Hyperexpression of recombinant CFTR in heterologous cells alters its physiological properties

Expression of gain-of-function CFTR in cystic fibrosis airway cells restores epithelial function better than wild-type or codon-optimized CFTR

Class Ia/b cystic fibrosis transmembrane regulator (CFTR) variants cause severe lung disease in 10% of cystic fibrosis (CF) patients and are untreatable with small-molecule pharmaceuticals. Genetic replacement of CFTR offers a cure, but its effectiveness is limited in vivo. We hypothesized that enhancing protein levels (using codon optimization) and/or activity (using gain-of-function variants) of CFTR would more effectively restore function to CF bronchial epithelial cells. Three different variants of the CFTR protein were tested: codon optimized (high codon adaptation index [hCAI]), a gain-of-function (GOF) variant (K978C), and a combination of both (hˆK978C). In human embryonic kidney (HEK293T) cells, initial results showed that hCAI and hˆK978C produced greater than 10-fold more CFTR protein and displayed ∼4-fold greater activity than wild-type (WT) CFTR. However, functionality was profoundly different in CF bronchial epithelial cells. Here, K978C CFTR more potently restored essential epithelial functions (anion transport, airway surface liquid height, and pH) than WT CFTR. hCAI and hˆK978C CFTRs had limited impact because of mislocalization in the cell. These data provide a proof of principle showing that GOF variants may be more effective than codon-optimized forms of CFTR for CF gene therapy.

Video abstract

Relationships among CFTR expression, HCO3- secretion, and host defense may inform gene- and cell-based cystic fibrosis therapies

Cystic fibrosis (CF) is caused by mutations in the gene encoding the cystic fibrosis transmembrane conductance regulator (CFTR) anion channel. Airway disease is the major source of morbidity and mortality. Successful implementation of gene- and cell-based therapies for CF airway disease requires knowledge of relationships among percentages of targeted cells, levels of CFTR expression, correction of electrolyte transport, and rescue of host defense defects. Previous studies suggested that, when ∼10-50% of airway epithelial cells expressed CFTR, they generated nearly wild-type levels of Cl(-) secretion; overexpressing CFTR offered no advantage compared with endogenous expression levels. However, recent discoveries focused attention on CFTR-mediated HCO3 (-) secretion and airway surface liquid (ASL) pH as critical for host defense and CF pathogenesis. Therefore, we generated porcine airway epithelia with varying ratios of CF and wild-type cells. Epithelia with a 50:50 mix secreted HCO3 (-) at half the rate of wild-type epithelia. Likewise, heterozygous epithelia (CFTR(+/-) or CFTR(+/∆F508)) expressed CFTR and secreted HCO3 (-) at ∼50% of wild-type values. ASL pH, antimicrobial activity, and viscosity showed similar relationships to the amount of CFTR. Overexpressing CFTR increased HCO3 (-) secretion to rates greater than wild type, but ASL pH did not exceed wild-type values. Thus, in contrast to Cl(-) secretion, the amount of CFTR is rate-limiting for HCO3 (-) secretion and for correcting host defense abnormalities. In addition, overexpressing CFTR might produce a greater benefit than expressing CFTR at wild-type levels when targeting small fractions of cells. These findings may also explain the risk of airway disease in CF carriers.

Antisera preparation and epitope mapping of a recombinant protein comprising three peptide fragments of the cystic fibrosis transmembrane conductance regulator

Antibodies targeting a single epitope of the cystic fibrosis transmembrane conductance regulator (CFTR) have been reported to influence the validity of immunological analyses; however, autoimmune mechanisms associated with CFTR epitopes are not well understood. In this study, antiserum raised against a multi-epitope recombinant protein composed of three peptide fragments of CFTR (r-CFTR-3P) was prepared and B cell epitope mapping of the protein was carried out using biosynthetic peptides. The r-CFTR-3P gene was cloned into the pSY621 expression plasmid and the protein was expressed in the BL21 strain of Escherichia coli. The rabbit r-CFTR-3P antiserum recognized the native CFTR antigen extracted from human sperm and the GST188 fusion peptides CFTR(25-36), CFTR(103-117), and CFTR(1387-1480) spanning different regions of CFTR. Four novel r-CFTR-3P B cell epitopes were identified: (29)RQRLEL(34), (104)RIIASY(109), (111)PDN(113), and (1447)VKLF(1450) of CFTR. Other proteins from various species shared sequence homology with the identified epitopes based on NCBI BLAST alignment. This study provides new tools for detecting CFTR protein and insight into the characteristics of minimal B cell epitopes of CFTR and associated immunological mechanisms.

Status of fluid and electrolyte absorption in cystic fibrosis

Salt and fluid absorption is a shared function of many of the body's epithelia, but its use is highly adapted to the varied physiological roles of epithelia-lined organs. These functions vary from control of hydration of outward-facing epithelial surfaces to conservation and regulation of total body volume. In the most general context, salt and fluid absorption is driven by active Na(+) absorption. Cl(-) is absorbed passively through various available paths in response to the electrical driving force that results from active Na(+) absorption. Absorption of salt creates a concentration gradient that causes water to be absorbed passively, provided the epithelium is water permeable. Key differences notwithstanding, the transport elements used for salt and fluid absorption are broadly similar in diverse epithelia, but the regulation of these elements enables salt absorption to be tailored to very different physiological needs. Here we focus on salt absorption by exocrine glands and airway epithelia. In cystic fibrosis, salt and fluid absorption by gland duct epithelia is effectively prevented by the loss of cystic fibrosis transmembrane conductance regulator (CFTR). In airway epithelia, salt and fluid absorption persists, in the absence of CFTR-mediated Cl(-) secretion. The contrast of these tissue-specific changes in CF tissues is illustrative of how salt and fluid absorption is differentially regulated to accomplish tissue-specific physiological objectives.

