, and driven by the underlying failure of CFTR function.NEW & NOTEWORTHY Cystic fibrosis transmembrane conductance regulatory protein (CFTR) genomic variants and expression of mRNA, protein, and electrogenic anion secretory activity of CFTR were characterized in dog gallbladder. Acquired inhibition of CFTR expression by gallbladder epithelium was identified as underpinning a naturally occurring muco-obstructive disease of the dog gallbladder that bears striking pathological similarity to animal models of cystic fibrosis.

Changes in cystic fibrosis transmembrane conductance regulator protein expression prior to and during elexacaftor-tezacaftor-ivacaftor therapy

Background: Defects in expression, maturation or function of the epithelial membrane glycoprotein CFTR are causative for the progressive disease cystic fibrosis. Recently, molecular therapeutics that improve CFTR maturation and functional defects have been approved. We aimed to verify whether we could detect an improvement of CFTR protein expression and maturation by triple therapy with elexacaftor-tezacaftor-ivacaftor (ELX/TEZ/IVA). Methods: Rectal suction biopsies of 21 p.Phe508del homozygous or compound heterozygous CF patients obtained pre- and during treatment with ELX/TEZ/IVA were analyzed by CFTR Western blot that was optimized to distinguish CFTR glycoisoforms. Findings: CFTR western immunoblot analysis revealed that-compared to baseline-the levels of CFTR protein increased by at least twofold in eight out of 12 patients upon treatment with ELX/TEZ/IVA compared to baseline (p < 0.02). However, polydispersity of the mutant CFTR protein was lower than that of the fully glycosylated wild type CFTR Golgi isoform, indicating an incompletely glycosylated p.Phe508el CFTR protein isoform C* in patients with CF which persists after ELX/TEZ/IVA treatment. Interpretation: Treatment with ELX/TEZ/IVA increased protein expression by facilitating the posttranslational processing of mutant CFTR but apparently did not succeed in generating the polydisperse spectrum of N-linked oligosaccharides that is characteristic for the wild type CFTR band C glycoisoform. Our results caution that the lower amounts or immature glycosylation of the C* glycoisoform observed in patients' biomaterial might not translate to fully restored function of mutant CFTR necessary for long-term provision of clinical benefit.

Early pathogenesis of cystic fibrosis gallbladder disease in a porcine model

Hepatobiliary disease causes significant morbidity in people with cystic fibrosis (CF), yet this problem remains understudied. We previously found that newborn CF pigs have microgallbladders with significant luminal obstruction in the absence of infection and consistent inflammation. In this study, we sought to better understand the early pathogenesis of CF pig gallbladder disease. We hypothesized that loss of CFTR would impair gallbladder epithelium anion/liquid secretion and increase mucin production. CFTR was expressed apically in non-CF pig gallbladder epithelium but was absent in CF. CF pig gallbladders lacked cAMP-stimulated anion transport. Using a novel gallbladder epithelial organoid model, we found that Cl- or HCO3- was sufficient for non-CF organoid swelling. This response was absent for non-CF organoids in Cl-/HCO3--free conditions and in CF. Single-cell RNA-sequencing revealed a single epithelial cell type in non-CF gallbladders that coexpressed CFTR, MUC5AC, and MUC5B. Despite CF gallbladders having increased luminal MUC5AC and MUC5B accumulation, there was no significant difference in the epithelial expression of gel-forming mucins between non-CF and CF pig gallbladders. In conclusion, these data suggest that loss of CFTR-mediated anion transport and fluid secretion contribute to microgallbladder development and luminal mucus accumulation in CF.

Diet-Induced Dysbiosis and Genetic Background Synergize With Cystic Fibrosis Transmembrane Conductance Regulator Deficiency to Promote Cholangiopathy in Mice

The most typical expression of cystic fibrosis (CF)-related liver disease is a cholangiopathy that can progress to cirrhosis. We aimed to determine the potential impact of environmental and genetic factors on the development of CF-related cholangiopathy in mice. Cystic fibrosis transmembrane conductance regulator (Cftr)^-/- mice and Cftr ^+/+ littermates in a congenic C57BL/6J background were fed a high medium-chain triglyceride (MCT) diet. Liver histopathology, fecal microbiota, intestinal inflammation and barrier function, bile acid homeostasis, and liver transcriptome were analyzed in 3-month-old males. Subsequently, MCT diet was changed for chow with polyethylene glycol (PEG) and the genetic background for a mixed C57BL/6J;129/Ola background (resulting from three backcrosses), to test their effect on phenotype. C57BL/6J Cftr ^-/- mice on an MCT diet developed cholangiopathy features that were associated with dysbiosis, primarily Escherichia coli enrichment, and low-grade intestinal inflammation. Compared with Cftr ^+/+ littermates, they displayed increased intestinal permeability and a lack of secondary bile acids together with a low expression of ileal bile acid transporters. Dietary-induced (chow with PEG) changes in gut microbiota composition largely prevented the development of cholangiopathy in Cftr ^-/- mice. Regardless of Cftr status, mice in a mixed C57BL/6J;129/Ola background developed fatty liver under an MCT diet. The Cftr ^-/- mice in the mixed background showed no cholangiopathy, which was not explained by a difference in gut microbiota or intestinal permeability, compared with congenic mice. Transcriptomic analysis of the liver revealed differential expression, notably of immune-related genes, in mice of the congenic versus mixed background. In conclusion, our findings suggest that CFTR deficiency causes abnormal intestinal permeability, which, combined with diet-induced dysbiosis and immune-related genetic susceptibility, promotes CF-related cholangiopathy.

Gallbladder and bile duct disease in Cystic Fibrosis

Cystic fibrosis (CF) is a multi-organ, clinically diverse disorder caused by mutations in the cystic fibrosis transmembrane conductance receptor (CFTR). Awareness of extra-pulmonary manifestations, including gastrointestinal and hepatobiliary disturbances, is an increasingly important part of providing high-quality care to patients with CF. Furthermore, biliary disorders, including gallbladder and bile duct disease, are common complications of CF. Therefore, a thorough understanding and efficient clinical evaluation of the gallbladder and biliary tree is an important aspect of integrated care for the patient with CF in order to prevent progression of undetected pathology. This best practice article summarizes the basis for gallbladder and bile duct pathology, describes the context and clinical presentation of biliary disease, and provides recommended approaches to delivery of high-quality care for patients with CF.

