The role of regulated CFTR trafficking in epithelial secretion

Localization and function of humanized F508del-CFTR in mouse intestine following activation of serum glucocorticoid kinase 1 and Trikafta

The CFTR modulator Trikafta has markedly improved lung disease for Cystic Fibrosis (CF) patients carrying the common delta F508 (F508del-CFTR) CFTR mutation. F508del-CFTR results in an apical trafficking defect and loss of function in CFTR-expressing epithelial cells. However, Trikafta has not resulted in improved gastrointestinal function in CF patients. A humanized mouse model of F508del-CFTR was recently generated to evaluate CFTR modulators and other compounds to treat human F508del-CFTR CF intestinal disease. Short-term (4 h) treatment of rats with Dexamethasone (Dex) potently activates serum glucocorticoid kinase 1 (SGK1) and increases CFTR apical traffic and ion transport in the native intestine. This study examined CFTR localization and ion transport in intestinal segments from humanized F508del-CFTR mice following treatment with Dex in the presence/absence of Trikafta. Dex treatment improved apical CFTR localization and function but was inconsistent along intestinal segments. Combined treatment with Dex and Trikafta was superior to Dex alone but inconsistently improved CFTR localization and function. These data suggest further optimization of humanized CF mouse models will be necessary to test the efficacy of compounds to treat human CF intestinal disease.

Easy-to-Build and Reusable Microfluidic Device for the Dynamic Culture of Human Bronchial Cystic Fibrosis Epithelia

Cystic fibrosis (CF) is one of the most frequent genetic diseases, caused by dysfunction of the CF transmembrane conductance regulator (CFTR) chloride channel. CF particularly affects the epithelium of the respiratory system. Therapies aim at rescuing CFTR defects in the epithelium, but CF genetic heterogeneity hinders the finding of a single and generally effective treatment. Therefore, in vitro models have been developed to study CF and guide patient therapy. Here, we show a CF model on-chip by coupling the feasibility of the human bronchial epithelium differentiated in vitro at the air-liquid interface and the innovation of microfluidics. We demonstrate that the dynamic flow enhanced cilia distribution and increased mucus quantity, thus promoting tissue differentiation in a short time. The microfluidic devices highlighted differences between CF and non-CF epithelia, as shown by electrophysiological measures, mucus quantity, viscosity, and the analysis of ciliary beat frequency. The described model on-chip may be a handy instrument for studying CF and setting up therapies. As a proof of principle, we administrated the corrector VX-809 on-chip and observed a decrease in mucus thickness and viscosity.

A conserved WXXE motif is an apical delivery determinant of ABC transporter C subfamily isoforms

ATP-binding cassette transporter isoform C7 (ABCC7), also designated as cystic fibrosis transmembrane conductance regulator (CFTR), is exclusively targeted to the apical plasma membrane of polarized epithelial cells. Although the apical localization of ABCC7 in epithelia is crucial for the Cl- excretion into lumens, the mechanism regulating its apical localization is poorly understood. In the present study, an apical localization determinant was identified in the N-terminal 80-amino acid long cytoplasmic region of ABCC7 (NT80). In HepG2 cells, overexpression of NT80 significantly disturbed the apical expression of ABCC7 in a competitive manner, suggesting the presence of a sorting determinant in this region. Deletion analysis identified a potential sorting information within a 20-amino acid long peptide (aa 41-60) of NT80. Alanine scanning mutagenesis of this region in full-length ABCC7 further narrowed down the apical localization determinant to four amino acids, W57DRE60. This WDRE sequence was conserved among vertebrate ABCC7 orthologs. Site-directed mutagenesis showed that W57 and E60 were critical for the apical expression of ABCC7, confirming a novel apical sorting determinant of ABCC7. Furthermore, a WXXE motif (tryptophan and glutamic acid residues with two-amino acid spacing) was found to be conserved among the N-terminal regions of apically localized ABCC members with 12-TM configuration. The significance of the WXXE motif was demonstrated for proper trafficking of ABCC4 to the apical plasma membrane.Key words: apical plasma membrane, sorting, ATP-binding cassette transporter, CFTR, MRP4.

Human epiblast lumenogenesis: From a cell aggregate to a lumenal cyst

The formation of a central lumen in the human epiblast is a critical step for development. However, because the lumen forms in the epiblast coincident with implantation, the molecular and cellular events of this early lumenogenesis process cannot be studied in vivo. Recent developments using new model systems have revealed insight into the underpinnings of epiblast formation. To provide an up-to-date comprehensive review of human epiblast lumenogenesis, we highlight recent findings from human and mouse models with an emphasis on new molecular understanding of a newly described apicosome compartment, a novel 'formative' state of pluripotency that coordinates with epiblast polarization, and new evidence about the physical and polarized trafficking mechanisms contributing to lumenogenesis.

Carbon nanoparticles adversely affect CFTR expression and toxicologically relevant pathways

Cystic fibrosis is an autosomal recessive disorder caused by mutations in the cystic fibrosis transmembrane conductance regulator (CFTR) that can lead to terminal respiratory failure. Ultrafine carbonaceous particles, which are ubiquitous in ambient urban and indoor air, are increasingly considered as major contributors to the global health burden of air pollution. However, their effects on the expression of CFTR and associated genes in lung epithelial cells have not yet been investigated. We therefore evaluated the effects of carbon nanoparticles (CNP), generated by spark-ablation, on the human bronchial epithelial cell line 16HBE14o− at air–liquid interface (ALI) culture conditions. The ALI-cultured cells exhibited epithelial barrier integrity and increased CFTR expression. Following a 4-h exposure to CNP, the cells exhibited a decreased barrier integrity, as well as decreased expression of CFTR transcript and protein levels. Furthermore, transcriptomic analysis revealed that the CNP-exposed cells showed signs of oxidative stress, apoptosis and DNA damage. In conclusion, this study describes spark-ablated carbon nanoparticles in a realistic exposure of aerosols to decrease CFTR expression accompanied by transcriptomic signs of oxidative stress, apoptosis and DNA damage.

Pharmacological Effects of Panduratin A on Renal Cyst Development in In Vitro and In Vivo Models of Polycystic Kidney Disease

Renal cyst expansion in polycystic kidney disease (PKD) involves abnormalities in both cyst-lining-cell proliferation and fluid accumulation. Suppression of these processes may retard the progression of PKD. Evidence suggests that the activation of 5' AMP-activated protein kinase (AMPK) inhibits cystic fibrosis transmembrane conductance regulator (CFTR)-mediated chloride secretion, leading to reduced progression of PKD. Here we investigated the pharmacological effects of panduratin A, a bioactive compound known as an AMPK activator, on CFTR-mediated chloride secretion and renal cyst development using in vitro and animal models of PKD. We demonstrated that AMPK was activated in immortalized normal renal cells and autosomal dominant polycystic kidney disease (ADPKD) cells following treatment with panduratin A. Treatment with panduratin A reduced the number of renal cyst colonies corresponding with a decrease in cell proliferation and phosphorylated p70/S6K, a downstream target of mTOR signaling. Additionally, panduratin A slowed cyst expansion via inhibition of the protein expression and transport function of CFTR. In heterozygous Han:Sprague-Dawley (Cy/+) rats, an animal model of PKD, intraperitoneal administration of panduratin A (25 mg/kg BW) for 5 weeks significantly decreased the kidney weight per body weight ratios and the cystic index. Panduratin A also reduced collagen deposition in renal tissue. Intraperitoneal administration of panduratin A caused abdominal bleeding and reduced body weight. However, 25 mg/kg BW of panduratin A via oral administration in the PCK rats, another non-orthologous PKD model, showed a significant decrease in the cystic index without severe adverse effects, indicating that the route of administration is critical in preventing adverse effects while still slowing disease progression. These findings reveal that panduratin A might hold therapeutic properties for the treatment of PKD.