Physiological and tissue-specific vectors for treatment of inherited diseases

After more than 1500 gene therapy clinical trials in the past two decades, the overall conclusion is that for gene therapy (GT) to be successful, the vector systems must still be improved in terms of delivery, expression and safety. The recent development of more efficient and stable vector systems has created great expectations for the future of GT. Impressive results were obtained in three primary immunodeficiencies and other inherited diseases such as congenital blindness, adrenoleukodystrophy or junctional epidermolysis bullosa. However, the development of leukemia in five children included in the GT clinical trials for X-linked severe combined immunodeficiency and the silencing of the therapeutic gene in the chronic granulomatous disease clearly showed the importance of improving safety and efficiency. In this review, we focus on the main strategies available to achieve physiological or tissue-specific expression of therapeutic transgenes and discuss the importance of controlling transgene expression to improve safety. We propose that tissue-specific and/or physiological viral vectors offer the best balance between efficiency and safety and will be the tools of choice for future clinical trials in GT of inherited diseases.

CFTR gene transfer to human cystic fibrosis pancreatic duct cells using a Sendai virus vector

Cystic fibrosis (CF) is a fatal inherited disease caused by the absence or dysfunction of the CF transmembrane conductance regulator (CFTR) Cl- channel. About 70% of CF patients are exocrine pancreatic insufficient due to failure of the pancreatic ducts to secrete a HCO3- -rich fluid. Our aim in this study was to investigate the potential of a recombinant Sendai virus (SeV) vector to introduce normal CFTR into human CF pancreatic duct (CFPAC-1) cells, and to assess the effect of CFTR gene transfer on the key transporters involved in HCO3- transport. Using polarized cultures of homozygous F508del CFPAC-1 cells as a model for the human CF pancreatic ductal epithelium we showed that SeV was an efficient gene transfer agent when applied to the apical membrane. The presence of functional CFTR was confirmed using iodide efflux assay. CFTR expression had no effect on cell growth, monolayer integrity, and mRNA levels for key transporters in the duct cell (pNBC, AE2, NHE2, NHE3, DRA, and PAT-1), but did upregulate the activity of apical Cl-/HCO3- and Na+/H+ exchangers (NHEs). In CFTR-corrected cells, apical Cl-/HCO3- exchange activity was further enhanced by cAMP, a key feature exhibited by normal pancreatic duct cells. The cAMP stimulated Cl-/HCO3- exchange was inhibited by dihydro-4,4'-diisothiocyanostilbene-2,2'-disulfonic acid (H2-DIDS), but not by a specific CFTR inhibitor, CFTR(inh)-172. Our data show that SeV vector is a potential CFTR gene transfer agent for human pancreatic duct cells and that expression of CFTR in CF cells is associated with a restoration of Cl- and HCO3- transport at the apical membrane.

Sendai virus-mediated CFTR gene transfer to the airway epithelium

The potential for gene therapy to be an effective treatment for cystic fibrosis has been hampered by the limited gene transfer efficiency of current vectors. We have shown that recombinant Sendai virus (SeV) is highly efficient in mediating gene transfer to differentiated airway epithelial cells, because of its capacity to overcome the intra- and extracellular barriers known to limit gene delivery. Here, we have identified a novel method to allow the cystic fibrosis transmembrane conductance regulator (CFTR) cDNA sequence to be inserted within SeV (SeV-CFTR). Following in vitro transduction with SeV-CFTR, a chloride-selective current was observed using whole-cell and single-channel patch-clamp techniques. SeV-CFTR administration to the nasal epithelium of cystic fibrosis (CF) mice (Cftr(G551D) and Cftr(tm1Unc)TgN(FABPCFTR)#Jaw mice) led to partial correction of the CF chloride transport defect. In addition, when compared to a SeV control vector, a higher degree of inflammation and epithelial damage was found in the nasal epithelium of mice treated with SeV-CFTR. Second-generation transmission-incompetent F-deleted SeV-CFTR led to similar correction of the CF chloride transport defect in vivo as first-generation transmission-competent vectors. Further modifications to the vector or the host may make it easier to translate these studies into clinical trials of cystic fibrosis.