Functions of the Gallbladder

The gallbladder stores and concentrates bile between meals. Gallbladder motor function is regulated by bile acids via the membrane bile acid receptor, TGR5, and by neurohormonal signals linked to digestion, for example, cholecystokinin and FGF15/19 intestinal hormones, which trigger gallbladder emptying and refilling, respectively. The cycle of gallbladder filling and emptying controls the flow of bile into the intestine and thereby the enterohepatic circulation of bile acids. The gallbladder also largely contributes to the regulation of bile composition by unique absorptive and secretory capacities. The gallbladder epithelium secretes bicarbonate and mucins, which both provide cytoprotection against bile acids. The reversal of fluid transport from absorption to secretion occurs together with bicarbonate secretion after feeding, predominantly in response to an adenosine 3',5'-cyclic monophosphate (cAMP)-dependent pathway triggered by neurohormonal factors, such as vasoactive intestinal peptide. Mucin secretion in the gallbladder is stimulated predominantly by calcium-dependent pathways that are activated by ATP present in bile, and bile acids. The gallbladder epithelium has the capacity to absorb cholesterol and provides a cholecystohepatic shunt pathway for bile acids. Changes in gallbladder motor function not only can contribute to gallstone disease, but also subserve protective functions in multiple pathological settings through the sequestration of bile acids and changes in the bile acid composition. Cholecystectomy increases the enterohepatic recirculation rates of bile acids leading to metabolic effects and an increased risk of nonalcoholic fatty liver disease, cirrhosis, and small-intestine carcinoid, independently of cholelithiasis. Among subjects with gallstones, cholecystectomy remains a priority in those at risk of gallbladder cancer, while others could benefit from gallbladder-preserving strategies. © 2016 American Physiological Society. Compr Physiol 6:1549-1577, 2016.

Regulation of the Axillary Osmidrosis-Associated ABCC11 Protein Stability by N-Linked Glycosylation: Effect of Glucose Condition

ATP-binding cassette C11 (ABCC11) is a plasma membrane protein involved in the transport of a variety of lipophilic anions. ABCC11 wild-type is responsible for the high-secretion phenotypes in human apocrine glands, such as that of wet-type ear wax, and the risk of axillary osmidrosis. We have previously reported that mature ABCC11 is a glycoprotein containing two N-linked glycans at Asn838 and Asn844. However, little is known about the role of N-linked glycosylation in the regulation of ABCC11 protein. In the current study, we investigated the effects of N-linked glycosylation on the protein level and localization of ABCC11 using polarized Madin-Darby canine kidney II cells. When the N-linked glycosylation in ABCC11-expressing cells was chemically inhibited by tunicamycin treatment, the maturation of ABCC11 was suppressed and its protein level was significantly decreased. Immunoblotting analyses demonstrated that the protein level of the N-linked glycosylation-deficient mutant (N838Q and N844Q: Q838/844) was about half of the ABCC11 wild-type level. Further biochemical studies with the Q838/844 mutant showed that this glycosylation-deficient ABCC11 was degraded faster than wild-type probably due to the enhancement of the MG132-sensitive protein degradation pathway. Moreover, the incubation of ABCC11 wild-type-expressing cells in a low-glucose condition decreased mature, glycosylated ABCC11, compared with the high-glucose condition. On the other hand, the protein level of the Q838/844 mutant was not affected by glucose condition. These results suggest that N-linked glycosylation is important for the protein stability of ABCC11, and physiological alteration in glucose may affect the ABCC11 protein level and ABCC11-related phenotypes in humans, such as axillary osmidrosis.

Bioelectric characterization of epithelia from neonatal CFTR knockout ferrets

Cystic fibrosis (CF) is a life-shortening, recessive, multiorgan genetic disorder caused by the loss of CF transmembrane conductance regulator (CFTR) chloride channel function found in many types of epithelia. Animal models that recapitulate the human disease phenotype are critical to understanding pathophysiology in CF and developing therapies. CFTR knockout ferrets manifest many of the phenotypes observed in the human disease, including lung infections, pancreatic disease and diabetes, liver disease, malnutrition, and meconium ileus. In the present study, we have characterized abnormalities in the bioelectric properties of the trachea, stomach, intestine, and gallbladder of newborn CF ferrets. Short-circuit current (ISC) analysis of CF and wild-type (WT) tracheas revealed the following similarities and differences: (1) amiloride-sensitive sodium currents were similar between genotypes; (2) responses to 4,4'-diisothiocyano-2,2'-stilbene disulphonic acid were 3.3-fold greater in CF animals, suggesting elevated baseline chloride transport through non-CFTR channels in a subset of CF animals; and (3) a lack of 3-isobutyl-1-methylxanthine (IBMX)/forskolin-stimulated and N-(2-Naphthalenyl)-((3,5-dibromo-2,4-dihydroxyphenyl)methylene)glycine hydrazide (GlyH-101)-inhibited currents in CF animals due to the lack of CFTR. CFTR mRNA was present throughout all levels of the WT ferret and IBMX/forskolin-inducible ISC was only observed in WT animals. However, despite the lack of CFTR function in the knockout ferret, the luminal pH of the CF ferret gallbladder, stomach, and intestines was not significantly changed relative to WT. The WT stomach and gallbladder exhibited significantly enhanced IBMX/forskolin ISC responses and inhibition by GlyH-101 relative to CF samples. These findings demonstrate that multiple organs affected by disease in the CF ferret have bioelectric abnormalities consistent with the lack of cAMP-mediated chloride transport.

The CF-modifying gene EHF promotes p.Phe508del-CFTR residual function by altering protein glycosylation and trafficking in epithelial cells

The three-base-pair deletion c.1521_1523delCTT (p.Phe508del, F508del) in the cystic fibrosis transmembrane conductance regulator (CFTR) is the most frequent disease-causing lesion in cystic fibrosis (CF). The CFTR gene encodes a chloride and bicarbonate channel at the apical membrane of epithelial cells. Altered ion transport of CFTR-expressing epithelia can be used to differentiate manifestations of the so-called CF basic defect. Recently, an 11p13 region has been described as a CF modifier by the North American CF Genetic Modifier Study Consortium. Selecting the epithelial-specific transcription factor EHF (ets homologous factor) as the likely candidate gene on 11p13, we have genotyped two intragenic microsatellites in EHF to replicate the 11p13 finding in the patient cohort of the European CF Twin and Sibling Study. We could observe an association of rare EHF haplotypes among homozygotes for c.1521_1523delCTT in CFTR, which exhibit a CF-untypical manifestation of the CF basic defect such as CFTR-mediated residual chloride secretion and low response to amiloride. We have reviewed transcriptome data obtained from intestinal epithelial samples of homozygotes for c.1521_1523delCTT in CFTR, which were stratified for their EHF genetic background. Transcripts that were upregulated among homozygotes for c.1521_1523delCTT in CFTR, who carry two rare EHF alleles, were enriched for genes that alter protein glycosylation and trafficking, both mechanisms being pivotal for the effective targeting of fully functional p.Phe508del-CFTR to the apical membrane of epithelial cells. We conclude that EHF modifies the CF phenotype by altering capabilities of the epithelial cell to correctly process the folding and trafficking of mutant p.Phe508del-CFTR.