The Distribution and Role of the CFTR Protein in the Intracellular Compartments

Cystic fibrosis is a hereditary disease that mainly affects secretory organs in humans. It is caused by mutations in the gene encoding CFTR with the most common phenylalanine deletion at position 508. CFTR is an anion channel mainly conducting Cl⁻ across the apical membranes of many different epithelial cells, the impairment of which causes dysregulation of epithelial fluid secretion and thickening of the mucus. This, in turn, leads to the dysfunction of organs such as the lungs, pancreas, kidney and liver. The CFTR protein is mainly localized in the plasma membrane; however, there is a growing body of evidence that it is also present in the intracellular organelles such as the endosomes, lysosomes, phagosomes and mitochondria. Dysfunction of the CFTR protein affects not only the ion transport across the epithelial tissues, but also has an impact on the proper functioning of the intracellular compartments. The review aims to provide a summary of the present state of knowledge regarding CFTR localization and function in intracellular compartments, the physiological role of this localization and the consequences of protein dysfunction at cellular, epithelial and organ levels. An in-depth understanding of intracellular processes involved in CFTR impairment may reveal novel opportunities in pharmacological agents of cystic fibrosis.

Tensin 1 (TNS1) is a modifier gene for low body mass index (BMI) in homozygous [F508del]CFTR patients

Cystic fibrosis (CF) is a life‐limiting autosomal recessive genetic disease caused by variants in the CFTR gene, most commonly by the [F508del] variant. Although CF is a classical Mendelian disease, genetic variants in several modifier genes have been associated with variation of the clinical phenotype for pulmonary and gastrointestinal function and urogenital development. We hypothesized that whole genome sequencing of a well‐phenotyped CF populations might identify novel variants in known, or hitherto unknown, modifier genes. Whole genome sequencing was performed on the Illumina HiSeq X platform for 98 clinically diagnosed cystic fibrosis patient samples from the Adult CF Clinic at the University of California San Diego (UCSD). We compared protein‐coding, non‐silent variants genome wide between CFTR [F508del] homozygotes vs CFTR compound heterozygotes. Based on a single variant score test, we found 3 SNPs in common variants (MAF >5%) that occurred at significantly different rates between homozygous [F508del]CFTR and compound heterozygous [F508del]CFTR patients. The 3 SNPs were all located in one gene on chromosome 2: Tensin 1 (TNS1: rs3796028; rs2571445: and rs918949). We observed significantly lower BMIs in homozygous [F508del]CFTR patients who were also homozygous for Tensin 1 rs918949 (T/T) (p = 0.023) or rs2571445 (G/G) (p = 0.02) variants. The Tensin 1 gene is thus a potential modifier gene for low BMI in CF patients homozygous for the [F508del]CFTR variant.

Association of cystic fibrosis transmembrane conductance regulator with epithelial sodium channel subunits carrying Liddle's syndrome mutations

The association of the cystic fibrosis transmembrane conductance regulator (CFTR) and epithelial sodium channel (ENaC) in the pathophysiology of cystic fibrosis (CF) is controversial. Previously, we demonstrated a close physical association between wild-type (WT) CFTR and WT ENaC. We have also shown that the F508del CFTR fails to associate with ENaC unless the mutant protein is rescued pharmacologically or by low temperature. In this study, we present the evidence for a direct physical association between WT CFTR and ENaC subunits carrying Liddle's syndrome mutations. We show that all three ENaC subunits bearing Liddle's syndrome mutations (both point mutations and the complete truncation of the carboxy terminus), could be coimmunoprecipitated with WT CFTR. The biochemical studies were complemented by fluorescence lifetime imaging microscopy (FLIM), a distance-dependent approach that monitors protein-protein interactions between fluorescently labeled molecules. Our measurements revealed significantly increased fluorescence resonance energy transfer between CFTR and all tested ENaC combinations as compared with controls (ECFP and EYFP cotransfected cells). Our findings are consistent with the notion that CFTR and ENaC are within reach of each other even in the setting of Liddle's syndrome mutations, suggestive of a direct intermolecular interaction between these two proteins.

Genetic effects of BDKRB2 and KNG1 on deep venous thrombosis after orthopedic surgery and the potential mediator

Deep venous thrombosis (DVT) is a common complication of orthopedic surgery. Genetic risk factors and high heritability carried a substantial risk of DVT. In this study, we aimed to investigate the potential association in the Han Chinese population between the polymorphisms of BDKRB2 and KNG1 and DVT after orthopedic surgery (DVTAOS). A total of 3,010 study subjects comprising 892 DVT cases and 2,118 controls were included in the study, and 39 single nucleotide polymorphisms (SNPs) in total (30 for BDKRB2 and 9 for KNG1) were chosen for genotyping. Two SNPs, rs710446 (OR = 1.27, P = 0.00016) and rs2069588 (OR = 1.29, P = 0.00056), were identified as significantly associated with DVTAOS. After adjusting for BMI, the significance of rs2069588 decreased (P = 0.0013). Haplotype analyses showed that an LD block containing rs2069588 significantly correlated with the DVTAOS risk. Moreover, bioinformatics analysis indicated that hsa-miR-758-5p and BDKRB2 formed miRNA/SNP target duplexes if the rs2069588 allele was in the T form, suggesting that rs2069588 may alter BDKRB2 expression by affecting hsa-miR-758-5p/single-nucleotide polymorphism target duplexes. Our results demonstrate additional evidence supporting that there is an important role for the KNG1 and BDKRB2 genes in the increased susceptibility of DVTAOS.

Specific autophagy and ESCRT components participate in the unconventional secretion of CFTR

The most common mutation in cystic fibrosis patients is a phenylalanine deletion at position 508 (ΔF508) in the CFTR (cystic fibrosis transmembrane conductance regulator) gene. This mutation impairs cell-surface trafficking of CFTR. During cellular stress, core-glycosylated CFTRΔF508 is transported to the cell surface from the endoplasmic reticulum (ER) via an unconventional route that bypasses the Golgi. However, the mechanisms for this unconventional secretory pathway of CFTR are not well delineated. Here, we report that components of the macroautophagy/autophagy and ESCRT (endosomal sorting complex required for transport) pathways are involved in unconventional secretion of CFTR. In mammalian cells, we found that autophagic pathways were modulated by conditions that also stimulate unconventional secretion, namely ER stress and an ER-to-Golgi transport blockade. Additionally, we found that knockdown of early autophagy components, ATG5 and ATG7, and treatment with pharmacological autophagy inhibitors, wortmannin and 3-methyladenine, abolished the unconventional secretion of CFTR that had been stimulated by ER stress and an ER-to-Golgi blockade. Interestingly, immunoelectron microscopy revealed that GORASP2/GRASP55, which mediates unconventional CFTR trafficking, is present in multivesicular bodies (MVB) and autophagosomal structures under ER stress conditions. A custom small-interfering RNA screen of mammalian ESCRT proteins that mediate MVB biogenesis showed that silencing of some ESCRTs, including MVB12B, inhibited unconventional CFTRΔF508 secretion. Furthermore, MVB12B overexpression partially rescued cell-surface expression and Cl^- channel function of CFTRΔF508. Taken together, these results suggest that components involved in early autophagosome formation and the ESCRT/MVB pathway play a key role in the stress-induced unconventional secretion of CFTR.

Trafficking Ion Transporters to the Apical Membrane of Polarized Intestinal Enterocytes

Epithelial cells lining the gastrointestinal tract require distinct apical and basolateral domains to function properly. Trafficking and insertion of enzymes and transporters into the apical brush border of intestinal epithelial cells is essential for effective digestion and absorption of nutrients. Specific critical ion transporters are delivered to the apical brush border to facilitate fluid and electrolyte uptake. Maintenance of these apical transporters requires both targeted delivery and regulated membrane recycling. Examination of altered apical trafficking in patients with Microvillus Inclusion disease caused by inactivating mutations in MYO5B has led to insights into the regulation of apical trafficking by elements of the apical recycling system. Modeling of MYO5B loss in cell culture and animal models has led to recognition of Rab11a and Rab8a as critical regulators of apical brush border function. All of these studies show the importance of apical membrane trafficking dynamics in maintenance of polarized epithelial cell function.

Molecular mechanisms of calcium signaling in the modulation of small intestinal ion transports and bicarbonate secretion

Background and Purpose:

Although Ca²⁺ signaling may stimulate small intestinal ion secretion, little is known about its critical role and the molecular mechanisms of Ca²⁺-mediated biological action.