Discovery of α-Aminoazaheterocycle-Methylglyoxal Adducts as a New Class of High-Affinity Inhibitors of Cystic Fibrosis Transmembrane Conductance Regulator Chloride Channels

The cystic fibrosis transmembrane conductance regulator (CFTR) represents the main Cl(-) channel in the apical membrane of epithelial cells for cAMP-dependent Cl(-) secretion. Here we report on the synthesis and screening of a small library of nontoxic alpha-aminoazaheterocycle-methylglyoxal adducts, inhibitors of wild-type (WT) CFTR and G551D-, G1349D-, and F508del-CFTR Cl(-) channels. In whole-cell patch-clamp experiments of Chinese hamster ovary (CHO) cells expressing WT-CFTR, we recorded rapid and reversible inhibition of forskolin-activated CFTR currents in the presence of the adducts 5a and 8a,b at 10 pM concentrations. Using iodide efflux experiments, we compared concentration-dependent inhibition of CFTR with glibenclamide (IC(50) = 14.7 microM), 3-[(3-trifluoromethyl)phenyl]-5-[(4-carboxyphenyl-)methylene]-2-thioxo-4-thiazolidinone (CFTR(inh)-172) (IC(50) = 1.2 microM), and alpha-aminoazaheterocycle-methylglyoxal adducts and identified compounds 5a (IC(50) = 71 pM), 8a,b (IC(50) = 2.5 nM), and 7a,b (IC(50) = 3.4 nM) as the most potent inhibitors of WT-CFTR channels. Similar ranges of inhibition were also found when these compounds were evaluated on CFTR channels with the cystic fibrosis mutations F508del (in temperature-corrected human airway epithelial F508del/F508del CF15 cells)-, G551D-, and G1349D-CFTR (expressed in CHO and COS-7 cells). No effect of compound 5a was detected on the volume-regulated or calcium-regulated iodide efflux. Picomolar inhibition of WT-CFTR with adduct 5a was also found using a 6-methoxy-N-(3-sulfopropyl)-quinolinium fluorescent probe applied to the human tracheobronchial epithelial cell line 16HBE14o-. Finally, we found comparable inhibition by 5a or by CFTR(inh)-172 of forskolin-dependent short-circuit currents in mouse colon. To the best of our knowledge, these new nontoxic alpha-aminoazaheterocycle-methylglyoxal adducts represent the most potent compounds reported to inhibit CFTR chloride channels.

NHE-RF1 protein rescues DeltaF508-CFTR function

In cystic fibrosis (CF), the DeltaF508-CFTR anterograde trafficking from the endoplasmic reticulum to the plasma membrane is inefficient. New strategies for increasing the delivery of DeltaF508-CFTR to the apical membranes are thus pathophysiologically relevant targets to study for CF treatment. Recent studies have demonstrated that PDZ-containing proteins play an essential role in determining polarized plasma membrane expression of ionic transporters. In the present study we have hypothesized that the PDZ-containing protein NHE-RF1, which binds to the carboxy terminus of CFTR, rescues DeltaF508-CFTR expression in the apical membrane of epithelial cells. The plasmids encoding DeltaF508-CFTR and NHE-RF1 were intranuclearly injected in A549 or Madin-Darby canine kidney (MDCK) cells, and DeltaF508-CFTR channel activity was functionally assayed using SPQ fluorescent probe. Cells injected with DeltaF508-CFTR alone presented a low chloride channel activity, whereas its coexpression with NHE-RF1 significantly increased both the basal and forskolin-activated chloride conductances. This last effect was lost with DeltaF508-CFTR deleted of its 13 last amino acids or by injection of a specific NHE-RF1 antisense oligonucleotide, but not by NHE-RF1 sense oligonucleotide. Immunocytochemical analysis performed in MDCK cells transiently transfected with DeltaF508-CFTR further revealed that NHE-RF1 specifically determined the apical plasma membrane expression of DeltaF508-CFTR but not that of a trafficking defective mutant potassium channel (KCNQ1). These data demonstrate that the modulation of the expression level of CFTR protein partners, like NHE-RF1, can rescue DeltaF508-CFTR activity.

Molecular characterization of two founder mutations causing long QT syndrome and identification of compound heterozygous patients

BACKGROUND

Mutations of at least six different genes have been found to cause long QT syndrome (LQTS), an inherited arrhythmic disorder characterized by a prolonged QT interval on the electrocardiogram (ECG), ventricular arrhythmias and risk of sudden death.

AIM

The aims were to define the yet undetermined phenotypic characteristics of two founder mutations and to study clinical features in compound heterozygotes identified during the course of the study.

METHODS

To maximize identification of the compound heterozygotes, we used an extended group of LQTS patients comprising 700 documented or suspected cases. Functional studies were carried out upon transient expression in COS-7 or HEK293 cells.

RESULTS

The KCNQ1 IVS7-2A>G (KCNQ1-FinB) mutation associated with a mean QTc interval of 464 ms and a complete loss-of-channel function. The HERG R176W (HERG-FinB) mutation caused a reduction in current density as well as slight acceleration of the deactivation kinetics in vitro, and its carriers had a mean QTc of 448 ms. The HERG R176W mutation was also present in 3 (0.9%) out of 317 blood donors. A total of six compound heterozygotes were identified who had the HERG R176W mutation in combination with a previously reported LQTS mutation (KCNQ1 G589D or IVS7-2A>G). When present simultaneously with an apparent LQTS-causing mutation, the HERG R176W mutation may exert an additional in vivo phenotypic effect.

CONCLUSIONS

The HERG R176W mutation represents a population-prevalent mutation predisposing to LQTS. Compound heterozygosity for mutant LQTS genes may modify the clinical picture in LQTS.

