Chloride secretion in the trachea of null cystic fibrosis mice: the effects of transfection with pTrial10-CFTR2

Expression of CFTR from a ciliated cell-specific promoter is ineffective at correcting nasal potential difference in CF mice

Successful gene therapy will require that the therapeutic gene be expressed at a sufficient level in the correct cell type(s). To improve the specificity of gene transfer for cystic fibrosis (CF) and other airway diseases, we have begun to develop cell-type specific promoters to target the expression of transgenes to specific airway cell types. Using a FOXJ1 promoter construct previously shown to direct transgene expression specifically to ciliated cells, we have generated transgenic mice expressing human cystic fibrosis transmembrane conductance regulator (CFTR) in the murine tracheal and nasal epithelia. RNA analysis demonstrated levels of CFTR expression is greater than or equal to the level of endogenous mouse CFTR. Immunoprecipitation and western blotting demonstrated the production of human CFTR protein, and immunochemistry confirmed that CFTR was expressed in the apical region of ciliated cells. The transgenic animals were bred to CFTR null mice (Cftr(tm1Unc)) to determine if expression of CFTR from the FOXJ1 promoter is capable of correcting the airway defects in Cl(-) secretion and Na(+) absorption that accompany CF. Isolated trachea from neonatal CF mice expressing the FOXJ1/CFTR transgene demonstrated a correction of forskolin-stimulated Cl(-) secretion. However, expression of human CFTR in ciliated cells of the nasal epithelia failed to significantly change the nasal bioelectrics of the CF mice.

Culture of murine nasal epithelia: model for cystic fibrosis

The ion transport defects reported for human cystic fibrosis (CF) airways are reproduced in nasal epithelia of the CF mouse. Although this tissue has been studied in vivo using the nasal potential difference technique and as a native tissue mounted in the Ussing chamber, little information is available on cultured murine nasal epithelia. We have developed a polarized cell culture model of primary murine nasal epithelia in which the CF tissue exhibits not only a defect in cAMP-mediated Cl- secretion but also the Na+ hyperabsorption and upregulation of the Ca2+-activated Cl- conductance observed in human airways. Both the wild-type and CF cultures were constituted predominantly of undifferentiated cuboidal columnar cells, with most cultures exhibiting a small number of ciliated cells. Although no goblet cells were observed, RT-PCR demonstrated the expression of Muc5ac RNA after approximately 22 days in culture. The CF tissue exhibited an adherent layer of mucus similar to the mucus plaques reported in the distal airways of human CF patients. Furthermore, we found that treatment of CF preparations with a Na+ channel blocker for 7 days prevented formation of mucus adherent to epithelial surfaces. The cultured murine nasal epithelial preparation should be an excellent model tissue for gene transfer studies and pharmacological studies of Na+ channel blockers and mucolytic agents as well as for further characterization of CF ion transport defects. Culture of nasal epithelia from DeltaF508 mice will be particularly useful in testing drugs that allow DeltaF508 CFTR to traffic to the membrane.

Emerging drug treatments for cystic fibrosis

Cystic fibrosis (CF) is one of the most common life-shortening inherited disorders. Mutations in the cystic fibrosis transmembrane regulator (CFTR) gene disrupt the localisation and function of the cAMP-mediated chloride channel. Most of the morbidity and mortality arise from the lung disease which is characterised by excessive inflammation and chronic infection. Research into the mechanisms of wild-type and mutant CFTR biogenesis suggest that multiple drug targets can be identified. This review explores the current understanding of the nature of the different mutant CFTR forms and the potential for repair of the chloride channel defect. High-throughput screening, pharmacogenomics and proteomics bring recent technological advances to the field.

