Ion transport characteristics of the murine trachea and caecum

Species-specific differences in mouse and human airway epithelial biology of recombinant adeno-associated virus transduction

Differences in airway epithelial biology between mice and humans have presented challenges to evaluating gene therapies for cystic fibrosis (CF) using murine models. In this context, recombinant adeno-associated virus (rAAV) type 2 and rAAV5 vectors have very different transduction efficiencies in human air-liquid interface (ALI) airway epithelia (rAAV2 approximately = rAAV5) as compared with mouse lung (rAAV5 >> rAAV2). It is unclear if these differences are due to species-specific airway biology or limitations of ALI cultures to reproduce in vivo airway biology. To this end, we compared rAAV2 and rAAV5 transduction biology in mouse and human ALI cultures, and investigated the utility of murine deltaF508 cystic fibrosis transmembrane conductance regulator (CFTR) ALI epithelia to study CFTR complementation. Our results demonstrate that mouse ALI epithelia retain in vivo preferences for rAAV serotype transduction from the apical membrane (rAAV5 >> rAAV2) not seen in human epithelia (rAAV2 approximately = rAAV5). Viral binding of rAAV2 and rAAV5 to the apical surface of mouse ALI airway epithelia was not significantly different, and proteasome-modulating agents significantly enhanced rAAV2 transduction to a level equivalent to that of rAAV5 in the presence of these agents, suggesting that the ubiquitin/proteasome pathway represents a more significant intracellular block for rAAV2 transduction of mouse airway epithelia. Interestingly, cAMP-inducible chloride currents were enhanced in deltaF508CFTR mouse ALI cultures, making this model incompatible with CFTR complementation studies. These studies emphasize species-specific differences in airway biology between mice and humans that significantly influence the use of mice as surrogate models for rAAV transduction and gene therapy for CF.

Electrophysiological properties of the airway: epithelium in the murine, ovalbumin model of allergic airway disease

The electrophysiological properties of cultured tracheal cells (CTCs) were examined in a murine (C57BL/6J), ovalbumin (OVA)-induced model of allergic airway disease (AAD) at early (3-day OVA-aerosol) and peak (10-day OVA-aerosol) periods of inflammation. Transepithelial potential difference, short-circuit current (Isc), and resistance (RT) were lower in CTCs from 10-day OVA-aerosol animals compared to CTCs from naïve mice. In cells cultured for 5 weeks, RT was greater in naive CTCs than in 10-day OVA-aerosol CTCs at all times (P < 0.01). The Isc response to mucosal amiloride (10(-4) mol/L) was increased in CTCs from 10-day OVA-aerosol mice compared to naïve mice (6.0 +/- 0.37 microA/cm2 versus 1.8 +/- 0.56 microA/cm2; P < 0.001) with intermediate values for CTCs from 3-day OVA-aerosol mice. The cAMP-induced increase in Isc was blunted in 10-day OVA-aerosol animals compared to CTCs from naïve mice (9 +/- 12% versus 39 +/- 7%; P < 0.01) with intermediate values for CTCs from 3-day OVA-aerosol mice. There was no difference in mannitol flux in naïve compared to 10-day OVA-aerosol CTCs. Similar results were found using intact tracheas mounted in a perfusion chamber. These data demonstrate changes in airway epithelial cell function in the OVA-induced model of AAD that may contribute to the pathogenesis of airway inflammation.

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.

Expression of nucleotide-regulated Cl(-) currents in CF and normal mouse tracheal epithelial cell lines