Pancreatic and biliary secretion are both altered in cystic fibrosis pigs

The pancreas, liver, and gallbladder are commonly involved in cystic fibrosis (CF), and acidic, dehydrated, and protein-rich secretions are characteristic findings. Pancreatic function studies in humans have been done by sampling the jejunal fluid. However, it has been difficult to separately study the function of pancreatic and biliary systems in humans with CF, because jejunal fluid contains a mixture of bile and pancreatic fluids. In contrast, pancreatic and biliary ducts open separately into the porcine intestine; therefore, biliary and pancreatic fluid can be individually analyzed in CF pigs. We studied newborn wild-type (WT) and CF pigs and found that CFTR was localized to the pancreatic ducts. We collected bile and pancreatic fluid and analyzed pancreatic enzymes with activity assays and immunoblot. Pancreatic enzyme expression was significantly decreased in CF compared with WT pigs. The volume and pH of pancreatic fluid were significantly lower and protein concentration was >5-fold higher in CF pigs. Secretin stimulation increased pancreatic fluid volume and pH in WT, but not CF, pigs. Baseline bile volume did not differ between WT and CF pigs, but volume did not increase in response to secretin in CF pigs. Bile pH was lower and protein concentration was twofold higher in CF pigs. These results indicate that pancreatic and biliary secretions are altered in CF pigs. Abnormal pancreatic and biliary secretion in CF may have important implications in disease pathogenesis.

The membrane-bound bile acid receptor TGR5 is localized in the epithelium of human gallbladders

TGR5 (Gpbar-1) is a plasma membrane-bound, G protein-coupled receptor for bile acids. TGR5 messenger RNA (mRNA) has been detected in many tissues, including rat cholangiocytes and mouse gallbladder. A role for TGR5 in gallstone formation has been suggested, because TGR5 knockout mice did not develop gallstones when fed a lithogenic diet. In this study, expression and localization of TGR5 was studied in human gallbladders. TGR5 mRNA and protein were detected in all 19 gallbladders. Although TGR5 mRNA was significantly elevated in the presence of gallstones, no such relation was found for TGR5 protein levels. In order to study the localization of TGR5 in human gallbladders, a novel antibody was generated. The receptor was localized in the apical membrane and the rab11-positive recycling endosome of gallbladder epithelial cells. Furthermore, the TGR5 staining colocalized with the cyclic adenosine monophosphate-regulated chloride channel cystic fibrosis transmembrane conductance regulator (CFTR) and the apical sodium-dependent bile salt uptake transporter, suggesting a functional coupling of TGR5 to bile acid uptake and chloride secretion. Stimulation with bile acids significantly increased cyclic adenosine monophosphate concentration in human gallbladder tissue. Incubation of gallbladder epithelial cells with a TGR5 agonist led to a rise of N-(ethoxycarbonylmethyl)-6-methoxyquinolinium bromide (MQAE)-fluorescence, suggestive of a decrease in intracellular chloride concentration. The TGR5 agonist-dependent increase in MQAE-fluorescence was absent in TGR5 knockout mice or in the presence of a CFTR inhibitor, indicating that TGR5 mediates chloride secretion via activation of CFTR. The presence of the receptor in both the plasma membrane and the recycling endosome indicate that TGR5 can be regulated by translocation.

CONCLUSION

The data suggest a role for TGR5 in bile acid-induced fluid secretion in biliary epithelial cells.

Airway ion transport impacts on disease presentation and severity in cystic fibrosis

OBJECTIVES

Abnormal airway ion transport is a feature of cystic fibrosis. The aim of this study was to investigate whether distinct components of ion transport are associated with the clinical expression and severity of the disease.

DESIGN AND METHODS

Univariate and multivariate analyses were used to study interaction effects between nasal potential difference parameters and clinical status, recorded at stable conditions, in 75 F508del homozygous young adults.

RESULTS

All patients demonstrated increased sodium and reduced chloride conductances. Less sodium transport abnormalities were related to better respiratory function and nutrition. Presentation with digestive symptoms at diagnosis was associated with lower chloride conductance. With an accuracy of 85% good nutritional status was linked to more preserved lung function, increasing age and more preserved chloride conductance.

CONCLUSIONS

Ion transport abnormalities have distinct clinical outcomes. Sodium conductance relates to respiratory function and nutrition; chloride conductance to nutrition and presentation with digestive symptoms at diagnosis.

CSN5 binds to misfolded CFTR and promotes its degradation

Cystic fibrosis is mainly caused by mutations that interfere with the biosynthetic folding of the cystic fibrosis transmembrane conductance regulator (CFTR) protein. The aim of this study was to find cellular proteins interacting with CFTR and regulating its processing. We have used a genetic screen in yeast to identify such proteins and identified CSN5 that interacted with the third cytoplasmic loop of CFTR. CSN5 is the 5th component of the COP9 signalosome, a complex of eight subunits that shares significant homologies to the lid subcomplex of the 26S proteasome and controls the stability of many proteins. The present study shows that CSN5 associates with the core-glycosylated form of CFTR and suggests that this association targets misfolded CFTR to the degradative pathway. Identifying CSN5 as a new component of the degradative pathway is an important step towards the goal of unraveling the sorting between misfolded and correctly folded CFTR proteins.