Key Results

Activation of muscarinic receptors by carbachol(CCh) stimulated mouse duodenal I_sc, which was significantly inhibited in Ca²⁺-free serosal solution and by several selective store-operated Ca²⁺ channels(SOC) blockers added to the serosal side of duodenal tissues. Furthermore, we found that CRAC/Orai channels may represent the molecular candidate of SOC in intestinal epithelium. CCh increased intracellular Ca²⁺ but not cAMP, and Ca²⁺ signaling mediated duodenal Cl^- and HCO₃^- secretion in wild type mice but not in CFTR knockout mice. CCh induced duodenal ion secretion and stimulated PI3K/Akt activity in duodenal epithelium, all of which were inhibited by selective PI3K inhibitors with different structures. CCh-induced Ca²⁺ signaling also stimulated the phosphorylation of CFTR proteins and their trafficking to the plasma membrane of duodenal epithelial cells, which were inhibited again by selective PI3K inhibitors.

Materials and Methods

Functional, biochemical and morphological experiments were performed to examine ion secretion, PI3K/Akt and CFTR activity of mouse duodenal epithelium. Ca²⁺ imaging was performed on HT-29 cells.

Conclusions and Implications

Ca²⁺ signaling plays a critical role in intestinal ion secretion via CRAC/Orai-mediated SOCE mechanism on the serosal side of epithelium. We also demonstrated the molecular mechanisms of Ca²⁺ signaling in CFTR-mediated secretion via novel PI3K/Akt pathway. Our findings suggest new perspectives for drug targets to protect the upper GI tract and control liquid homeostasis in the small intestine.

Effects of plasma kallikrein deficiency on haemostasis and thrombosis in mice: Murine Ortholog of the Fletcher Trait

Plasma kallikrein is a multifunctional serine protease involved in contact activation of coagulation. Deficiency in humans is characterised by prolonged activated partial thromboplastin time (aPTT); however, the balance between thrombosis and haemostasis is not fully understood. A study of plasma kallikrein-deficient mice revealed increased aPTT, without prolonged bleeding time. Prekallikrein antisense oligonucleotide (ASO) treatment in mice suggested potential for a positive therapeutic index. The current goal was to further define the role of plasma kallikrein in coagulation. Blood pressure and heart rate were normal in plasma kallikrein-deficient mice, and mice were completely protected from occlusion (100 ± 1.3% control flow) in 3.5% FeCl3 -induced arterial thrombosis versus heterozygotes (20 ± 11.4%) and wild-type littermates (8 ± 0%). Vessels occluded in 8/8 wild-type, 7/8 heterozygotes, and 0/8 knockouts. Anti-thrombotic protection was less pronounced in 5% FeCl3-induced arterial injury. Integrated blood flow was 8 ± 0% control in wild-type and heterozygotes, and significantly (p<0.01) improved to 43 ± 14.2% in knockouts. The number of vessels occluded was similar in all genotypes. Thrombus weight was significantly reduced in knockouts (−47%) and heterozygotes (−23%) versus wild-type in oxidative venous thrombosis. Average tail bleeding time increased modestly in knockout mice compared to wild-type. Average renal bleeding times were similar in all genotypes. These studies confirm and extend studies with prekallikrein ASO, and demonstrate that plasma kallikrein deletion prevents occlusive thrombus formation in mice with a minimal role in provoked bleeding. Additional support for the significance of the intrinsic pathway in the coagulation cascade is provided, as well as for a potential new anti-thrombotic approach.

Clinical and laboratory profile of children with Cystic Fibrosis: Experience of a tertiary care center in Pakistan

Objective:

To determine the clinical presentation, diagnostic investigations and laboratory workup done in admitted children with cystic fibrosis at Aga Khan University Hospital Karachi, Pakistan.

Methods:

This is athree years retrospective study from January 2013 to December 2015 conducted at The Aga Khan University Hospital Karachi Pakistan, enrolling admitted patient from birth to 15 years of either gender, diagnosed with CF on the basis of clinical features and positive sweat chloride test. Different clinical presentations were noted including initial presentations. Sweat chloride values more than 60mmol/L were labeled as positive and consistent with diagnosis of CF. Available Delta F-508 mutation analyses were noted. Relevant laboratory and radiological investigations including sputum culture and HR-CT chest findings were documented. Results were analyzed using SPSS version 20.

Results:

Total 43 children were selected according to the inclusion criteria. Chronic cough (69.76%) was the most common initial clinical presentation. Mean age at onset of symptoms was 14.41± 26.18 months and mean age at diagnosis was 47.20 ± 45.80 months Respiratory features were most common in our cohort including chronic productive cough (90.71%), recurrent bronchopneumonia (72.09%) and asthma like presentation (44.19%) with wheezing and cough. 86% patients presented with failure to thrive. Gastroenterological features including steatorrhea were seen in 55.81% patients and 44.19% patients had abdominal distension. Mean sweat chloride value in our population was 82.70± 22.74. Gene analysis for Delta F-508 was identified in 12 (27.90%) patients. Bronchiectatic pulmonary changes on HRCT were seen in 18 patients (41.86%). Pseudomonas grew in 12 patients (27.90%) in sputum cultures at the time of diagnosis.

Conclusion:

Respiratory presentations predominate in CF children followed by gastrointestinal features. Nearly half of our patient had bronchiectatic changes on CT scan chest and more than quarter had pseudomonas colonization in the airways at the time of diagnosis. Delta F-508 mutation was found to be uncommon in our study population. There is significant delay in diagnosing patients with CF.

In silico search for modifier genes associated with pancreatic and liver disease in Cystic Fibrosis

Cystic Fibrosis is the most common lethal autosomal recessive disorder in the white population, affecting among other organs, the lung, the pancreas and the liver. Whereas Cystic Fibrosis is a monogenic disease, many studies reveal a very complex relationship between genotype and clinical phenotype. Indeed, the broad phenotypic spectrum observed in Cystic Fibrosis is far from being explained by obvious genotype-phenotype correlations and it is admitted that Cystic Fibrosis disease is the result of multiple factors, including effects of the environment as well as modifier genes. Our objective was to highlight new modifier genes with potential implications in the lung, pancreatic and liver outcomes of the disease. For this purpose we performed a system biology approach which combined, database mining, literature mining, gene expression study and network analysis as well as pathway enrichment analysis and protein-protein interactions. We found that IFI16, CCNE2 and IGFBP2 are potential modifiers in the altered lung function in Cystic Fibrosis. We also found that EPHX1, HLA-DQA1, HLA-DQB1, DSP and SLC33A1, GPNMB, NCF2, RASGRP1, LGALS3 and PTPN13, are potential modifiers in pancreas and liver, respectively. Associated pathways indicate that immune system is likely involved and that Ubiquitin C is probably a central node, linking Cystic Fibrosis to liver and pancreatic disease. We highlight here new modifier genes with potential implications in Cystic Fibrosis. Nevertheless, our in silico analysis requires functional analysis to give our results a physiological relevance.

Review: The plasma kallikrein/kinin and renin angiotensin systems in blood pressure regulation in sepsis

The hemodynamics of septic shock after endotoxinemiai s influenced by the plasma kallikrein/kinin and the renin angiotensin systems. In recent years, new information has improved understanding of the protein/biologically active peptide interactions between these two systems. The plasma kallikrein/kinin system, more commonly known as the contact system, has undergone a re-evaluation as to how it assembles on cell membranes for physiological and pathophysiological activation and as to its role in Gram-negative sepsis. It has been proposed that it counterbalances the plasma renin angiotensin system. Furthermore, more knowledge about the renin angiotensin system has become available on how it either opposes the actions of the kallikrein/kinin system or, in some cases, summates with it. Understanding the interactions between these two systems may lead to development of better pharmacological treatments for endotoxin-induced shock.