Transport properties of the human intestinal anion exchanger DRA (down-regulated in adenoma) in transfected HEK293 cells

Electroneutral NaCl absorption in the intestine is mediated by parallel Na+/H+ and Cl-/HCO3- exchange. Mutations in the down-regulated in adenoma (DRA) gene cause congenital chloride diarrhoea but the transport characteristics of human DRA have not been studied in a heterologous human expression system. A N-terminal enhanced green fluorescence protein (EGFP)-tagged human DRA construct was therefore expressed stably in HEK293 cells. Cl-/HCO3- exchange was assessed by measuring intracellular pH and intracellular Cl- using fluorescent dyes. Expression of DRA resulted in the appearance of EGFP fluorescence and DRA immunoreactivity consistent with a location in the plasma membrane and possibly structures below the plasma membrane. DRA mediated electroneutral Cl-/HCO3- exchange but OH- was not transported and SO4(2-)/HCO3- exchange was minimal. In the presence of 5% CO2/HCO3- the apparent affinity of DRA for Cl- in transfected HEK cells was 23-36 mM, which is lower than that reported for rabbit ileal brush border membrane vesicles and for oocytes injected with human DRA. DRA was inhibited by 4 mM DIDS (45+/-11%), by 50 microM tenidap (71+/-8%) and by 100 microM glibenclamide (59+/-22% inhibition of HCO3- transport and 79+/-3% inhibition of Cl- transport). The effects of DIDS and tenidap were not additive to those of glibenclamide.

Disease genes: flattery and deception

Completion of the human genome project raises the possibility of genetically based treatments for a multitude of human diseases. As yet only a handful of patients have benefited clinically from this approach. Why gene transfer is such a complex issue is discussed in this article. Theoretically, the easiest diseases to treat are single gene recessive diseases, where, presumably, gene delivery to somatic cells is all that is required. Two prime candidates for gene therapy are severe combined immunodeficiency disease (SCID) and cystic fibrosis (CF). Attempts to treat both of these diseases by gene therapy commenced in the late 1980s. Some clinical benefit has been recorded with SCID, but none, as yet, has been recorded with CF.

Optimisation of real-time quantitative RT-PCR for the evaluation of non-viral mediated gene transfer to the airways

Naked plasmid DNA and DNA/liposome complexes are currently being considered as gene therapy treatments for cystic fibrosis (CF) pulmonary disease. Current methods of gene delivery to the airways result only in transient correction of the CF ion transport defect, and disease treatment is likely to require repeated administrations of vector. However, it is unclear if repeat administration will be tolerated by CF individuals. Technologies including TaqMan (Applied Biosystems) real-time quantitative reverse transcription-polymerase chain reaction (RT-PCR) can be used to determine the efficacy of gene transfer formulations. TaqMan RT-PCR assays were designed and optimised to detect plasmid vector-derived and endogenous gene expression. Subsequently, these assays were used to quantify vector-derived mRNA after delivery of naked DNA and DNA/liposome formulations expressing human and murine cystic fibrosis transmembrane conductance regulator (CFTR) to the mouse airways. Vector-derived mRNA was detected in samples following the delivery of naked DNA or DNA/liposomes to the mouse airways, and no reduction in vector-derived mRNA was observed upon repeat administration, a finding that is consistent with the murine and human CFTR being tolerated by the mouse. Although it remains to be seen if CF patients can tolerate long-term expression of wild-type CFTR, these data demonstrate that TaqMan RT-PCR is an effective tool to accurately quantify transgene expression in the airways.

A prime-boost immunisation regimen using DNA followed by recombinant modified vaccinia virus Ankara induces strong cellular immune responses against the Plasmodium falciparum TRAP antigen in chimpanzees

Two chimpanzees were vaccinated intramuscularly against malaria using plasmid DNA expressing the pre-erythrocytic antigens thrombospondin related adhesion protein (PfTRAP) and liver stage specific antigen-1 (PfLSA-1) of Plasmodium falciparum together with GM-CSF protein. A recombinant modified vaccinia virus Ankara (MVA) expressing PfTRAP was injected intramuscularly 6 weeks later to boost the immune response. This sequence of antigen delivery induced a specific and long-lasting T cell and antibody response to PfTRAP as detected by ELISPOT assay and ELISA. Antibody responses were detected after four DNA injections, and were boosted by injection of recombinant MVA expressing PfTRAP. Interferon-gamma secreting antigen-specific T cells were detected in both animals, but only after boosting with recombinant MVA. By screening a panel of PfTRAP-derived peptides, an epitope was identified that was recognized by cytotoxic T lymphocytes in one of the chimpanzees studied. T cells specific for this epitope were present in PBMCs and liver-infiltrating lymphocytes at a frequency of between 1 in 200 and 1 in 500. The high immunogenicity of this prime-boost regimen in chimpanzees supports further assessment of this delivery strategy for the induction of protection against P. falciparum malaria in humans.