Impact of nutrition on phenotype in CFTR-deficient mice

To elucidate the impact of nutrition in cystic fibrosis (CF), we compared the phenotypic traits of Cftr -/- mice fed either a lipid-enriched liquid diet (Peptamen) or a standard chow combined with polyethylenglycol osmotic laxative (PEG), two strategies commonly used to prevent intestinal obstruction in CF mice. Survival, growth, liver, and ventilatory status were determined in Cftr -/- and Cftr +/+ mice, followed-up until 120 d. Ventilation was recorded in conscious animals using whole-body plethysmography. We found that the survival rate was similar in Peptamen and PEG Cftr -/- mice. Cftr -/- mice had lower minute ventilation than Cftr +/+ mice, whatever the diet. Both Cftr -/- and Cftr +/+ mice fed Peptamen displayed preadult growth delay compared with PEG-treated animals. Despite subsequent growth catch-up, Cftr -/- mice remained smaller than Cftr +/+ mice, whatever the diet. All Peptamen fed Cftr -/- mice showed hepatomegaly and liver steatosis, which also occurred but to a lesser extent in Peptamen fed Cftr +/+ animals. Therefore, while both treatment strategies are similarly efficient to avoid high mortality at weaning, Peptamen induces preadult growth delay and liver steatosis. These effects of diet are important to consider in future animal studies and also prompt to evaluate high-energy diets in CF patients.

Rab27a negatively regulates CFTR chloride channel function in colonic epithelia: Involvement of the effector proteins in the regulatory mechanism

Cystic fibrosis, an autosomal recessive disorder, is caused by the disruption of biosynthesis or function of CFTR. CFTR regulatory mechanisms include channel transport to plasma membrane and protein-protein interactions. Rab proteins are small GTPases involved in vesicle transport, docking, and fusion. The colorectal epithelial HT-29 cells natively express CFTR and respond to cAMP with an increase in CFTR-mediated currents. DPC-inhibited currents could be completely eliminated with CFTR-specific SiRNA. Over-expression of Rab27a inhibited, while isoform specific SiRNA and Rab27a antibody stimulated CFTR-mediated currents in HT-29 cells. CFTR activity is inhibited both by Rab27a (Q78L) (constitutive active GTP-bound form of Rab27a) and Rab27a (T23N) (constitutive negative form that mimics the GDP-bound form). Rab27a mediated effects could be reversed by Rab27a-binding proteins, the synaptotagmin-like protein (SLP-5) and Munc13-4 accessory protein (a putative priming factor for exocytosis). The SLP reversal of Rab27a effect was restricted to C2A/C2B domains while the SHD motif imparted little more inhibition. The CFTR-mediated currents remain unaffected by Rab3 though SLP-5 appears to weakly bind it. The immunoprecipitation experiments suggest protein-protein interactions between Rab27a and CFTR. Rab27a appears to impair CFTR appearance at the cell surface by trapping CFTR in the intracellular compartments. Munc13-4 and SLP-5, on the other hand, limit Rab27a availability to CFTR, thus minimizing its effect on channel function. These observations decisively prove that Rab27a is involved in CFTR channel regulation through protein-protein interactions involving Munc13-4 and SLP-5 effector proteins, and thus could be a potential target for cystic fibrosis therapy.

Altered hepatobiliary gene expressions in PFIC1: ATP8B1 gene defect is associated with CFTR downregulation

Recent reports in patients with PFIC1 have indicated that a gene defect in ATP8B1 could cause deregulations in bile salt transporters through decreased expression and/or activity of FXR. This study aimed to: (1) define ATP8B1 expression in human hepatobiliary cell types, and (2) determine whether ATP8B1 defect affects gene expressions related to bile secretion in these cells. ATP8B1 expression was detected by RT-PCR in hepatocytes and cholangiocytes isolated from normal human liver and gallbladder. ATP8B1 mRNA levels were 20- and 200-fold higher in bile duct and gallbladder epithelial cells, respectively, than in hepatocytes. RT-PCR analyses of the liver from two patients with PFIC1, one with PFIC2, one with biliary atresia, showed that, compared to normal liver, hepatic expressions of FXR, SHP, CYP7A1, ASBT were decreased at least by 90% in all cholestatic disorders. In contrast, NTCP transcripts were less decreased (by < or = 30% vs. 97%) in PFIC1 as compared with other cholestatic disorders, while BSEP transcripts, in agreement with BSEP immunohistochemical signals, were normal or less decreased (by 50% vs. 97%). CFTR hepatic expression was decreased (by 80%), exclusively in PFIC1, while bile duct mass was not reduced, as ascertained by cytokeratin-19 immunolabeling. In Mz-ChA-2 human biliary epithelial cells, a significant decrease in CFTR expression was associated with ATP8B1 invalidation by siRNA. In conclusion, cholangiocytes are a major site ofATP8B1 hepatobiliary expression. A defect of ATP8B1 along with CFTR downregulation can impair the contribution of these cells to bile secretion, and potentially explain the extrahepatic cystic fibrosis-like manifestations that occur in PFIC1.

Rab4GTPase modulates CFTR function by impairing channel expression at plasma membrane

Cystic fibrosis (CF), an autosomal recessive disorder, is caused by the disruption of biosynthesis or the function of a membrane cAMP-activated chloride channel, CFTR. CFTR regulatory mechanisms include recruitment of channel proteins to the cell surface from intracellular pools and by protein-protein interactions. Rab proteins are small GTPases involved in regulated trafficking controlling vesicle docking and fusion. Rab4 controls recycling events from endosome to the plasma membrane, fusion, and degradation. The colorectal cell line HT-29 natively expresses CFTR and responds to cAMP stimulation with an increase in CFTR-mediated currents. Rab4 over-expression in HT-29 cells inhibits both basal and cAMP-stimulated CFTR-mediated currents. GTPase-deficient Rab4Q67L and GDP locked Rab4S22N both inhibit channel activity, which appears characteristically different. Active status of Rab4 was confirmed by GTP overlay assay, while its expression was verified by Western blotting. The pull-down and immunoprecipitation experiments suggest that Rab4 physically interacts with CFTR through protein-protein interaction. Biotinylation with cell impermeant NHS-Sulfo-SS-Biotin implies that Rab4 impairs CFTR expression at cell surface. The enhanced cytosolic CFTR indicates that Rab4 expression restrains CFTR appearance at the cell membrane. The study suggests that Rab4 regulates the channel through multiple mechanisms that include protein-protein interaction, GTP/GDP exchange, and channel protein trafficking. We propose that Rab4 is a dynamic molecule with a significant role in CFTR function.