Identification of intestinal ion transport defects in microvillus inclusion disease

Loss of function mutations in the actin motor myosin Vb (Myo5b) lead to microvillus inclusion disease (MVID) and death in newborns and children. MVID results in secretory diarrhea, brush border (BB) defects, villus atrophy, and microvillus inclusions (MVIs) in enterocytes. How loss of Myo5b results in increased stool loss of chloride (Cl(-)) and sodium (Na(+)) is unknown. The present study used Myo5b loss-of-function human MVID intestine, polarized intestinal cell models of secretory crypt (T84) and villus resembling (CaCo2BBe, C2BBe) enterocytes lacking Myo5b in conjunction with immunofluorescence confocal stimulated emission depletion (gSTED) imaging, immunohistochemical staining, transmission electron microscopy, shRNA silencing, immunoblots, and electrophysiological approaches to examine the distribution, expression, and function of the major BB ion transporters NHE3 (Na(+)), CFTR (Cl(-)), and SLC26A3 (DRA) (Cl(-)/HCO3 (-)) that control intestinal fluid transport. We hypothesized that enterocyte maturation defects lead villus atrophy with immature secretory cryptlike enterocytes in the MVID epithelium. We investigated the role of Myo5b in enterocyte maturation. NHE3 and DRA localization and function were markedly reduced on the BB membrane of human MVID enterocytes and Myo5bKD C2BBe cells, while CFTR localization was preserved. Forskolin-stimulated CFTR ion transport in Myo5bKD T84 cells resembled that of control. Loss of Myo5b led to YAP1 nuclear retention, retarded enterocyte maturation, and a cryptlike phenotype. We conclude that preservation of functional CFTR in immature enterocytes, reduced functional expression of NHE3, and DRA contribute to Cl(-) and Na(+) stool loss in MVID diarrhea.

Genetic variation and exercise-induced muscle damage: implications for athletic performance, injury and ageing

Prolonged unaccustomed exercise involving muscle lengthening (eccentric) actions can result in ultrastructural muscle disruption, impaired excitation-contraction coupling, inflammation and muscle protein degradation. This process is associated with delayed onset muscle soreness and is referred to as exercise-induced muscle damage. Although a certain amount of muscle damage may be necessary for adaptation to occur, excessive damage or inadequate recovery from exercise-induced muscle damage can increase injury risk, particularly in older individuals, who experience more damage and require longer to recover from muscle damaging exercise than younger adults. Furthermore, it is apparent that inter-individual variation exists in the response to exercise-induced muscle damage, and there is evidence that genetic variability may play a key role. Although this area of research is in its infancy, certain gene variations, or polymorphisms have been associated with exercise-induced muscle damage (i.e. individuals with certain genotypes experience greater muscle damage, and require longer recovery, following strenuous exercise). These polymorphisms include ACTN3 (R577X, rs1815739), TNF (-308 G>A, rs1800629), IL6 (-174 G>C, rs1800795), and IGF2 (ApaI, 17200 G>A, rs680). Knowing how someone is likely to respond to a particular type of exercise could help coaches/practitioners individualise the exercise training of their athletes/patients, thus maximising recovery and adaptation, while reducing overload-associated injury risk. The purpose of this review is to provide a critical analysis of the literature concerning gene polymorphisms associated with exercise-induced muscle damage, both in young and older individuals, and to highlight the potential mechanisms underpinning these associations, thus providing a better understanding of exercise-induced muscle damage.

Dynamically Regulated CFTR Expression and Its Functional Role in Cutaneous Wound Healing

The physiological role of cystic fibrosis transmembrane conductance regulator (CFTR) in keratinocytes and skin wound healing is completely unknown. The present study shows that CFTR is expressed in the multiple layers of keratinocytes in mouse epidermis and exhibits a dynamic expression pattern in a dorsal skin wound healing model, with diminishing levels observed from day 3 to day 5 and re-appearing from day 7 to day 10 after wounding. Knockdown of CFTR in cultured human keratinocytes promotes cell migration but inhibits differentiation, while overexpression of CFTR suppresses migration but enhances differentiation, indicating an important role of CFTR in regulating keratinocyte behavior. In addition, we have demonstrated a direct association of CFTR with epithelial junction formation as knockdown of CFTR downregulates the expression of adhesion molecules, such as E-cadherin, ZO-1 and β-catenin, and disrupts the formation of cell junction, while overexpression of CFTR enhances cell junction formation. More importantly, we have shown that ΔF508cftr-/- mice with defective CFTR exhibit delayed wound healing as compared to wild type mice, indicating that normal function of CFTR is critical for wound repair. Taken together, the present study has revealed a previously undefined role of CFTR in regulating skin wound healing processes, which may have implications in injury repair of other epithelial tissues.

Disease-modifying genes and monogenic disorders: experience in cystic fibrosis

The mechanisms responsible for the determination of phenotypes are still not well understood; however, it has become apparent that modifier genes must play a considerable role in the phenotypic heterogeneity of Mendelian disorders. Significant advances in genetic technologies and molecular medicine allow huge amounts of information to be generated from individual samples within a reasonable time frame. This review focuses on the role of modifier genes using the example of cystic fibrosis, the most common lethal autosomal recessive disorder in the white population, and discusses the advantages and limitations of candidate gene approaches versus genome-wide association studies. Moreover, the implications of modifier gene research for other monogenic disorders, as well as its significance for diagnostic, prognostic, and therapeutic approaches are summarized. Increasing insight into modifying mechanisms opens up new perspectives, dispelling the idea of genetic disorders being caused by one single gene.

Microfluidic approaches for epithelial cell layer culture and characterisation

In higher eukaryotes, epithelial cell layers line most body cavities and form selective barriers that regulate the exchange of solutes between compartments. In order to fulfil these functions, the cells assume a polarised architecture and maintain two distinct plasma membrane domains, the apical domain facing the lumen and the basolateral domain facing other cells and the extracellular matrix. Microfluidic biochips offer the unique opportunity to establish novel in vitro models of epithelia in which the in vivo microenvironment of epithelial cells is precisely reconstituted. In addition, analytical tools to monitor biologically relevant parameters can be directly integrated on-chip. In this review we summarise recently developed biochip designs for culturing epithelial cell layers. Since endothelial cell layers, which line blood vessels, have similar barrier functions and polar organisation as epithelial cell layers, we also discuss biochips for culturing endothelial cell layers. Furthermore, we review approaches to integrate tools to analyse and manipulate epithelia and endothelia in microfluidic biochips; including methods to perform electrical impedance spectroscopy; methods to detect substances undergoing trans-epithelial transport via fluorescence, spectrophotometry, and mass spectrometry; techniques to mechanically stimulate cells via stretching and fluid flow-induced shear stress; and methods to carry out high-resolution imaging of vesicular trafficking using light microscopy. Taken together, this versatile microfluidic toolbox enables novel experimental approaches to characterise epithelial monolayers.

Serum- and glucocorticoid-induced protein kinase 1 (SGK1) increases the cystic fibrosis transmembrane conductance regulator (CFTR) in airway epithelial cells by phosphorylating Shank2E protein

The glucocorticoid dexamethasone increases cystic fibrosis transmembrane conductance regulator (CFTR) abundance in human airway epithelial cells by a mechanism that requires serum- and glucocorticoid-induced protein kinase 1 (SGK1) activity. The goal of this study was to determine whether SGK1 increases CFTR abundance by phosphorylating Shank2E, a PDZ domain protein that contains two SGK1 phosphorylation consensus sites. We found that SGK1 phosphorylates Shank2E as well as a peptide containing the first SGK1 consensus motif of Shank2E. The dexamethasone-induced increase in CFTR abundance was diminished by overexpression of a dominant-negative Shank2E in which the SGK1 phosphorylation sites had been mutated. siRNA-mediated reduction of Shank2E also reduced the dexamethasone-induced increase in CFTR abundance. Taken together, these data demonstrate that the glucocorticoid-induced increase in CFTR abundance requires phosphorylation of Shank2E at an SGK1 consensus site.

Purification, crystallization and preliminary X-ray data collection of the N-terminal domain of the 26S proteasome regulatory subunit p27 and its complex with the ATPase domain of Rpt5 from Mus musculus

The protein 26S proteasome regulatory subunit p27 is one of the four chaperones that help in the assembly of the 19S regulatory particle (RP) of the 26S proteasome. In the present work, the N-terminus of p27 (residues 1-128) from Mus musculus was cloned, expressed, purified and crystallized alone and in complex with the C-terminal ATPase domain of Rpt5 (residues 173-442). The crystals of p27((1-128)) diffracted to 1.7 Å resolution and belonged to space group P212121, with unit-cell parameters a = 26.79, b = 30.39, c = 145.06 Å. Resolution-dependent Matthews coefficient probability analysis suggested the presence of only one molecule per asymmetric unit, with 40.5% solvent content and a VM value of 2.02 Å(3) Da(-1). The crystal of the p27((1-128))-Rpt5((173-442)) complex diffracted to 4 Å resolution and belonged to space group P222, with unit-cell parameters a = 75.93, b = 76.08, c = 336.85 Å. The presence of four heterodimers in the asymmetric unit with 53.2% solvent content and a VM value of 2.63 Å(3) Da(-1) or five heterodimers in the asymmetric unit with 41.5% solvent content and a VM value of 2.10 Å(3) Da(-1) is assumed.