The dominant route for Cl(-) secretion in mouse tracheal epithelium is via Cl(-) channels different from the cystic fibrosis (CF) transmembrane conductance regulator (CFTR), the channel that is defective in CF. It has been proposed that the use of purinergic agonists to activate these alternative channels in human airways may be beneficial in CF. In the present study, two conditionally immortal epithelial cell lines were established from the tracheae of mice possessing the tsA58 T antigen gene, one of which [MTE18-(-/-)] was homozygous for a knockout of CFTR and the other [MTE7b-(+/-)] heterozygous for CFTR expression. In Ussing chamber studies, amiloride (10(-4) M) and a cocktail of cAMP-activating agents (forskolin, IBMX, and dibutyryl cAMP) resulted in small changes in the short-circuit current (I(sc)) and resistance of both cell lines, with larger increases in I(sc) being elicited by ionomycin (10(-6) M). Both cell lines expressed P(2)Y(2) receptors and responded to the purinergic agonists ATP, UTP, and 5'-adenylylimidodiphosphate (10(-4) M) with an increase in I(sc). This response could be inhibited by DIDS and was abolished in the presence of Cl(-)-free Ringer solution. Reducing the mucosal Cl(-) concentration increased the response to UTP of both cell lines, with a significantly greater increase in MTE18-(-/-) cells. Pretreatment of these cells with thapsigargin caused a direct increase in I(sc) and inhibited the response to UTP. These data suggest that both cell lines express purinergic-regulated Cl(-) currents and may prove valuable tools in studying the properties of this pathway.

A primary culture model of differentiated murine tracheal epithelium

The goal of this study was to develop a primary culture model of differentiated murine tracheal epithelium. When grown on semipermeable membranes at an air interface, dissociated murine tracheal epithelial cells formed confluent polarized epithelia with high transepithelial resistances ( approximately 12 kOmega. cm(2)) that remained viable for up to 80 days. Immunohistochemistry and light and electron microscopy demonstrated that the cells were epithelial in nature (cytokeratin positive, vimentin and alpha-smooth muscle actin negative) and differentiated to form ciliated and secretory cells from day 8 after seeding onward. With RT-PCR, expression of the cystic fibrosis transmembrane conductance regulator (Cftr) and murine beta-defensin (Defb) genes was detected (Defb-1 was constitutively expressed, whereas Defb-2 expression was induced by exposure to lipopolysaccharide). Finally, Ussing chamber experiments demonstrated an electrophysiological profile compatible with functional amiloride-sensitive sodium channels and cAMP-stimulated CFTR chloride channels. These data indicate that primary cultures of murine tracheal epithelium have many characteristics similar to those of murine tracheal epithelium in vivo. This method will facilitate the establishment of primary cultures of airway epithelium from transgenic mouse models of human diseases.

Development of mouse models for cystic fibrosis

Using gene targeting in embryonal stem cells it is now possible to create accurate genetic models of inherited human disease in the mouse. The value of an animal model of cystic fibrosis is in providing clarification of disease pathogenesis, genotype-phenotype correlation, the identification of other relevant genetic factors, and as the optimal test system for novel therapeutic intervention. Correction of the basic defect by a somatic gene therapy approach is an attractive approach to disease treatment. CF mouse models have been described which display the characteristic electrophysiological defect and thus both safety and efficacy can be monitored in these animals. Modulation of Cftr levels in transgenic animals and the results on disease phenotype give some indication of the level of gene expression necessary to give clinical effect.

Scaling of transepithelial potential difference in the mammalian trachea

Tracheal epithelia of different mammalian species differ widely with regard to the relative rates of Na+ absorption and Cl- secretion. However, the short circuit current, a measure of total ion transport, appears to be consistently greater in large than in small mammals. Thus, we hypothesized that the in vivo tracheal electrical potential difference (PD) would vary among species as a function of body mass (M). To test this hypothesis we measured PD in ten mammalian species that ranged 1000-fold in mass. The results in mV (mean +/- SE, lumen negative) were: 11.4 +/- 1.0 in mice; 11.6 +/- 1.2 in gerbils; 12.9 +/- 1.4 in rats; 19.3 +/- 0.9 in guinea pigs; 27.2 +/- 2.2 in ferrets; 23.0 +/- 1.6 in cats; 27.0 +/- 0.6 in rabbits; 32.5 +/- 2.6 in dogs; 37.0 +/- 1.9 in sheep; and 49.0 +/- 3.3 in pigs. Log-log correlation analysis of mean PD (in mV) and M (in kg) yielded PD = 20.9 M0.19 (r = 0.96, P < 0.001). Analysis of published short circuit current (SCC, in microA/cm2) data revealed a similar relationship: SCC = 38.2 M0.21. Thus, the transepithelial electrical potential and active charge transport by the tracheal epithelium are allometric variables that may have direct physiological significance. These results raise questions regarding the importance of net osmotic solute and water transport across the tracheal epithelium.