The gallbladder and biliary tract in cystic fibrosis

Chronic liver disease is a major complication of cystic fibrosis. Its incidence and severity are variable, and diagnosis relies on a combination of clinical evaluation, biochemical testing, and radiologic assessment. Identifying patients who have early disease is critical, and the administration of ursodeoxycholic acid appears to be beneficial. The pathogenesis is incompletely understood, and factors that contribute to the variability in incidence and severity are unknown. Fortunately, only a small proportion of individuals progress to advanced liver disease; however, in this population, there is significant morbidity and impairment in quality of life. Liver transplantation can be performed successfully in patients with end-stage liver disease. Future treatments involve targeted gene therapy and activation of mutant forms of the cystic fibrosis transmembrane conductance regulator.

Misprocessing of theCFTRprotein leads to mild cystic fibrosis phenotype

Cystic fibrosis (CF) is mainly caused by mutations that interfere with the biosynthetic folding of the cystic fibrosis transmembrane conductance regulator (CFTR) chloride channel. The aim of this study was to determine the mechanism of dysfunction of a disease-causing mutation associated with variable phenotypes. In order to attain these objectives, we studied the effect of the p.L206W mutation on CFTR protein production and function, and we examined the genotype-phenotype correlation of [p.L206W]+[p.F508del] patients. We showed that p.L206W is a processing (class II) mutation since the CFTR biosynthetic pathway was severely impaired, whereas single-channel measurements indicated ion conductance similar to the wild-type protein. These data raise the larger question of the phenotypic variability of class II mutants, including p.F508del. Since multiple potential partners could modify the processing of the CFTR protein during its course to the cell surface, environmental and other genetic factors might contribute to this variability.

La fonction biliaire et sa régulation

Biliary function is essential for intestinal absorption of fat, homeostasis of cholesterol and elimination of diverse metabolic end-products. Bile is elaborated in hepatocyte canaliculi and modified by cholangiocytes through both secretion and absorption processes. The main determinant of bile formation is an osmotic filtration process resulting from active transport of bile acids and other osmotic solutes. Most of the membrane transporters ensuring bile formation have now been identified. The expression of these membrane transporters is regulated in particular through transcriptional mechanisms under the control of nuclear receptors activated by ligands, such as bile acids, which act as endogenous steroids synthesized from cholesterol in hepatocytes. Monogenic cholestatic diseases illustrate the key role of membrane transporters in biliary function. Bile acids are potent modulators of transporters and thus trigger an adaptative response to cholestasis. The extent of this adaptative response could explain the compelling phenotypic variability of cholestatic diseases in childhood and adults. The firstline medical treatment is currently ursodeoxycholic acid. In case of failure of this medical treatment, liver transplantation is required. Recent progress in the molecular pathogenesis of bile formation and cholestatic liver diseases is expected to provide the design for drugs targeted to the molecular abnormalities responsible of cholestatic diseases.

Kidneys sans glomeruli

The evolution of the vertebrate kidney records three occasions, each separated by about 50 million years, when fish have abandoned glomeruli to produce urine by tubular mechanisms. The recurring dismissal of glomeruli suggests a mechanism of aglomerular urine formation intrinsic to renal tubules. Indeed, the transepithelial secretion of organic solutes and of inorganic solutes such as sulfate, phosphate, and magnesium can all drive secretory water flow in renal proximal tubules of fish. However, the secretion of NaCl via secondary active transport of Cl is the primary mover of secretory water flow in, surprisingly, proximal tubules of both glomerular and aglomerular fish. In filtering kidneys, the tubular secretion of solute and water is overshadowed by reabsorptive transport activities, but secretion progressively comes to light as glomerular filtration decreases. Thus the difference between glomerular and aglomerular urine formation is more a difference of degree than of kind. At low rates of glomerular filtration in seawater fish, NaCl-coupled water secretion serves to increase the renal excretory capacity by increasing the luminal volume into which waste, excess, and toxic solutes can be secreted. The reabsorption of NaCl and water in the distal nephron and urinary bladder concentrates unwanted solutes for excretion while minimizing renal water loss. In aglomerular fish, NaCl-coupled water secretion across proximal tubules replaces glomerular filtration to increase renal excretory capacity. A review of the literature suggests that tubular secretion of NaCl and water is an early function of the vertebrate proximal tubule that has been retained throughout evolution. Active transepithelial Cl secretion takes place in gall bladders studied as models of the mammalian proximal tubule and in proximal tubules of amphibians and apparently also of mammals. The tubular secretion of Cl is also observed in mammalian distal tubules. The evidence consistent with and for Cl secretion in, respectively, proximal and distal tubules of the mammalian kidney calls for a reexamination of basic assumptions in renal physiology that may lead to new opportunities for managing some forms of renal disease.

Guanylin regulates chloride secretion in the human gallbladder via the bile fluid

BACKGROUND & AIMS

The biliary epithelium of bile ducts and gallbladder modifies the composition of primary hepatic bile by absorption and secretion of an electrolyte-rich fluid. The underlying transport mechanisms, however, are still incompletely understood. We investigated the expression, the cellular localization, and the functional role of guanylin, a bioactive intestinal peptide involved in the cystic fibrosis transmembrane conductance regulator (CFTR)-regulated electrolyte/water secretion, in the human gallbladder.

METHODS

Peptide-specific antibodies were raised to localize guanylin and its affiliated signaling proteins, i.e., the guanylin receptor, guanylate cyclase C (GC-C), cGMP-dependent protein kinase type II (cGKII), and CFTR in the human gallbladder and cholangiocarcinoma cells (Mz-Cha-1) by RT-PCR, Western blot, and immunocytochemistry. A sensitive ELISA was used to assess the range of guanylin concentration in human bile fluid. The functional role of guanylin was investigated in subconfluent Mz-Cha-1 cell monolayers by isotope efflux experiments.

RESULTS

Guanylin and its affiliated signaling proteins are highly expressed in the human gallbladder. Guanylin is localized to secretory epithelial cells of the gallbladder and is present in the bile, whereas GC-C, cGKII, and CFTR are confined exclusively to the apical membrane of the same epithelial cells. Functional studies in Mz-Cha-1 cells identify guanylin as a specific regulator of biliary Cl(-) secretion that very likely is mediated by an intracellular increase of cGMP-concentration.

CONCLUSIONS

Based on the present findings and on the functional role of guanylin in other epithelia, it is likely that gallbladder epithelial cells synthesize and release guanylin into the bile to regulate electrolyte secretion by a paracrine/luminocrine signaling pathway.