LMTK2-mediated phosphorylation regulates CFTR endocytosis in human airway epithelial cells

Cystic fibrosis transmembrane conductance regulator (CFTR) is a Cl⁻-selective ion channel expressed in fluid-transporting epithelia. Lemur tyrosine kinase 2 (LMTK2) is a transmembrane protein with serine and threonine but not tyrosine kinase activity. Previous work identified CFTR as an in vitro substrate of LMTK2, suggesting a functional link. Here we demonstrate that LMTK2 co-immunoprecipitates with CFTR and phosphorylates CFTR-Ser⁷³⁷ in human airway epithelial cells. LMTK2 knockdown or expression of inactive LMTK2 kinase domain increases cell surface density of CFTR by attenuating its endocytosis in human airway epithelial cells. Moreover, LMTK2 knockdown increases Cl⁻ secretion mediated by the wild-type and rescued ΔF508-CFTR. Compared with the wild-type CFTR, the phosphorylation-deficient mutant CFTR-S737A shows increased cell surface density and decreased endocytosis. These results demonstrate a novel mechanism of the phospho-dependent inhibitory effect of CFTR-Ser⁷³⁷ mediated by LMTK2 via endocytosis and inhibition of the cell surface density of CFTR Cl⁻ channels. These data indicate that targeting LMTK2 may increase the cell surface density of CFTR Cl⁻ channels and improve stability of pharmacologically rescued ΔF508-CFTR in patients with cystic fibrosis.

Cigarette smoke-induced Ca2+ release leads to cystic fibrosis transmembrane conductance regulator (CFTR) dysfunction

Chronic obstructive pulmonary disease affects 64 million people and is currently the fourth leading cause of death worldwide. Chronic obstructive pulmonary disease includes both emphysema and chronic bronchitis, and in the case of chronic bronchitis represents an inflammatory response of the airways that is associated with mucus hypersecretion and obstruction of small airways. Recently, it has emerged that exposure to cigarette smoke (CS) leads to an inhibition of the cystic fibrosis transmembrane conductance regulator (CFTR) Cl(-) channel, causing airway surface liquid dehydration, which may play a role in the development of chronic bronchitis. CS rapidly clears CFTR from the plasma membrane and causes it to be deposited into aggresome-like compartments. However, little is known about the mechanism(s) responsible for the internalization of CFTR following CS exposure. Our studies revealed that CS triggered a rise in cytoplasmic Ca(2+) that may have emanated from lysosomes. Furthermore, chelation of cytoplasmic Ca(2+), but not inhibition of protein kinases/phosphatases, prevented CS-induced CFTR internalization. The macrolide antibiotic bafilomycin A1 inhibited CS-induced Ca(2+) release and prevented CFTR clearance from the plasma membrane, further linking cytoplasmic Ca(2+) and CFTR internalization. We hypothesize that CS-induced Ca(2+) release prevents normal sorting/degradation of CFTR and causes internalized CFTR to reroute to aggresomes. Our data provide mechanistic insight into the potentially deleterious effects of CS on airway epithelia and outline a hitherto unrecognized signaling event triggered by CS that may affect the long term transition of the lung into a hyper-inflammatory/dehydrated environment.

Role of Factor XII in hemostasis and thrombosis: clinical implications

The plasma coagulation system reacts quickly to limit blood loss from injury sites but also contributes to vascular thrombosis. In current models of hemostatic balance, normal coagulation and thrombosis represent two sides of the same coin, however, recent data from gene-deleted murine models have challenged this dogma. Deficiency of coagulation Factor XII (Hageman factor), a serine protease that initiates the intrinsic pathway of coagulation, severely impairs arterial thrombus formation but is not associated with excessive bleeding. These findings suggest that fibrin-generating mechanisms that operate during pathologic thrombus formation involve pathways distinct from those that are active during normal hemostasis. As Factor XII selectively contributes to thrombus formation in occlusive disease, but not to normal hemostasis, inhibition of this protease may offer a novel treatment strategy for prevention of arterial thrombosis with minimal or no risk of bleeding.

Biochemical properties of urea transporters

Urea and urea transporters (UT) are critical to the production of concentrated urine and hence in maintaining body fluid balance. The UT-A1 urea transporter is the major and most important UT isoform in the kidney. Native UT-A1, expressed in the terminal inner medullary collecting duct (IMCD) epithelial cells, is a glycosylated protein with two glycoforms of 117 and 97 kDa. Vasopressin is the major hormone in vivo that rapidly increases urea permeability in the IMCD through increases in phosphorylation and apical plasma-membrane accumulation of UT-A1. The cell signaling pathway for vasopressin-mediated UT-A1 phosphorylation and activity involves two cAMP-dependent signaling pathways: protein kinase A (PKA) and exchange protein activated by cAMP (Epac). In this chapter, we will discuss UT-A1 regulation by phosphorylation, ubiquitination, and glycosylation.

Exocrine cell-derived microparticles in response to lipopolysaccharide promote endocrine dysfunction in cystic fibrosis

BACKGROUND

Diabetes in cystic fibrosis (CF) is a result of exocrine pancreas alteration followed by endocrine dysfunction at a later stage. Microparticles (MPs) are plasma membrane fragments shed from stimulated or damaged cells that act as cellular effectors. Our aim was to identify a new form of interaction between exocrine and endocrine pancreatic cells mediated by exocrine MPs, in the context of recurrent infection in CF.

METHODS

MPs from either human exocrine CFTRΔF508-mutated (CFPAC-1) cells or exocrine normal pancreatic (PANC-1) cells were collected after treatment by LPS from Pseudomonas aeruginosa and applied to rat endocrine normal insulin-secreting RIN-m5F cells. MP membrane integration in target cells was established by confocal microscopy and flow cytometry using PKH26 lipid probe. Apoptosis, lysosomal activity, insulin secretion were measured after 18 h. MP-mediated NF-κB activation was measured in HEK-Blue reporter cells by SEAP reporter gene system and in RIN-m5F cells by Western blot. In endocrine normal cells, CFTR inhibition was achieved using Inhibitor-172.

RESULTS

Compared to PANC-1, MPs from CFPAC-1 significantly reduced insulin secretion and lysosomal activity in RIN-m5F. MPs induced NF-κB activation by increasing the level of IκB phosphorylation. Moreover, the inhibition of NF-κB activation using specific inhibitors was associated with a restored insulin secretion. Interestingly, CFTR inhibition in normal RIN-m5F cells promoted apoptosis and decreased insulin secretion.

CONCLUSIONS

During recurrent infections associated with CF, exocrine MPs may contribute to endocrine cell dysfunction via NF-κB pathways. Membrane CFTR dysfunction is associated with decreased insulin secretion.

c-Cbl reduces stability of rescued ∆F508-CFTR in human airway epithelial cells: Implications for cystic fibrosis treatment

CFTR is a PKA activated Cl^- channel expressed in the apical membrane of fluid transporting epithelia. We previously demonstrated that c-Cbl decreases CFTR stability in the plasma membrane by facilitating its endocytosis and lysosomal degradation in human airway epithelium. The most common mutation associated with cystic fibrosis, deletion of Phe508 (∆F508), leads to a temperature sensitive biosynthetic processing defect in the CFTR protein. Mature ∆F508-CFTR that has been rescued by low temperature or chemical chaperones is partially functional as a Cl^- channel but has decreased plasma membrane stability due to altered post-maturational trafficking. Our present data demonstrate that c-Cbl controls the post-maturational trafficking of rescued ∆F508-CFTR. Partial depletion of c-Cbl increased stability of the plasma membrane associated mature ∆F508-CFTR and the ∆F508-CFTR mediated Cl^- secretion. These data indicate that correcting the post-maturational trafficking of ∆F508-CFTR may represent a therapeutic approach complementary to the biosynthetic rescue. Because c-Cbl functions as an adaptor and scaffolding protein during CFTR endocytosis, we propose that interfering with the c-Cbl mediated endocytic recruitment of ∆F508-CFTR may increase stability of ∆508-CFTR in the plasma membrane after its biosynthetic rescue.