Alterations in airway ion transport in NKCC1-deficient mice

Airways of Na(+)-K(+)-2Cl(-) (NKCC1)-deficient mice (-/-) were studied in Ussing chambers to determine the role of the basolateral NKCC1 in transepithelial anion secretion. The basal short-circuit current (I(sc)) of tracheae and bronchi from adult mice did not differ between NKCC1-/- and normal mice, whereas NKCC1-/- tracheae from neonatal mice exhibited a significantly reduced basal I(sc). In normal mouse tracheae, sensitivity to the NKCC1 inhibitor bumetanide correlated inversely with the age of the mouse. In contrast, tracheae from NKCC1-/- mice at all ages were insensitive to bumetanide. The anion secretory response to forskolin did not differ between normal and NKCC1-/- tissues. However, when larger anion secretory responses were induced with UTP, airways from the NKCC1-/- mice exhibited an attenuated response. Ion substitution and drug treatment protocols suggested that HCO secretion compensated for reduced Cl(-) secretion in NKCC1-/- airway epithelia. The absence of spontaneous airway disease or pathology in airways from the NKCC1-/- mice suggests that the NKCC1 mutant mice are able to compensate adequately for absence of the NKCC1 protein.

Genistein activates CFTR-mediated Cl(-) secretion in the murine trachea and colon

The action of the isoflavone genistein on the cystic fibrosis transmembrane conductance regulator (CFTR) has been studied in many cell systems but not in intact murine tissues. We have investigated the action of genistein on murine tissues from normal and cystic fibrosis (CF) mice. Genistein increased the short-circuit current (I(sc)) in tracheal (16.4 +/- 2.8 microA/cm(2)) and colonic (40.0 +/- 4.4 microA/cm(2)) epithelia of wild-type mice. This increase was inhibited by furosemide, diphenylamine-2-carboxylate, and glibenclamide, but not by DIDS. In contrast, genistein produced no significant change in the I(sc) of the tracheal epithelium (0.9 +/- 1.1 microA/cm(2)) and decreased the I(sc) of colons from CF null (-13.1 +/- 2.3 microA/cm(2)) and DeltaF508 mice (-10.3 +/- 1.3 microA/cm(2)). Delivery of a human CFTR cDNA-liposome complex to the airways of CF null mice restored the genistein response in the tracheas to wild-type levels. Tracheas from DeltaF508 mice were also studied: 46% of trachea showed no response to genistein, whereas 54% gave an increase in I(sc) similar to that in wild type. We conclude that genistein activates CFTR-mediated Cl(-) secretion in the murine trachea and distal colon.

Repeat administration of DNA/liposomes to the nasal epithelium of patients with cystic fibrosis

The major cause of mortality in patients with cystic fibrosis (CF) is lung disease. Expression of the cystic fibrosis transmembrane conductance regulator (CFTR) gene product in the airways is a potential treatment. Clinical studies in which the CFTR cDNA was delivered to the respiratory epithelia of CF patients have resulted in modest, transient gene expression. It seems likely that repeated administration of the gene transfer vector will be required for long-term gene expression. We have undertaken a double-blinded study in which multiple doses of a DNA/liposome formulation were delivered to the nasal epithelium of CF patients. Ten subjects received plasmid DNA expressing the CFTR cDNA complexed with DC-Chol/DOPE cationic liposomes, whilst two subjects received placebo. Each subject received three doses, administered 4 weeks apart. There was no evidence of inflammation, toxicity or an immune response towards the DNA/liposomes or the expressed CFTR. Nasal epithelial cells were collected 4 days after each dose for a series of efficacy assays including quantitation of vector-specific DNA and mRNA, immunohistochemistry of CFTR protein, bacterial adherence, and detection of halide efflux ex vivo. Airway ion transport was also assessed in vivo by repeated nasal potential difference (PD) measurements. On average, six of the treated subjects were positive for CFTR gene transfer after each dose. All subjects positive for CFTR function were also positive for plasmid DNA, plasmid-derived mRNA and CFTR protein. The efficacy measures suggest that unlike high doses of recombinant adenoviral vectors, DNA/liposomes can be successfully re-administered without apparent loss of efficacy.