Cystic fibrosis transmembrane conductance regulator (CFTR)-mediated residual chloride secretion does not protect against early chronic Pseudomonas aeruginosa infection in F508del homozygous cystic fibrosis patients

Cystic fibrosis (CF) disease severity is characterized by a broad variability that has been attributed, in addition to the CF transmembrane conductance regulator (CFTR) genotype, to modulating factors such as CFTR-mediated residual chloride (Cl-) secretion. Moreover, CFTR has been suggested to function as a receptor for Pseudomonas aeruginosa (PA). In this study, we investigated whether or not the presence of residual Cl- secretion protects against early chronic PA colonization of patients' airways. Excluding influences on the phenotype caused by different CFTR mutations, we evaluated a cohort of F508del homozygous individuals with respect to the correlation between residual Cl- secretion and the age of onset of PA colonization as an important marker of clinical phenotype. A group with early chronic PA colonization before the age of 7 y (n = 14) was compared with a cohort that had no initial PA detection at least until the age of 13 y (n = 10). We determined the Cl- transport properties by using the intestinal current measurement in rectal suction biopsies. Residual Cl- secretion, most likely due to the CFTR Cl- channel, was observed in 63% of subjects, more frequently in early chronically PA colonized than among late or not colonized patients. These results demonstrate the presence of some active F508del-CFTR in the apical cell membrane and imply that factors other than the CFTR-mediated residual Cl- secretion determine the age of onset of PA colonization.

Liver transplantation for cholestasis associated with cystic fibrosis in the pediatric population

The most common hepatic complications of cystic fibrosis (CF) are steatosis, fibrosis, biliary cirrhosis, atretic gallbladder, cholelithiasis, and sclerosing cholangitis. Cholestatic liver disease is a slow progressive disorder, but will stabilize for many patients. CF patients may suffer from the consequences of their liver disease and without liver transplantation, variceal hemorrhage, malnutrition, or end-stage liver disease can lead to death. Prospective data were collected and reviewed on 311 liver transplants performed in 283 patients at the Children's Medical Center of Dallas between October 1984 and November 2000. Ten children received an orthotopic liver transplant (OTLX) for end-stage liver disease associated with cystic fibrosis. Pulmonary function tests were obtained preoperatively in all cases. There were nine boys and one girl. Six are currently alive, and four are dead. Both patient and graft survival was 5.75 yr. Among those currently alive, mean patient and graft survival is 7.71 yr (range 0.10-12.62 yr). Mean patient and graft survival of those who died was 2.35 yr (range 0.78-5.33 yr). No survivor required re-transplantation and currently, all have normal serum aminotransferase values. Chronic sinusitis was not a significant pre- or post-transplant morbidity, although systematic radiographic evaluation of the sinuses did not occur. Pulmonary deaths occurred in three patients from pulmonary hemorrhage, pulmonary infection with Aspergillus and Candida glabrata, and acute bronchopneumonia associated with polymicrobial sepsis because of Pseudomonas, Klebsiella, and Candida albicans 1.44, 0.78, and 1.83 yr, respectively, after transplantation. The fourth death was associated with chronic rejection, and occurred 5.33 yr after transplantation. All non-survivors were below the 5th percentile for height and weight at the time of liver transplantation. Mean age at transplantation was 9.72 yr (range 1.23-19.09, median 9.61). Survivors were transplanted at a younger age than non-survivors (mean of 9.21 yr vs. 10.66 yr), and had shorter waiting times from diagnosis of end-stage liver disease to transplantation (6.87 months vs. 13.83 months). Eighty percentage (n = 8) of patients had pretransplant variceal bleeds (83% of survivors, 75% of non-survivors). While all non-survivors had a history of meconium ileus and preoperative need of pancreatic enzymes, only 67% of those alive experienced these complications. Preoperative forced vital capacity FVC was 103% for survivors and 95% for non-survivors. The corresponding numbers for forced expiratory flow (FEF) 25-75 were 74-84% respectively. Preoperative Aspergillus was identified in 30% of patients (n = 3). Two of these patients are alive. Cystic fibrosis constitutes an indication for 3.5% of pediatric liver transplants. Evaluation and transplantation for end-stage liver disease associated with cystic fibrosis should be undertaken at an early age. Most deaths were associated with pulmonary/septic events, and occurred less than 2 yr after OLTX. Those children who did not survive had poor growth and nutrition, prolonged waiting times prior to transplantation, were transplanted at an older age, and had a higher incidence of pancreatic insufficiency and meconium ileus. The presence of Aspergillus in the sputum does not constitute a contraindication for OLTX.

Normal function of the cystic fibrosis conductance regulator protein can be associated with homozygous (Delta)F508 mutation

Cystic fibrosis (CF) is caused by mutations of the gene encoding for the CFTR (CF transmembrane conductance regulator) protein. The most frequent mutation, the (Delta)F508 mutation, results in a defective cAMP-regulated chloride transport in the epithelial cells. The spectrum of clinical manifestations in patients bearing homozygous (Delta)F508 mutations can vary considerably, suggesting that, in the patients with a mild disease, CFTR could be partly functional. To test this hypothesis, we explored in nasal ciliated epithelial cells (NCC) of 9 control subjects and 23 (Delta)F508 homozygous patients the anion conductive pathway by a halide sensitive fluorescent dye assay SPQ (6-methoxy-N-3'-sulfopropylquinolinium) and the CFTR transcript levels by RT-PCR. As 50% represented the lowest fraction of the control subjects NCC demonstrating a cAMP-dependent conductance, a CF patient was considered as "cAMP responder" if at least 50% of the NCC tested displayed a cAMP-dependent conductive pathway. According to these criteria, 8 of the 23 patients were considered as cAMP responders. They had a significantly less severe disease considering the respiratory function and infectious status. The amount of CFTR mRNA did not differ between the control subjects and the patients. No statistical correlation could be found between the transcript level and the expression of a cAMP conductive pathway. This cAMP-dependent Cl(-) conductance detected in homozygous NCC could be due to a residual CFTR activity and may explain the mild phenotypes observed in some (Delta)F508 homozygous patients.