Tgf-β1 inhibits Cftr biogenesis and prevents functional rescue of ΔF508-Cftr in primary differentiated human bronchial epithelial cells

CFTR is an integral transmembrane glycoprotein and a cAMP-activated Cl(-) channel. Mutations in the CFTR gene lead to Cystic Fibrosis (CF)-an autosomal recessive disease with majority of the morbidity and mortality resulting from airway infection, inflammation, and fibrosis. The most common disease-associated mutation in the CFTR gene-deletion of Phe508 (ΔF508) leads to a biosynthetic processing defect of CFTR. Correction of the defect and delivery of ΔF508-CFTR to the cell surface has been highly anticipated as a disease modifying therapy. Compared to promising results in cultured cell this approach was much less effective in CF patients in an early clinical trial. Although the cause of failure to rescue ΔF508-CFTR in the clinical trial has not been determined, presence of factor(s) that interfere with the rescue in vivo could be considered. The cytokine TGF-β1 is frequently elevated in CF patients. TGF-β1 has pleiotropic effects in different disease models and genetic backgrounds and little is known about TGF-β1 effects on CFTR in human airway epithelial cells. Moreover, there are no published studies examining TGF-β1 effects on the functional rescue of ΔF508-CFTR. Here we found that TGF-β1 inhibits CFTR biogenesis by reducing mRNA levels and protein abundance in primary differentiated human bronchial epithelial (HBE) cells from non-CF individuals. TGF-β1 inhibits CFTR biogenesis without compromising the epithelial phenotype or integrity of HBE cells. TGF-β1 also inhibits biogenesis and impairs the functional rescue of ΔF508-CFTR in HBE cells from patients homozygous for the ΔF508 mutation. Our data indicate that activation of TGF-β1 signaling may inhibit CFTR function in non-CF individuals and may interfere with therapies directed at correcting the processing defect of ΔF508-CFTR in CF patients.

Regulated recycling of mutant CFTR is partially restored by pharmacological treatment

Efficient trafficking of the cystic fibrosis transmembrane conductance regulator (CFTR) to and from the cell surface is essential for maintaining channel density at the plasma membrane (PM) and ensuring proper physiological activity. The most common mutation, F508del, exhibits reduced surface expression and impaired function despite treatment with currently available pharmacological small molecules, called correctors. To gain more detailed insight into whether CFTR enters compartments that allow corrector stabilization in the cell periphery, we investigated the peripheral trafficking itineraries and kinetics of wild type (WT) and F508del in living cells using high-speed fluorescence microscopy together with fluorogen activating protein detection. We directly visualized internalization and accumulation of CFTR WT from the PM to a perinuclear compartment that colocalized with the endosomal recycling compartment (ERC) markers Rab11 and EHD1, reaching steady-state distribution by 25 minutes. Stimulation by protein kinase A (PKA) depleted this intracellular pool and redistributed CFTR channels to the cell surface, elicited by reduced endocytosis and active translocation to the PM. Corrector or temperature rescue of F508del also resulted in targeting to the ERC and exhibited subsequent PKA-stimulated trafficking to the PM. Corrector treatment (24 hours) led to persistent residence of F508del in the ERC, while thermally destabilized F508del was targeted to lysosomal compartments by 3 hours. Acute addition of individual correctors, C4 or C18, acted on peripheral trafficking steps to partially block lysosomal targeting of thermally destabilized F508del. Taken together, corrector treatment redirects F508del trafficking from a degradative pathway to a regulated recycling route, and proteins that mediate this process become potential targets for improving the efficacy of current and future correctors.

Insulin-like growth factor 1 (IGF-1) enhances the protein expression of CFTR

Low levels of insulin-like growth factor 1 (IGF-1) have been observed in the serum of cystic fibrosis (CF) patients. However, the effects of low serum IGF-1 on the cystic fibrosis transmembrane conductance regulator (CFTR), whose defective function is the primary cause of cystic fibrosis, have not been studied. Here, we show in human cells that IGF-1 increases the steady-state levels of mature wildtype CFTR in a CFTR-associated ligand (CAL)- and TC10-dependent manner; moreover, IGF-1 increases CFTR-mediated chloride transport. Using an acceptor photobleaching fluorescence resonance energy transfer (FRET) assay, we have confirmed the binding of CAL and CFTR in the Golgi. We also show that CAL overexpression inhibits forskolin-induced increases in the cell-surface expression of CFTR. We found that IGF-1 activates TC10, and active TC10 alters the functional association between CAL and CFTR. Furthermore, IGF-1 and active TC10 can reverse the CAL-mediated reduction in the cell-surface expression of CFTR. IGF-1 does not increase the expression of ΔF508 CFTR, whose processing is arrested in the ER. This finding is consistent with our observation that IGF-1 alters the functional interaction of CAL and CFTR in the Golgi. However, when ΔF508 CFTR is rescued with low temperature or the corrector VRT-325 and proceeds to the Golgi, IGF-1 can increase the expression of the rescued ΔF508 CFTR. Our data support a model indicating that CAL-CFTR binding in the Golgi inhibits CFTR trafficking to the cell surface, leading CFTR to the degradation pathway instead. IGF-1-activated TC10 changes the interaction of CFTR and CAL, allowing CFTR to progress to the plasma membrane. These findings offer a potential strategy using a combinational treatment of IGF-1 and correctors to increase the post-Golgi expression of CFTR in cystic fibrosis patients bearing the ΔF508 mutation.

Dexamethasone regulates CFTR expression in Calu-3 cells with the involvement of chaperones HSP70 and HSP90

Background

Dexamethasone is widely used for pulmonary exacerbation in patients with cystic fibrosis, however, not much is known about the effects of glucocorticoids on the wild-type cystic fibrosis channel transmembrane regulator (CFTR). Our aim was to determine the effects of dexamethasone treatment on wild-type CFTR expression.

Methods and Results

Dose–response (1 nM to 10 µM) and time course (3 to 48 h) curves were generated for dexamethasone for mRNA expression in Calu-3 cells using a real-time PCR. Within 24 h, dexamethasone (10 nM) showed a 0.3-fold decrease in CFTR mRNA expression, and a 3.2-fold increase in αENaC mRNA expression compared with control groups. Dexamethasone (10 nM) induced a 1.97-fold increase in the total protein of wild-type CFTR, confirmed by inhibition by mifepristone. To access surface protein expression, biotinylation followed by Western blotting showed that dexamethasone treatment led to a 2.35-fold increase in the amount of CFTR in the cell surface compared with the untreated control groups. Once protein translation was inhibited with cycloheximide, dexamethasone could not increase the amount of CFTR protein. Protein stability was assessed by inhibition of protein synthesis with cycloheximide (50 µg/ml) at different times in cells treated with dexamethasone and in untreated cells. Dexamethasone did not alter the degradation of wild-type CFTR. Assessment of the B band of CFTR within 15 min of metabolic pulse labeling showed a 1.5-fold increase in CFTR protein after treatment with dexamethasone for 24 h. Chaperone 90 (HSP90) binding to CFTR increased 1.55-fold after treatment with dexamethasone for 24 h, whereas chaperone 70 (HSP70) binding decreased 0.30 fold in an immunoprecipitation assay.

Conclusion

Mature wild-type CFTR protein is regulated by dexamethasone post transcription, involving cotranslational mechanisms with HSP90 and HSP70, which enhances maturation and expression of wild-type CFTR.

Tissue-specific control of CFTR endocytosis by Dab2: Cargo recruitment as a therapeutic target

Clathrin-mediated endocytosis dynamically regulates cell membrane abundance of CFTR and plays an essential role in CFTR-dependent Cl(-) conductance in fluid-transporting epithelia. It requires two closely related, but distinct processes: assembly of the clathrin coat and recruitment of cargo proteins for endocytosis. The assembly polypeptide-2 complex (AP-2) is the prototypical endocytic adaptor responsible for optimal clathrin coat formation. Disabled-2 (Dab2) is a clathrin associated sorting protein (CLASP) that also mediates clathrin assembly and cargo selection. Both of these complexes have clearly been shown to play roles in CFTR endocytosis in cells that endogenously express the channel. However, their precise functions exhibit cell-specific differences. While Dab2 appears to play a central role in CFTR recruitment to the clathrin coat in airway epithelial cells, it does not play a direct role in CFTR endocytosis in intestinal epithelial cells. Here, we review our current understanding of the role of Dab2 in CFTR endocytosis in different tissues. Next, we present new data demonstrating the role of Dab2 in endocytosis of the most commonly mutated CFTR gene product, ∆F508-CFTR, in human airwy epithelial cells. Finally we discuss the potential therapeutic implications of targeting the functional interaction between ∆F508-CFTR and Dab2.