Ion-transporting activity in the murine colonic epithelium of normal animals and animals with cystic fibrosis

Electrogenic ion transport in the isolated colonic epithelium from normal and transgenic mice with cystic fibrosis (CF mice) has been investigated under short-circuit current (Isc) conditions. Normal tissues showed chloride secretion in response to carbachol or forskolin, which was sensitive to the Na-K-2Cl cotransport inhibitor, frusemide. Responses to both agents were maintained for at least 12 h in vitro, but the responses to carbachol changed in format throughout this period. By contrast CF colons failed to show the normal secretory responses to carbachol and forskolin, most preparations showing a decrease in Isc that was immediately reversed by frusemide. In CF colons addition of Ba2+ ions or tetraethylammonium (TEA+) to the apical bathing solution antagonised the reduction in Isc caused by the secretagogues. It is concluded that the reduction in Isc in CF colons is due to electrogenic K+ secretion and this was confirmed by flux studies using rubidium-86. In normal colons exposed to TEA+ the responses to forskolin were greater, but not significantly so, presumably because the minor K(+)-secretory responses are dominated by major chloride-secretory responses. Again rubidium-86 fluxes showed an increase of K+ secretion in normal colons receiving forskolin. Since the amiloride-sensitive current was not different in CF and normal colons there was no evidence that the CF mice were stressed in a way that increased mineralocorticoid levels and hence K+ secretion. Knowledge of the phenotype of the colonic epithelium of the CF mouse sets the baseline from which attempts at gene therapy for the gut must be judged.

Long-term survival of the exon 10 insertional cystic fibrosis mutant mouse is a consequence of low level residual wild-type Cftr gene expression

Recently we have created a mouse model of cystic fibrosis (CF) by insertional gene targeting to exon 10. In common with CF subjects, this model displays a low incidence of meconium ileus. This contrasts strikingly with the very high level of fatal intestinal obstruction in the three other CF mouse models so far described. We investigate here the molecular basis of this difference in phenotype. We show that the partial duplication consequent upon insertional gene targeting allows exon skipping and aberrant splicing to produce normal Cftr mRNA, but at levels greatly reduced compared with wild-type mice. Furthermore, instead of the predicted mutant Cftr transcript, a novel mRNA is produced that utilizes cryptic splice sites in the disrupting plasmid sequence. However, we have previously shown that these mice display the ion transport defect characteristic of CF, and mutant animals can be distinguished from their normal littermates on this basis. Consistent with this, residual CFTR function has recently been observed for several "mild" mutations in CF individuals who display pancreatic sufficiency but still develop lung disease. We conclude that (i) residual wild-type mRNA in the exon 10 insertional mutant mouse ameliorates the severity of the intestinal phenotype observed in the absolute "null" CF mice, (ii) the presence of low-level residual wild-type Cftr mRNA does not correct the CF ion transport defect, and (iii) the long-term survival of this insertional mutant mouse provides the opportunity to address the factors important in development of lung disease.

Anomalies in ion transport in CF mouse tracheal epithelium

The cystic fibrosis (CF) mouse trachea has become a model for gene transfer. To characterize ion transport properties of tracheal epithelium from normal and CF mice, tracheas were excised, mounted in Ussing chambers, and basal properties and responses to pharmacological agents and/or ion substitution protocols measured. No difference in basal short-circuit (Isc) was observed between normal (29.1 +/- 3.8 muA/cm2, n = 21) and CF (34.7 +/- 4.5 muA/cm2, n = 16) tracheas. The relative contribution of Na+ transport to basal Isc was small (30-40%). Ionomycin stimulated large increases in Isc in both normal and CF murine tracheas [change in Isc (delta Isc) with ionomycin: 30.5 +/- 8.8 muA/cm2, n = 11, normal; 27.3 +/- 6.7 muA/cm2, n = 6, CF]. Unexpectedly, forskolin increased Isc in both CF and normal amiloride-pretreated tracheas (delta Isc: 10.5 +/- 2.1 muA/cm2, n = 21, normal; 13 +/- 2.3 muA/cm2, n = 16, CF). Forskolin was observed to increase intracellular Ca2+ in both normal and CF tracheal cells, suggesting this as a mechanism to induce Cl- secretion. These similarities in ion transport, in part reflecting the dominance of Ca(2+)-regulated Cl- conductance, suggest that the murine trachea is not an ideal target for assessment of CF correction by gene transfer.