Liver disease in pediatric patients with cystic fibrosis is associated with glutathione S-transferase P1 polymorphism

Liver disease in patients with cystic fibrosis (CF) is inconstant and has not yet been clearly related to any specific risk factor. While the expression of cystic fibrosis transmembrane conductance regulator (CFTR) is restricted to the biliary epithelium in the liver, recent findings indicate that CFTR modulates reduced glutathione (GSH) transport and that CFTR dysfunction creates an imbalance in the antioxidant defense. Among liver detoxifying enzymes, the glutathione S-transferases (GSTs) play a key role in the protection against oxidative stress. Because oxidative injury contributes to the development of liver disease, we hypothesized that 2 members of the GST superfamily, GSTM1 and GSTP1, which are expressed in the biliary epithelium, could influence the hepatic status in patients with CF. The potential impact of GSTM1 and GSTP1 gene polymorphisms was assessed in 106 children with CF (mean age, 11.5 years). Based on polymerase chain reaction/restriction fragment length polymorphism analysis, we found that the frequency of GSTP1-Ile(105)/Ile(105) genotype was significantly higher in patients with CF with liver disease than in those without (P <.03). Among the youngest patients, aged 6 years, GSTP1-Ile(105)/Ile(105) genotype was associated with a 8-fold increase in the risk of liver disease compared with other GSTP1 genotypes (P =.002). No association between the GSTM1 genotype and liver status was documented. In conclusion, GSTP1-Ile(105)-encoding allele contributes to hepatic dysfunction in CF. Identification of this polymorphism may have prognostic value and prompt early treatment in patients with CF with an increased risk of liver disease.

beta2 adrenoceptor gene polymorphisms in cystic fibrosis lung disease

The cystic fibrosis membrane conductance regulator can be activated through beta2-adrenoceptor (beta2AR) stimulation. We tested the hypothesis that coding sequence polymorphisms in the beta2AR gene contribute to the disease state in patients with cystic fibrosis. The Arg16Gly, Gln27Glu, and Thr164Ile beta2AR polymorphisms were studied by specific polymerase chain reaction and restriction fragment length polymorphism analysis in 126 cystic fibrosis patients. Forced expiratory volume in 1 s was significantly (P < 0.05) reduced in cystic fibrosis patients carrying the Gly16 allele in either homozygous or heterozygous form (Gly16Gly + Arg16Gly) compared to patients homozygous for the Arg16 allele (60.3 +/- 3.5% versus 75.7 +/- 4.9% predicted). Similarly, forced vital capacity and flows at lower lung volumes were significantly (P < 0.05 and P < 0.01) lower in cystic fibrosis patients carrying the Gly16 allele. In addition, the Gly16 allele was associated with a greater 5 year decline in pulmonary function (P < 0.01). Bronchodilator responses to albuterol were not significantly different between the groups. The Thr164Ile variant was found in four patients; these patients had markedly reduced pulmonary function. Isoproterenol-stimulated cyclic AMP formation was significantly blunted in cystic fibrosis patients carrying either the Gly16 allele or Thr164Ile genotype compared to cystic fibrosis patients homozygous for the respective Arg16 alleles. These data provide the first evidence suggesting that polymorphisms of the beta2AR gene contribute to clinical severity and disease progression in cystic fibrosis.

Chloride conductance and genetic background modulate the cystic fibrosis phenotype of Delta F508 homozygous twins and siblings

To investigate the impact of chloride (Cl(-)) permeability, mediated by residual activity of the cystic fibrosis transmembrane conductance regulator (CFTR) or by other Cl(-) channels, on the manifestations of cystic fibrosis (CF), we determined Cl(-) transport properties of the respiratory and intestinal tracts in Delta F508 homozygous twins and siblings. In the majority of patients, cAMP and/or Ca(2+)-regulated Cl(-) conductance was detected in the airways and intestine. Our finding of cAMP-mediated Cl(-) conductance suggests that, in vivo, at least some Delta F508 CFTR can reach the plasma membrane and affect Cl(-) permeability. In respiratory tissue, the expression of basal CFTR-mediated Cl(-) conductance, demonstrated by 30% of Delta F508 homozygotes, was identified as a positive predictor of milder CF disease. In intestinal tissue, 4,4'-diisothiocyanatostilbene-2,2'-disulfonic acid-insensitive (DIDS-insensitive) Cl(-) secretion, which is indicative of functional CFTR channels, correlated with a milder phenotype, whereas DIDS-sensitive Cl(-) secretion was observed mainly in more severely affected patients. The more concordant Cl(-) secretory patterns within monozygous twins compared with dizygous pairs imply that genes other than CFTR significantly influence the manifestation of the basic defect.

Conformational and temperature-sensitive stability defects of the delta F508 cystic fibrosis transmembrane conductance regulator in post-endoplasmic reticulum compartments

Deletion of phenylalanine at position 508 (DeltaF508) is the most common cystic fibrosis (CF)-associated mutation in the CF transmembrane conductance regulator (CFTR), a cAMP-regulated chloride channel. The consensus notion is that DeltaF508 imposes a temperature-sensitive folding defect and targets newly synthesized CFTR for degradation at endoplasmic reticulum (ER). A limited amount of CFTR activity, however, appears at the cell surface in the epithelia of homozygous DeltaF508 CFTR mice and patients, suggesting that the ER retention is not absolute in native tissues. To further elucidate the reasons behind the inability of DeltaF508 CFTR to accumulate at the plasma membrane, its stability was determined subsequent to escape from the ER, induced by reduced temperature and glycerol. Biochemical and functional measurements show that rescued DeltaF508 CFTR has a temperature-sensitive stability defect in post-ER compartments, including the cell surface. The more than 4-20-fold accelerated degradation rate between 37 and 40 degrees C is, most likely, due to decreased conformational stability of the rescued DeltaF508 CFTR, demonstrated by in situ protease susceptibility and SDS-resistant thermoaggregation assays. We propose that the decreased stability of the spontaneously or pharmacologically rescued mutant may contribute to its inability to accumulate at the cell surface. Thus, therapeutic efforts to correct the folding defect should be combined with stabilization of the native DeltaF508 CFTR.