Pre-clinical and clinical endpoint assays for cystic fibrosis gene therapy

The credibility and hence value of pre-clinical and clinical cystic fibrosis gene therapy studies depend on the assays used to evaluate gene transfer. Awareness of assay suitability, sensitivity and variability is therefore crucial to the design of experimental programmes. Here, we review the assays that are in use to assess the efficacy of gene transfer in pre-clinical and clinical CF gene therapy research, highlight their weaknesses and suggest possible new strategies that may help to overcome current limitations.

Transfected β3- but Not β2-Adrenergic Receptors Regulate Cystic Fibrosis Transmembrane Conductance Regulator Activity via a New Pathway Involving the Mitogen-Activated Protein Kinases Extracellular Signal-Regulated Kinases

We have shown previously that in a heterologous mammalian expression system A549 cells, beta3-adrenoceptor (beta3-AR) stimulation regulates the activity of cystic fibrosis transmembrane conductance regulator (CFTR) chloride channel. The present investigation was carried out to determine the signaling pathway involved in this regulation. A549 cells were intranuclearly injected with plasmids encoding human CFTR and beta3-AR. CFTR activity was functionally assessed by microcytofluorimetry. The application of 1 microM 4-[3-t-butylamino-2-hydroxypropoxy]benzimidazol-2-1 hydrochloride (CGP-12177), a beta3-AR agonist, produced a CFTR activation that was not abolished by protein kinase A inhibitors. In pertussis toxin-pretreated cells, the CFTR activation induced by CGP-12177 was abolished. The overexpression of beta-adrenoceptor receptor kinase, an inhibitor of betagamma subunits, abolished the CGP-12177-induced CFTR activation, suggesting the involvement of betagamma subunits of Gi/o proteins. The pretreatment of A549 cells with selective inhibitors of either phosphoinositide 3-kinase (PI3K), wortmannin, and 2-(4-morpholinyl)-8-phenyl-1(4H)-benzopyran-4-one hydrochloride (LY294002), or extracellular signal-regulated kinases 1 and 2 (ERK1/2) mitogen-activated protein kinase (MAPK), 2'-amino-3'-methoxyflavone (PD98059), and 1,4-diamino-2,3-dicyano-1,4-bis(2-aminophynyltio)butadiene (U0126), abolished the effects of CGP-12177 on the CFTR activity. Immunohistochemical assays showed that only the cells expressing beta3-AR exhibited MAPK activation in response to CGP-12177. Furthermore, CFTR activity increased in cells pretreated with 10% fetal bovine serum both in A549 cells injected only with CFTR and in T84 cells, which endogenously express CFTR, indicating that CFTR activity can be regulated by the MAPK independently of the beta3-AR stimulation. In conclusion, we have demonstrated that CFTR is regulated through a Gi/o/PI3K/ERK1/2 MAPK signaling cascade dependently or not on an activation of beta3-ARs. This pathway represents a new regulation for CFTR.

Role of alphavbeta5 integrins and vitronectin in Pseudomonas aeruginosa PAK interaction with A549 respiratory cells

Bacterial adherence to mammalian cells and their internalization are thought to participate in Pseudomonas aeruginosa pathogenicity. In this study, we explored the role of alpha5beta1 and alphavbeta5 integrins and their natural ligands, fibronectin (Fn) and vitronectin (Vn), in P. aeruginosa interaction with epithelial cells by using the PAK reference bacterial strain, A549 respiratory, and SKOV-3 human ovarian cell lines. The host cell cytoskeleton and cellular tyrosine kinases seem to be solicited during the PAK-respiratory cell interaction: cytochalasin D and genistein decreased the bacterial adherence and internalization. Blocking antibodies to alphavbeta5 integrins were the only antibodies tested to have inhibitory activity against PAK adherence to A549 cells. PAK internalization by A549 and SKOV-3 cells was markedly decreased in the presence of blocking antibodies to Vn and alphavbeta5 integrins. Addition of Vn in excess restored PAK invasion of both A549 and SKOV-3 cells in the presence of anti-Vn antibodies. Immunofluorescence experiments revealed that, in the presence of bacteria, the Vn fibrillar network disappeared, and alphavbeta5 staining was concentrated in sites where adherent bacteria were present. Taken together, these findings suggest that alphavbeta5 integrins, and their natural ligand Vn, are involved in PAK entry into human epithelial cells.

A Common Antitussive Drug, Clobutinol, Precipitates the Long QT Syndrome 2

QT prolongation, a classic risk factor for arrhythmias, can result from a mutation in one of the genes governing cardiac repolarization and also can result from the intake of a medication acting as blocker of the cardiac K(+) channel human ether-a-go-go-related gene (HERG). Here, we identified the arrhythmogenic potential of a nonopioid antitussive drug, clobutinol. The deleterious effects of clobutinol were suspected when a young boy, with a diagnosis of congenital long QT syndrome, experienced arrhythmias while being treated with this drug. Using the patch-clamp technique, we showed that clobutinol dose-dependently inhibited the HERG K(+) current with a half-maximum block concentration of 2.9 microM. In the proband, we identified a novel A561P HERG mutation. Two others long QT mutations (A561V and A561T) had been reported previously at the same position. None of the three mutants led to a sizeable current in heterologous expression system. When coexpressed with wild-type (WT) HERG channels, the three Ala561 mutants reduced the trafficking of WT and mutant heteromeric channels, resulting in decreased K(+) current amplitude (dominant-negative effects). In addition, A561P but not A561V and A561T mutants induced a approximately -11 mV shift of the current activation curve and accelerated deactivation, thereby partially counteracting the dominant-negative effects. A561P mutation and clobutinol effects on the human ventricular action potential characteristics were simulated using the Priebe-Beuckelmann model. Our work shows that clobutinol has limited effects on WT action potential but should be classified as a "drug to be avoided by congenital long QT patients" rather than as a "drug with risk of torsades de pointes".