Functional interaction between CFTR and the sodium-phosphate co-transport type 2a in Xenopus laevis oocytes

Background

A growing number of proteins, including ion transporters, have been shown to interact with Cystic Fibrosis Transmembrane conductance Regulator (CFTR). CFTR is an epithelial chloride channel that is involved in Cystic Fibrosis (CF) when mutated; thus a better knowledge of its functional interactome may help to understand the pathophysiology of this complex disease. In the present study, we investigated if CFTR and the sodium-phosphate co-transporter type 2a (NPT2a) functionally interact after heterologous expression of both proteins in Xenopus laevis oocytes.

Methodology/Findings

NPT2a was expressed alone or in combination with CFTR in X. laevis oocytes. Using the two-electrode voltage-clamp technique, the inorganic phosphate-induced current (IPi) was measured and taken as an index of NPT2a activity. The maximal IPi for NPT2a substrates was reduced when CFTR was co-expressed with NPT2a, suggesting a decrease in its expression at the oolemna. This was consistent with Western blot analysis showing reduced NPT2a plasma membrane expression in oocytes co-expressing both proteins, whereas NPT2a protein level in total cell lysate was the same in NPT2a- and NPT2a+CFTR-oocytes. In NPT2a+CFTR- but not in NPT2a-oocytes, IPi and NPT2a surface expression were increased upon PKA stimulation, whereas stimulation of Exchange Protein directly Activated by cAMP (EPAC) had no effect. When NPT2a-oocytes were injected with NEG2, a short amino-acid sequence from the CFTR regulatory domain that regulates PKA-dependent CFTR trafficking to the plasma membrane, IPi values and NPT2a membrane expression were diminished, and could be enhanced by PKA stimulation, thereby mimicking the effects of CFTR co-expression.

Conclusion/Perspectives

We conclude that when both CFTR and NPT2a are expressed in X. laevis oocytes, CFTR confers to NPT2a a cAMPi-dependent trafficking to the membrane. This functional interaction raises the hypothesis that CFTR may play a role in phosphate homeostasis.

Regulation of transport in the connecting tubule and cortical collecting duct

The central goal of this overview article is to summarize recent findings in renal epithelial transport,focusing chiefly on the connecting tubule (CNT) and the cortical collecting duct (CCD).Mammalian CCD and CNT are involved in fine-tuning of electrolyte and fluid balance through reabsorption and secretion. Specific transporters and channels mediate vectorial movements of water and solutes in these segments. Although only a small percent of the glomerular filtrate reaches the CNT and CCD, these segments are critical for water and electrolyte homeostasis since several hormones, for example, aldosterone and arginine vasopressin, exert their main effects in these nephron sites. Importantly, hormones regulate the function of the entire nephron and kidney by affecting channels and transporters in the CNT and CCD. Knowledge about the physiological and pathophysiological regulation of transport in the CNT and CCD and particular roles of specific channels/transporters has increased tremendously over the last two decades.Recent studies shed new light on several key questions concerning the regulation of renal transport.Precise distribution patterns of transport proteins in the CCD and CNT will be reviewed, and their physiological roles and mechanisms mediating ion transport in these segments will also be covered. Special emphasis will be given to pathophysiological conditions appearing as a result of abnormalities in renal transport in the CNT and CCD.

A cis-acting five-amino-acid motif controls targeting of ABCC2 to the apical plasma membrane domain

ATP-binding cassette transporter isoform C2 (ABCC2) is exclusively targeted to the apical plasma membrane of polarized cells. Although apical localization of ABCC2 in hepatocytes is crucial for the biliary excretion of a variety of metabolites, the mechanism regulating its apical targeting is poorly understood. In the present study, an apical targeting signal was identified in the first cytoplasmic loop domain (CLD1) of ABCC2 in HepG2 cells. Overexpression of CLD1 significantly disturbed the apical targeting of FLAG-ABCC2 in a competitive manner, suggesting the presence of a saturable sorting machinery in HepG2 cells. Next, deletion analysis identified a potential targeting sequence within a 20-amino-acid long peptide (aa 272-291) of CLD1. Alanine scanning mutagenesis of this region in full-length ABCC2 further narrowed down the apical targeting determinant to five amino acids, S(283)QDAL(287). Of these, S(283) and L(287) were found to be conserved among vertebrate ABCC2 orthologs. Site-directed mutagenesis showed that both S(283) and L(287) were crucial for the targeting specificity of ABCC2. Introducing this apical targeting sequence into the corresponding region of ABCC1, an exclusively basolateral protein, caused the hybrid ABCC1 to partially localize in the apical membrane. Thus, the CLD1 of ABCC2 contains a novel apical sorting determinant, and a saturable sorting machinery is present in polarized HepG2 cells.

Disabled-2 protein facilitates assembly polypeptide-2-independent recruitment of cystic fibrosis transmembrane conductance regulator to endocytic vesicles in polarized human airway epithelial cells

Cystic fibrosis transmembrane conductance regulator (CFTR) is a cAMP-activated Cl(-) channel expressed in the apical plasma membrane of fluid-transporting epithelia, where the plasma membrane abundance of CFTR is in part controlled by clathrin-mediated endocytosis. The protein networks that control CFTR endocytosis in epithelial cells have only been partially explored. The assembly polypeptide-2 complex (AP-2) is the prototypical endocytic adaptor critical for optimal clathrin coat formation. AP-2 is essential for recruitment of cargo proteins bearing the YXXΦ motif. Although AP-2 interacts directly with CFTR in vitro and facilitates CFTR endocytosis in some cell types, it remains unknown whether it is critical for CFTR uptake into clathrin-coated vesicles (CCVs). Disabled-2 (Dab2) is a clathrin-associated sorting protein (CLASP) that contributes to clathrin recruitment, vesicle formation, and cargo selection. In intestinal epithelial cells Dab2 was not found to play a direct role in CFTR endocytosis. By contrast, AP-2 and Dab2 were shown to facilitate CFTR endocytosis in human airway epithelial cells, although the specific mechanism remains unknown. Our data demonstrate that Dab2 mediates AP-2 independent recruitment of CFTR to CCVs in polarized human airway epithelial cells. As a result, it facilitates CFTR endocytosis and reduces CFTR abundance and stability in the plasma membrane. These effects are mediated by the DAB homology domain. Moreover, we show that in human airway epithelial cells AP-2 is not essential for CFTR recruitment to CCVs.

Dab2 is a key regulator of endocytosis and post-endocytic trafficking of the cystic fibrosis transmembrane conductance regulator

CFTR (cystic fibrosis transmembrane conductance regulator) is expressed in the apical membrane of epithelial cells. Cell-surface CFTR levels are regulated by endocytosis and recycling. A number of adaptor proteins including AP-2 (μ2 subunit) and Dab2 (Disabled-2) have been proposed to modulate CFTR internalization. In the present study we have used siRNA (small interfering RNA)-mediated silencing of these adaptors to test their roles in the regulation of CFTR cell-surface trafficking and stability in human airway epithelial cells. The results indicate that μ2 and Dab2 performed partially overlapping, but divergent, functions. While μ2 depletion dramatically decreased CFTR endocytosis with little effect on the half-life of the CFTR protein, Dab2 depletion increased the CFTR half-life ~3-fold, in addition to inhibiting CFTR endocytosis. Furthermore, Dab2 depletion inhibited CFTR trafficking from the sorting endosome to the recycling compartment, as well as delivery of CFTR to the late endosome, thus providing a mechanistic explanation for increased CFTR expression and half-life. To test whether two E3 ligases were required for the endocytosis and/or down-regulation of surface CFTR, we siRNA-depleted CHIP [C-terminus of the Hsc (heat-shock cognate) 70-interacting protein] and c-Cbl (casitas B-lineage lymphoma). We demonstrate that CHIP and c-Cbl depletion have no effect on CFTR endocytosis, but c-Cbl depletion modestly enhanced the half-life of CFTR. The results of the present study define a significant role for Dab2 both in the endocytosis and post-endocytic fate of CFTR.