Ca2+-activated Cl- channels can substitute for CFTR in stimulation of pancreatic duct bicarbonate secretion

This study addresses the mechanisms by which a defect in CFTR impairs pancreatic duct bicarbonate secretion in cystic fibrosis. We used control (PANC-1) and CFTR-deficient (CFPAC-1; DeltaF508 mutation) cell lines and measured HCO3- extrusion by the rate of recovery of intracellular pH after an alkaline load and recorded whole cell membrane currents using patch clamp techniques. 1) In PANC-1 cells, cAMP causes parallel activation of Cl- channels and of HCO3- extrusion by DIDS-sensitive and Na+-independent Cl-/HCO3- exchange, both effects being inhibited by Cl- channel blockers NPPB and glibenclamide. 2) In CFPAC-1 cells, cAMP fails to stimulate Cl-/HCO3- exchange and Cl- channels, except after promoting surface expression of DeltaF508-CFTR by glycerol treatment. Instead, raising intracellular Ca2+ concentration to 1 micromol/l or stimulating purinergic receptors with ATP (10 and 100 micromol/l) leads to parallel activation of Cl- channels and HCO3- extrusion. 3) K+ channel function is required for coupling cAMP- and Ca2+-dependent Cl- channel activation to effective stimulation of Cl-/HCO3- exchange in control and CF cells, respectively. It is concluded that stimulation of pancreatic duct bicarbonate secretion via Cl-/HCO3- exchange is directly correlated to activation of apical membrane Cl- channels. Reduced bicarbonate secretion in cystic fibrosis results from defective cAMP-activated Cl- channels. This defect is partially compensated for by an increased sensitivity of CF cells to purinergic stimulation and by alternative activation of Ca2+-dependent Cl- channels, mechanisms of interest with respect to possible treatment of cystic fibrosis and of related chronic pancreatic diseases.

An anion channel in guinea pig gallbladder epithelial cells is highly permeable to HCO(-)(3)

In guinea pig gallbladder epithelium, a secretion of fluid, secondary to an electrogenic secretion of Cl(-) and HCO(-)(3), is elicited in the presence of a high intracellular concentration of adenosine 3'-5'-cyclic monophosphate (cAMP). The aim of this study was to analyze the effects of secretagogues on the activity of anionic channels in isolated epithelial cells using the patch-clamp technique and measuring the electrical potential difference of the cellular membrane (pd(cm)). In cell-attached configuration, with the microelectrode filled with a solution of N-methylglucamine-Cl, or in inside-out configuration (symmetrical solution), it was possible to demonstrate the presence of an 18-pS Cl(-) channel with linear current/voltage (I/V) relationship and voltage independence; this channel is not activated by cAMP (cell-attached configuration). In inside-out configuration (symmetrical solution), another anionic channel with a conductance of 2.8 pS, voltage independence, and a linear I/V relationship was also identified. This channel was stimulated by cAMP (cell-attached configuration) and by PKA + ATP + cAMP (inside-out configuration). The channel was inhibited by NPPB (10(-5) M), but not by other anionic inhibitors. Measurements of the pd(cm) value suggested that in isolated cells, as in whole tissue, cAMP activates conductance for both Cl(-) and HCO(-)(3). The selectivity of the channel was gluconate < SO(2-)(4) < Cl(-) < Br(-) < I(-) < HCO(-)(3) < SCN(-) and the P(HCO(3))/P(Cl) was 2.6. Some features of the channel resemble those of the cystic fibrosis transmembrane conductance regulator (CFTR) chloride channel and RT-PCR performed on mRNA from isolated epithelial cells detected the presence of a CFTR homologue mRNA. The results obtained indicate that this channel is responsible for the HCO(-)(3) conductance activated by cAMP.

Expression of cystic fibrosis transmembrane conductance regulator in liver tissue from patients with cystic fibrosis

The authors examined the expression of cystic fibrosis transmembrane conductance regulator (CFTR) and its relationship to histopathological changes in cystic fibrosis (CF) liver tissue. Immunohistochemistry was used to examine expression of CFTR, intercellular adhesion molecule-1 (ICAM-1) and liver cell-type markers in liver cryosections in 11 patients with CF-associated liver disease, and non-CF controls with (n = 17) and without (n = 3) liver disease. In CF patients prominent inflammatory infiltrates were not found, yet hepatic stellate cells were identified within fibrotic areas around bile ducts. Proliferating bile ducts displayed ICAM-1 immunoreactivity in 3 cases, but bile ducts were otherwise negative. In 2 patients homozygous for R764X and for 1112delT no CFTR immunoreactivity was detected. Bile-duct epithelial cells in patients carrying the DeltaF508 mutation displayed aberrant cytoplasmic immunolocalization of CFTR, as determined with confocal laser scanning microscopy, in contrast to the distinct CFTR expression at the luminal surface seen in controls. No clear relationship between CFTR expression and fibrosis or inflammation was evidenced in CF patients. In conclusion, these findings are consistent with an impairment of DeltaF508 CFTR processing in intrahepatic biliary epithelium. ICAM-1 expression on bile-duct epithelial cells and inflammatory infiltrates were rare findings in CF liver tissue, indicating that immunological mechanisms are unlikely to be involved in initiation of CF-associated liver disease.

Residual chloride secretion in intestinal tissue of deltaF508 homozygous twins and siblings with cystic fibrosis. The European CF Twin and Sibling Study Consortium

BACKGROUND & AIMS

Cholinergic stimulation of chloride secretion is impaired in the intestines of patients with cystic fibrosis (CF). However, intestinal chloride secretion has been observed in patients with mild CF mutations. The aim of this study was to investigate residual Cl(-) secretion in the intestine of DeltaF508 homozygous CF patients, and examine the contribution of cystic fibrosis transmembrane conductance regulator (CFTR) and alternative Cl(-) conductances. Twins and siblings with identical CFTR genotypes were investigated to determine the impact of factors other than CFTR on chloride secretion.

METHODS

Chloride secretion in rectal tissue was investigated by applying Ca(2+) and adenosine 3',5'-cyclic monophosphate (cAMP)-linked agonists before and after the inhibition of alternative Cl(-) conductances with 4,4'-diisothiocyanostilbene-2, 2'-disulfonic acid (DIDS).

RESULTS

cAMP-mediated Cl(-) secretion was observed in 73% of patients, and 20% showed DIDS-sensitive Ca(2+)-activated Cl(-) secretion. This DIDS-sensitive alternative chloride conductance was seen only in CF patients who also responded to cAMP agonists. Chloride secretion was more concordant within monozygous twins than within dizygous pairs.

CONCLUSIONS

These results suggest the presence of CFTR-mediated Cl(-) secretion in a subgroup of patients, implying that a portion of deltaF508 CFTR can be processed in vivo and function as a chloride channel in the apical membrane of intestinal cells. Moreover, a considerable number of deltaF508 homozygous patients express chloride conductances other than CFTR in their intestinal epithelia.