Partial correction of endogenous DeltaF508 CFTR in human cystic fibrosis airway epithelia by spliceosome-mediated RNA trans-splicing

Spliceosome-mediated RNA trans-splicing (SMaRT) was investigated as a means for functionally correcting endogenous DeltaF508 cystic fibrosis transmembrane conductance regulator (CFTR) transcripts using in vitro human cystic fibrosis (CF) polarized airway epithelia and in vivo human CF bronchial xenografts. Recombinant adenovirus (Ad.CFTR-PTM) encoding a pre-therapeutic molecule (PTM) targeted to CFTR intron 9 corrected transepithelial cyclic AMP (cAMP)-sensitive short-circuit current (Isc) in DeltaF508 homozygous epithelia to a level 16% of that observed in normal human bronchial epithelia. Molecular analyses using RT-PCR and western blotting confirmed SMaRT-mediated partial correction of endogenous DeltaF508 messenger RNA (mRNA) transcripts and protein. In an in vivo model of DeltaF508 CF airway epithelia, human CF bronchial xenografts infected with Ad.CFTR-PTM also demonstrated partial correction of CFTR-mediated Cl- permeability at a level 22% of that seen in non-CF xenografts. These results provide functional evidence for SMaRT-mediated repair of mutant endogenous CFTR mRNA in intact polarized CF airway epithelial models.

Novel SCN5A mutation leading either to isolated cardiac conduction defect or Brugada syndrome in a large French family

BACKGROUND

The SCN5A gene encoding the human cardiac sodium channel alpha subunit plays a key role in cardiac electrophysiology. Mutations in SCN5A lead to a large spectrum of phenotypes, including long-QT syndrome, Brugada syndrome, and isolated progressive cardiac conduction defect (Lenègre disease).

METHODS AND RESULTS

In the present study, we report the identification of a novel single SCN5A missense mutation causing either Brugada syndrome or an isolated cardiac conduction defect in the same family. A G-to-T mutation at position 4372 was identified by direct sequencing and was predicted to change a glycine for an arginine (G1406R) between the DIII-S5 and DIII-S6 domain of the sodium channel protein. Among 45 family members, 13 were carrying the G1406R SCN5A mutation. Four individuals from 2 family collateral branches showed typical Brugada phenotypes, including ST-segment elevation in the right precordial leads and right bundle branch block. One symptomatic patient with the Brugada phenotype required implantation of a cardioverter-defibrillator. Seven individuals from 3 other family collateral branches had isolated cardiac conduction defects but no Brugada phenotype. Three flecainide test were negative. One patient with an isolated cardiac conduction defect had an episode of syncope and required pacemaker implantation. An expression study of the G1406R-mutated SCN5A showed no detectable Na(+) current but normal protein trafficking.

CONCLUSIONS

We conclude that the same mutation in the SCN5A gene can lead either to Brugada syndrome or to an isolated cardiac conduction defect. Our findings suggest that modifier gene(s) may influence the phenotypic consequences of a SCN5A mutation.

Inactivating properties of recombinant ROMK2 channels expressed in mammalian cells

Biophysical properties of ROMK2 channel were investigated at physiological temperature, after reexpression of the recombinant ROMK2 protein in a mammalian cell expression system (COS-7). We observed that ROMK2 induced an inwardly rectifying K(+) current whether polyvalent cations were present or not. Above +10 mV, ROMK2-induced current exhibited a voltage- and time-dependent decay, consistent with an inactivation process. Inactivation of ROMK2-induced current was also seen in inside out patch from ROMK2-expressing Xenopus oocyte. In COS-7 cells, inactivation was found to account for most of the inward rectification. Mg(2+) and spermine modulated rectification by accelerating inactivation kinetics independently of membrane potential. These results establish for the first time ROMK2 properties in a mammalian cell expression system.

Targeting transgene expression for cystic fibrosis gene therapy

We have developed an expression cassette for cystic fibrosis (CF) gene therapy using control elements from the human cytokeratin 18 gene (KRT18, also known as K18). KRT18 is naturally expressed in a spatial pattern similar to that of CFTR, the gene mutated in CF. We delivered a KRT18-driven lacZ plasmid complexed with cationic liposomes intravenously to mice and examined expression in various tissues. We found expression in nasal and bronchial epithelium, airway submucosal glands, gall bladder, and kidneys. Expression was low in pancreas and gut, and absent from liver and alveolar lung. This is consistent with the expression pattern reported for a K18lacZ transgenic mouse. Following delivery of a cytomegalovirus (CMV) major immediate-early promoter/enhancer-driven lacZ plasmid, we found expression in bronchi, submucosal glands, alveolar cells, liver, and kidney. We did not detect expression in nose, pancreas, gall bladder, or gut. Using fluorescently labeled plasmid delivered by means of liposomes, we identified the liver, alveolar lung, and kidneys as the major plasmid deposition sites. Our data demonstrate that a KRT18-driven expression vector delivered systemically can target gene expression to CF-affected tissues, despite an uneven distribution of plasmid DNA. A KRT18-based vector may be a useful alternative to viral promoter-based vectors in clinical gene therapy trials to treat CF.