A murine tracheal culture system to investigate parameters affecting gene therapy for cystic fibrosis

Cystic fibrosis (CF) is a life-threatening condition caused by mutations in the cystic fibrosis transmembrane conductance regulator gene (CFTR). Delivery of the CFTR gene to the airways offers a potential treatment for CF but requires improvement in efficiency to obtain clinical benefit. We have developed a murine tracheal culture system that maintains tissue integrity as judged by normal histological appearance, high transepithelial resistance and electrophysiological responses similar to fresh tissue. This ex vivo system allows precise control of gene delivery parameters to a structure that retains the in vivo cellular architecture. We have demonstrated correction of CFTR-dependent Cl- secretion following ex vivo delivery of the CFTR gene to tracheas from CF null mice. We have used this system to examine parameters affecting liposome-mediated gene delivery to the upper airway such as plasmid dose. We have also found that a contact time of 1 min for the transfection mixture is sufficient to achieve significant DNA binding and maximal reporter gene expression.

Importance of basolateral K+ conductance in maintaining Cl- secretion in murine nasal and colonic epithelia

1. Epithelia lining the nasal passages and descending colon of wild-type and cystic fibrosis (CF) mice were examined by the short-circuit current technique. Additionally, intracellular Ca2+ ion determinations were made in nasal epithelial cells. Forskolin produced anion secretory currents in wild-type and CF nasal epithelia. It produced similar effects in wild-type colonic epithelia, but not in colonic epithelia from CF mice. 2. After electrogenic Na+ transport was blocked with amiloride and electrogenic Cl- secretion was stimulated with forskolin, the ability of K+ channel blockers to inhibit the forskolin-induced Cl- current was determined. The order of efficiency for nasal epithelium was: Ba2+ > clofilium >>> TEA = azimilide >>> trans-6-cyano-4-(N-ethylsulphonyl-N-methylamino)-3-hydroxy-2, 2-dimethyl-chromane (293B) = charybdotoxin, whereas for the colonic epithelium the order was: Ba2+ = 293B >>> azimilide = TEA >>> clofilium = charybdotoxin. 3. 1-Ethyl-2-benzimdazolinone (1-EBIO) was able to generate large Cl--secretory currents in colonic epithelia which were partially sensitive to charybdotoxin, with the remaining current being inhibited by 293B. In nasal epithelia 1-EBIO produced only a small transient effect on current. 4. Forskolin released intracellular Ca2+ in nasal epithelial cells; this activity was attenuated when more powerful Ca2+-releasing agents were applied first. 5. It is concluded that an action on basolateral cAMP-sensitive K+ channels is an important determinant of the maintained responses to forskolin in nasal and colonic epithelia, in addition to the effects on the cystic fibrosis transmembrane conductance regulator (CFTR) in the apical membrane. In CF nasal epithelia the activation of calcium-activated chloride channels (CACs) substitutes for the effect on CFTR. On the basis of the different orders of potency of the blocking agents and the differential response to 1-EBIO it is concluded that the cAMP-sensitive K+ channels are different in the airways and the gut.

Restoration by intratracheal gene transfer of bicarbonate secretion in cystic fibrosis mouse gallbladder

Gallbladders from cystic fibrosis (CF) mice (Cftrtm1Cam and Cftrtm2Cam) were examined with the short-circuit current technique. The tissues failed to show any electrogenic anion transport in response to forskolin (cAMP stimulus) but responded to the Ca2+ ionophore ionomycin. Administration of the plasmid pTrial10-CFTR2 complexed with cationic liposomes (3beta-[N-(dimethylaminoethane)-carbamoyl]cholesterol and L-alpha-phosphatidylethanolamine dioleolyl) to the airways restored the phenotype of CF gallbladders to that of the wild type, but did not do so when given orally. Formation of human CFTR mRNA in gallbladders of transfected CF null mice was demonstrated. Using the reporter genes pCMV-luc and pCMV-LacZ, we showed that 1) the intratracheal route was more effective than the oral,intravenous, intramuscular, subcutaneous, or intraperitoneal routes in expressing luciferase activity in the gallbladder and 2) beta-galactosidase staining after pCMV-LacZ was confined to the columnar epithelium lining the gallbladder without any discernible activity in it smooth muscle. The discovery of an unusual route for gene transfer to the biliary system may give useful insight into counteracting the consequences of biliary fibrosis in human CF patients.