Non-invasive liposome-mediated gene delivery can correct the ion transport defect in cystic fibrosis mutant mice

We report gene transfer to the Edinburgh insertional mutant mouse (cf/cf), delivering CFTR cDNA-liposome complexes into the airways by nebulization. We show full restoration of cAMP related chloride responses in some animals and demonstrate, in the same tissues, human CFTR cDNA expression. Overall, a range of correction was seen with restoration of about 50% of the deficit between wild type mice and untreated cf/cf controls. We report modest correction in the intestinal tract following direct instillation and provide initial encouraging safety data for both the respiratory and intestinal tract following the liposome mediated gene delivery. The non-viral nature and potentially lower immunogenicity of DNA-liposomes suggest that this may offer a therapeutic alternative to adenoviral therapies.

Production of a severe cystic fibrosis mutation in mice by gene targeting

We have used gene targeting in embryonic stem cells to introduce an HPRT mini-gene into the coding sequence of the murine cystic fibrosis gene (cftr). This insertion introduces a termination codon in frame with the cftr coding sequence to terminate prematurely the CFTR protein within the first nucleotide binding domain. Animals homozygous for the cftr disruption fail to thrive and display a range of symptoms including meconium ileus, distal intestinal obstructions, gastrointestinal mucus accumulation and blockage of pancreatic ducts. The animals also show lacrimal gland pathology. Tracheal and caecal transepithelial current measurements demonstrate the lack of a cAMP activatable Cl- channel. These animals will prove useful for the evaluation of new therapeutic drugs and gene therapy strategies.

Correction of the ion transport defect in cystic fibrosis transgenic mice by gene therapy

Cystic fibrosis (CF) is a lethal inherited disorder affecting about 1 in 2,000 Caucasians. The major cause of morbidity is permanent lung damage resulting from ion transport abnormalities in airway epithelia that lead to mucus accumulation and bacterial colonization. CF is caused by mutations in the cystic fibrosis transmembrane conductance regulator (CFTR) gene that encodes a cyclic-AMP-regulated chloride channel. Cyclic-AMP-regulated chloride conductances are altered in airway epithelia from CF patients, suggesting that the functional expression of CFTR in the airways of CF patients may be a strategy for treatment. Transgenic mice with a disrupted cftr gene are appropriate for testing gene therapy protocols. Here we report the use of liposomes to deliver a CFTR expression plasmid to epithelia of the airway and to alveoli deep in the lung, leading to the correction of the ion conductance defects found in the trachea of transgenic (cf/cf) mice. These studies illustrate the feasibility of gene therapy for the pulmonary aspects of CF in humans.

Cystic fibrosis in the mouse by targeted insertional mutagenesis

Cystic fibrosis is a fatal genetic disorder which afflicts 50,000 people worldwide. A viable animal model would be invaluable for investigating and combating this disease. The mouse cystic fibrosis transmembrane conductance regulator gene was disrupted in embryonal stem cells using an insertional gene targeting vector. Germ-line chimaeras were derived and the offspring of heterozygous crosses studied. These homozygous mutant mice survive beyond weaning. In vivo electrophysiology demonstrates the predicted defect in chloride ion transport in these mice and can distinguish between each genotype. Histological analysis detects important hallmarks of human disease pathology, including abnormalities of the colon, lung and vas deferens. This insertional mouse mutation provides a valid model system for the development and testing of therapies for cystic fibrosis patients.