Regulated Expression of the Human CFTR Gene in Epithelial Cells

We developed an epithelium-specific, inducible cystic fibrosis transmembrane conductance regulator (CFTR) expression system. In this system we used a human cytokeratin 18 expression cassette to drive epithelium-specific expression of the reverse tetracycline transactivator (rtTA), which turns on CFTR expression from a Tet-inducible promoter in the presence of doxycycline. CFTR expression was monitored by reverse-transcription polymerase chain reaction, immunostaining, and Western blotting. We confirmed that protein expression was dose-dependent in double stable transfected cell lines, with no detectable protein in the absence of doxycycline. However, low levels of CFTR mRNA could be detected in the uninduced state. When clones capable of inducing high levels of CFTR expression were analyzed, we observed a decrease in cell proliferation, consistent with reports in other cell lines (NIH3T3 and BTS). We generated transgenic mice expressing rtTA from the K18 expression cassette and demonstrated that the system retained its tissue specificity for lacZ reporter expression in vivo. When mice were induced with doxycycline, high levels of expression were found in the trachea, upper bronchi, and submucosal glands. Therefore, this inducible system can improve our understanding of the role of CFTR in the lung and should help in the design of safe and effective CF therapies.

AKAP proteins anchor cAMP-dependent protein kinase to KvLQT1/IsK channel complex

In cardiac myocytes, the slow component of the delayed rectifier K(+) current (I(Ks)) is regulated by cAMP. Elevated cAMP increases I(Ks) amplitude, slows its deactivation kinetics, and shifts its activation curve. At the molecular level, I(Ks) channels are composed of KvLQT1/IsK complexes. In a variety of mammalian heterologous expression systems maintained at physiological temperature, we explored cAMP regulation of recombinant KvLQT1/IsK complexes. In these systems, KvLQT1/IsK complexes were totally insensitive to cAMP regulation. cAMP regulation was not restored by coexpression with the dominant negative isoform of KvLQT1 or with the cystic fibrosis transmembrane regulator. In contrast, coexpression of the neuronal A kinase anchoring protein (AKAP)79, a fragment of a cardiac AKAP (mAKAP), or cardiac AKAP15/18 restored cAMP regulation of KvLQT1/IsK complexes inasmuch as cAMP stimulation increased the I(Ks) amplitude, increased its deactivation time constant, and negatively shifted its activation curve. However, in cells expressing an AKAP, the effects of cAMP stimulation on the I(Ks) amplitude remained modest compared with those previously reported in cardiac myocytes. The effects of cAMP stimulation were fully prevented by including the Ht31 peptide (a global disruptor of protein kinase A anchoring) in the intracellular medium. We concluded that cAMP regulation of I(Ks) requires protein kinase A anchoring by AKAPs, which therefore participate with the channel protein complex underlying I(Ks).

ClC-2 contributes to native chloride secretion by a human intestinal cell line, Caco-2

It has been previously determined that ClC-2, a member of the ClC chloride channel superfamily, is expressed in certain epithelial tissues. These findings fueled speculation that ClC-2 can compensate for impaired chloride transport in epithelial tissues affected by cystic fibrosis and lacking the cystic fibrosis transmembrane conductance regulator. However, direct evidence linking ClC-2 channel expression to epithelial chloride secretion was lacking. In the present studies, we show that ClC-2 transcripts and protein are present endogenously in the Caco-2 cell line, a cell line that models the human small intestine. Using an antisense strategy we show that ClC-2 contributes to native chloride currents in Caco-2 cells measured by patch clamp electrophysiology. Antisense ClC-2-transfected monolayers of Caco-2 cells exhibited less chloride secretion (monitored as iodide efflux) than did mock transfected monolayers, providing the first direct molecular evidence that ClC-2 can contribute to chloride secretion by the human intestinal epithelium. Further, examination of ClC-2 localization by confocal microscopy revealed that ClC-2 contributes to secretion from a unique location in this epithelium, from the apical aspect of the tight junction complex. Hence, these studies provide the necessary rationale for considering ClC-2 as a possible therapeutic target for diseases affecting intestinal chloride secretion such as cystic fibrosis.