Escherichia coli-induced epithelial hyporesponsiveness to secretagogues is associated with altered CFTR localization

Both pathogenic and commensal strains of Escherichia coli colonize the human intestinal tract. Pathogenic strains differ only in the expression of virulence factors, many of which comprise a type III secretion system (TTSS). Little is known regarding the effect of E. coli on the intestinal epithelial response to the secretagogues that drive ion secretion, despite its importance in causing clinically significant diarrhoea. Using Ussing chambers to measure electrogenic ion transport of T84 intestinal epithelial cell monolayers, we found that all strains of E. coli tested (pathogenic, commensal, probiotic and lab strain) significantly reduced cAMP-dependent ion secretion after 4-8 h exposure. Enteropathogenic E. coli mutants lacking a functional TTSS caused similar hyposecretion while not causing significant apoptosis (as shown by caspase-3 cleavage) or necrosis (lactate dehydrogenase release), as did the commensal strain F18, indicating that epithelial cell death was not the cause of hyposecretion. Enteropathogenic E. coli and the TTSS mutant significantly reduced cell surface expression of the apical anion channel, cystic fibrosis transmembrane conductance regulator, which is likely the mechanism behind the pathogen-induced hyposecretion. However, F18 did not cause cystic fibrosis transmembrane conductance regulator mislocalization and the commensal-induced mechanism remains unclear.

Epithelial membrane transporters expression in the developing to adult mouse vomeronasal organ and olfactory mucosa

To contribute clarifying mechanisms operating in nose chemosensory epithelia and their developmental patterns, we analyzed the expression of different epithelial membrane transporters as well as the Clara cell secretory protein, CC26 in the olfactory, vomeronasal organ (VNO), and respiratory epithelia of embryonic (E13-E19) and postnatal (P1-P60) mice by means of immunohistochemistry and reverse transcriptase-polymerase chain reaction. Results showed that CC26, cAMP-activated chloride channel (CFTR), and the water channel protein aquaporin 2, 3, 4, and 5 (AQP2, AQP3, AQP4, and AQP5) are expressed in developing to adult chemosensory epithelia with differential timing; moreover, their pattern of expression is not identical in VNO and olfactory epithelia as well as the corresponding associated glands; co-localization experiments using olfactory marker protein showed that CFTR, CC26, and AQP4 are not expressed in olfactory neurones. CFTR is expressed in sustentacular cells of the VNO and olfactory epithelium as well as blood vessels of the underlying mucosa, and VNO (but not Bowman's) glands; a similar pattern (excluding blood vessels) is present for AQP2; AQP4 is found in the two chemosensory epithelia and in Bowman's glands. AQP3 is expressed in the olfactory epithelium and the associated Bowman's glands, but not in the VNO chemosensory epithelium and glands. AQP5 is expressed in the olfactory epithelium and both Bowman's and VNO glands. These results indicate that water/ions handling as well as antioxidant mechanisms operating at the surface and/or inside the nose chemosensory epithelia start developing in utero and are maintained up to sexual maturity.

Role of adaptive immunity in alcoholic liver disease

Stimulation of innate immunity is increasingly recognized to play an important role in the pathogenesis of alcoholic liver disease (ALD), while the contribution of adaptive immunity has received less attention. Clinical and experimental data show the involvement of Th-1 and Th-17 T-lymphocytes in alcoholic hepatitis. Nonetheless, the mechanisms by which alcohol triggers adaptive immunity are still incompletely characterized. Patients with advanced ALD have circulating IgG and T-lymphocytes recognizing epitopes derived from protein modification by hydroxyethyl free radicals and end products of lipid-peroxidation. High titers of IgG against lipid peroxidation-derived antigens are associated with an increased hepatic production of proinflammatory cytokines/chemokines. Moreover, the same antigens favor the breaking of self-tolerance towards liver constituents. In particular, autoantibodies against cytochrome P4502E1 (CYP2E1) are evident in a subset of ALD patients. Altogether these results suggest that allo- and autoimmune reactions triggered by oxidative stress might contribute to hepatic inflammation during the progression of ALD.

Rab11b regulates the trafficking and recycling of the epithelial sodium channel (ENaC)

Expression of the epithelial sodium channel (ENaC) at the apical membrane of cortical collecting duct (CCD) principal cells is modulated by regulated trafficking mediated by vesicle insertion and retrieval. Small GTPases are known to facilitate vesicle trafficking, recycling, and membrane fusion events; however, little is known about the specific Rab family members that modify ENaC surface density. Using a mouse CCD cell line that endogenously expresses ENaC (mpkCCD), the channel was localized to both Rab11a- and Rab11b-positive endosomes by immunoisolation and confocal fluorescent microscopy. Expression of a dominant negative (DN) form of Rab11a or Rab11b significantly reduced the basal and cAMP-stimulated ENaC-dependent sodium (Na(+)) transport. The greatest reduction in Na(+) transport was observed with the expression of DN-Rab11b. Furthermore, small interfering RNA-mediated knockdown of each Rab11 isoform demonstrated the requirement for Rab11b in ENaC surface expression. These data indicate that Rab11b, and to a lesser extent Rab11a, is involved in establishing the constitutive and cAMP-stimulated Na(+) transport in mpkCCD cells.

Highly adaptable and sensitive protease assay based on fluorescence resonance energy transfer

Proteases are widely used in analytical sciences and play a central role in several widespread diseases. Thus, there is an immense need for highly adaptable and sensitive assays for the detection and monitoring of various proteolytic enzymes. We established a simple protease fluorescence resonance energy transfer (pro-FRET) assay for the determination of protease activities, which could in principle be adapted for the detection of all proteases. As proof of principle, we demonstrated the potential of our method using trypsin and enteropeptidase in complex biological mixtures. Briefly, the assay is based on the cleavage of a FRET peptide substrate, which results in a dramatic increase of the donor fluorescence. The assay was highly sensitive and fast for both proteases. The detection limits for trypsin and enteropeptidase in Escherichia coli lysate were 100 and 10 amol, respectively. The improved sensitivity for enteropeptidase was due to the application of an enzyme cascade, which leads to signal amplification. The pro-FRET assay is highly specific as even high concentrations of other proteases did not result in significant background signals. In conclusion, this sensitive and simple assay can be performed in complex biological mixtures and can be easily adapted to act as a versatile tool for the sensitive detection of proteases.

Enhanced Ca2+ entry due to Orai1 plasma membrane insertion increases IL-8 secretion by cystic fibrosis airways

Cystic fibrosis (CF) is caused by mutations in the gene encoding the CF transmembrane conductance regulator (CFTR). The most common mutation, ΔF508, causes retention of CFTR in the endoplasmic reticulum (ER). Some CF abnormalities can be explained by altered Ca(2+) homeostasis, although it remains unknown how CFTR influences calcium signaling. This study examined the novel hypothesis that store-operated calcium entry (SOCE) through Orai1 is abnormal in CF. The significance of Orai1-mediated SOCE for increased interleukin-8 (IL-8) expression in CF was also investigated. CF and non-CF human airway epithelial cell line and primary cells (obtained at lung transplantation) were used in Ca(2+) imaging, electrophysiology, and fluorescence imaging experiments to explore differences in Orai1 function in CF vs. non-CF cells. Protein expression and localization was assessed by Western blots, cell surface biotinylation, ELISA, and image correlation spectroscopy (ICS). We show here that store-operated Ca(2+) entry (SOCE) is elevated in CF human airway epithelial cells (hAECs; ≈ 1.8- and ≈ 2.5-fold for total Ca(2+)(i) increase and Ca(2+) influx rate, respectively, and ≈ 2-fold increase in the I(CRAC) current) and is caused by increased exocytotic insertion (≈ 2-fold) of Orai1 channels into the plasma membrane, which is normalized by rescue of ΔF508-CFTR trafficking to the cell surface. Augmented SOCE in CF cells is a major factor leading to increased IL-8 secretion (≈ 2-fold). CFTR normally down-regulates the Orai1/stromal interaction molecule 1 (STIM1) complex, and loss of this inhibition due to the absence of CFTR at the plasma membrane helps to explain the potentiated inflammatory response in CF cells.