Repair of CFTR mRNA by spliceosome-mediated RNA trans-splicing

Most messenger RNA precursors (pre-mRNA) undergo cis-splicing in which introns are excised and the adjoining exons from a single pre-mRNA are ligated together to form mature messenger RNA. This reaction is driven by a complex known as the spliceosome. Spliceosomes can also combine sequences from two independently transcribed pre-mRNAs in a process known as trans-splicing. Spliceosome-mediated RNA trans-splicing (SMaRT) is an emerging technology in which RNA pre-therapeutic molecules (PTMs) are designed to recode a specific pre-mRNA by suppressing cis-splicing while enhancing trans-splicing between the PTM and its pre-mRNA target. This study examined the feasibility of SMaRT as a potential therapy for genetic diseases to correct mutations using cystic fibrosis (CF) as an example. We used several versions of a cystic fibrosis transmembrane conductance regulator (CFTR) mini-gene expressing mutant (deltaF508) pre-mRNA targets and tested this against a number of PTMs capable of binding to the CFTR target intron 9 and trans-splicing in the normal coding sequences for exons 10-24 (containing F508). When 293T cells were cotransfected with both constructs, they produced a trans-spliced mRNA in which normal exon 10-24 replaced mutant exon 10. To test whether SMaRT produced mature CFTR protein, proteins were immunoprecipitated from lysates of cotransfected cells and detected by Western blotting and PKA-phosphorylation. Tryptic phosphopeptide mapping confirmed the identity of CFTR. This proof-of-concept study demonstrates that exon replacement by SMaRT can repair an abnormal pre-mRNA associated with a genetic disease.

Forskolin-induced apical membrane insertion of virally expressed, epitope-tagged CFTR in polarized MDCK cells

Channel gating of the cystic fibrosis transmembrane conductance regulator (CFTR) is activated in response to cAMP stimulation. In addition, CFTR activation may also involve rapid insertion of a subapical pool of CFTR into the plasma membrane (PM). However, this issue has been controversial, in part because of the difficulty in distinguishing cell surface vs. intracellular CFTR. Recently, a fully functional, epitope-tagged form of CFTR (M2-901/CFTR) that can be detected immunologically in nonpermeabilized cells was characterized (Howard M, Duvall MD, Devor DC, Dong J-Y, Henze K, and Frizzell RA. Am J Physiol Cell Physiol 269: C1565-C1576, 1995; and Schultz BD, Takahashi A, Liu C, Frizzell RA, and Howard M. Am J Physiol Cell Physiol 273: C2080-C2089, 1997). We have developed replication-defective recombinant adenoviruses that express M2-901/CFTR and used them to probe cell surface CFTR in forskolin (FSK)-stimulated polarized Madin-Darby canine kidney (MDCK) cells. Virally expressed M2-901/CFTR was functional and was readily detected on the apical surface of FSK-stimulated polarized MDCK cells. Interestingly, at low multiplicity of infection, we observed FSK-stimulated insertion of M2901/CFTR into the apical PM, whereas at higher M2-901/CFTR expression levels, no increase in surface expression was detected using indirect immunofluorescence. Immunoelectron microscopy of unstimulated and FSK-stimulated cells confirmed the M2-901/CFTR redistribution to the PM upon FSK stimulation and demonstrates that the apically inserted M2-901/CFTR originates from a population of subapical vesicles. Our observations may reconcile previous conflicting reports regarding the effect of cAMP stimulation on CFTR trafficking.

Differential expression of KvLQT1 isoforms across the human ventricular wall

Long Q-T mutant (KvLQT1) K(+) channels associate with their regulatory subunit IsK to produce the slow component of the delayed rectifier potassium (I(Ks)) cardiac current. The amplitude of KvLQT1 current depends on the expression of a KvLQT1 splice variant (isoform 2) that exerts strong dominant negative effects on the full-length KvLQT1 protein (isoform 1). We used RNase protection assays to determine the relative expression of KvLQT1 isoforms 1 and 2 and IsK mRNAs in human ventricular layers. Overall expression of KvLQT1 and IsK genes was similar in the three layers. However, there was a significant difference in the ratio between KvLQT1 isoforms 1 and 2. Isoform 2 represented 25.2 +/- 2.3%, 31.7 +/- 1.2%, and 24.9 +/- 1.7% of total KvLQT1 expression in left ventricular endocardial, midmyocardial, and epicardial tissues, respectively. Similar data were obtained from right ventricular samples. COS-7 cells were intranuclearly injected with KvLQT1 isoforms 1 or 2 plus IsK cDNAs, using two different isoform 2-to-isoform 1 ratios. Cells injected with an isoform 2-to-isoform 1 ratio mimicking that in the midmyocardium showed a K(+) current with approximately 75% reduced amplitude compared with those injected with a ratio mimicking that in the epicardium. Our results suggest that differential expression of KvLQT1 isoform 2 in endocardial, midmyocardial, and epicardial tissues is responsible for differential I(Ks) amplitude and contributes to the regional action potential heterogeneity observed across the ventricular wall.

Activation of the epithelial Na+ channel (ENaC) requires CFTR Cl- channel function

It is increasingly being recognized that cells coordinate the activity of separate ion channels that allow electrolytes into the cell. However, a perplexing problem in channel regulation has arisen in the fatal genetic disease cystic fibrosis, which results from the loss of a specific Cl- channel (the CFTR channel) in epithelial cell membranes. Although this defect clearly inhibits the absorption of Na+ in sweat glands, it is widely accepted that Na+ absorption is abnormally elevated in defective airways in cystic fibrosis. The only frequently cited explanation for this hypertransport is that the activity of an epithelial Na+ channel (ENaC) is inversely related to the activity of the CFTR Cl- channel. However, we report here that, in freshly isolated normal sweat ducts, ENaC activity is dependent on, and increases with, CFTR activity. Surprisingly, we also find that the primary defect in Cl- permeability in cystic fibrosis is accompanied secondarily by a Na+ conductance in this tissue that cannot be activated. Thus, reduced salt absorption in cystic fibrosis is due not only to poor Cl- conductance but also to poor Na+ conductance.

Mutations in a dominant-negative isoform correlate with phenotype in inherited cardiac arrhythmias

The long QT syndrome is characterized by prolonged cardiac repolarization and a high risk of sudden death. Mutations in the KCNQ1 gene, which encodes the cardiac KvLQT1 potassium ion (K+) channel, cause both the autosomal dominant Romano-Ward (RW) syndrome and the recessive Jervell and Lange-Nielsen (JLN) syndrome. JLN presents with cardiac arrhythmias and congenital deafness, and heterozygous carriers of JLN mutations exhibit a very mild cardiac phenotype. Despite the phenotypic differences between heterozygotes with RW and those with JLN mutations, both classes of variant protein fail to produce K+ currents in cultured cells. We have shown that an N-terminus-truncated KvLQT1 isoform endogenously expressed in the human heart exerts strong dominant-negative effects on the full-length KvLQT1 protein. Because RW and JLN mutations concern both truncated and full-length KvLQT1 isoforms, we investigated whether RW or JLN mutations would have different impacts on the dominant-negative properties of the truncated KvLQT1 splice variant. In a mammalian expression system, we found that JLN, but not RW, mutations suppress the dominant-negative effects of the truncated KvLQT1. Thus, in JLN heterozygous carriers, the full-length KvLQT1 protein encoded by the unaffected allele should not be subject to the negative influence of the mutated truncated isoform, leaving some cardiac K+ current available for repolarization. This is the first report of a genetic disease in which the impact of a mutation on a dominant-negative isoform correlates with the phenotype.

beta3-adrenoceptor control the cystic fibrosis transmembrane conductance regulator through a cAMP/protein kinase A-independent pathway

In human cardiac myocytes, we have previously identified a functional beta3-adrenoceptor in which stimulation reduces action potential duration. Surprisingly, in cardiac biopsies obtained from cystic fibrosis patients, beta3-adrenoceptor agonists produced no effects on action potential duration. This result suggests the involvement of cystic fibrosis transmembrane conductance regulator (CFTR) chloride current in the electrophysiological effects of beta3-adrenoceptor stimulation in non-cystic fibrosis tissues. We therefore investigated the control of CFTR activity by human beta3-adrenoceptors in a recombinant system: A549 human cells were intranuclearly injected with plasmids encoding CFTR and beta3-adrenoceptors. CFTR activity was functionally assayed using the 6-methoxy-N-(3-sulfopropyl)quinolinium fluorescent probe and the patch-clamp technique. Injection of CFTR-cDNA alone led to the expression of a functional CFTR protein activated by cAMP or cGMP. Co-expression of CFTR (but not of mutated DeltaF508-CFTR) with high levels of beta3-adrenoceptor produced an increased halide permeability under base-line conditions that was not further sensitive to cAMP or beta3-adrenoceptor stimulation. Patch-clamp experiments confirmed that CFTR channels were permanently activated in cells co-expressing CFTR and a high level of beta3-adrenoceptor. Permanent CFTR activation was not associated with elevated intracellular cAMP or cGMP levels. When the expression level of beta3-adrenoceptor was lowered, CFTR was not activated under base-line conditions but became sensitive to beta3-adrenoceptor stimulation (isoproterenol plus nadolol, SR 58611, or CGP 12177). This later effect was not prevented by protein kinase A inhibitors. Our results provide molecular evidence that CFTR but not mutated DeltaF508-CFTR is regulated by beta3-adrenoceptors expression through a protein kinase A-independent pathway.

Mechanisms of carbon acquisition for endosymbiont photosynthesis in Anthozoa

In contrast to free-living photoautotrophs, endosymbiontic dinoflagellates of the genus Symbiodinium must absorb their inorganic carbon from the cytoplasm of their host anthozoan cell rather then from seawater. The purpose of this paper is to review the present knowledge on the source of dissolved inorganic carbon supply for endosymbiont photosynthesis and the transport mechanisms involved. Symbiodinium spp., generally known as zooxanthellae, live within the endodermal cells of their hosts, corals and sea anemones. They are separated from the surrounding seawater by the host tissues (oral ectodermal cell layer, collagenous basal membrane, endodermal cell, and perisymbiotic vesicles). The symbiotic association is therefore faced with the problem of delivering dissolved inorganic carbon to an endodermal site of consumption from an, essentially, ectodermal site of availability. Studies using original methods demonstrated that neither the internal medium (coelenteric fluid) nor paracellular diffusion could supply enough dissolved inorganic carbon for endosymbiont photosynthesis. A transepithelial active mechanism must be present in the host tissues to maintain the photosynthetic rate under saturating irradiance. A pharmacological approach led to propose a working model of dissolved inorganic carbon transport from seawater to zooxanthellae. This vectorial transport generates a pH gradient across the epithelium. The role of this gradient as well as the physiological adaptation of Symbiodinium spp. to symbiotic life are discussed.Key words: carbon concentrating mechanism, anthozoan, dinoflagellates, anion transport, symbiosis, transepithelial transport.