Altered sensitivity to amiloride in cystic fibrosis. Observations using cultured sweat glands

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.

Bioelectric properties of cystic fibrosis airways obtained at heart-lung transplantation

BACKGROUND

The basic defect in cystic fibrosis centres on abnormal ion transport in affected tissues such as the respiratory tract. Heart-lung transplantation provides a limited supply of native lower airways from these patients. The feasibility of in vitro studies of bioelectric properties and ion fluxes in lower airways, obtained at heart-lung transplantation from patients with cystic fibrosis, has been assessed. Comparison was made with airways from patients without cystic fibrosis.

METHODS AND RESULTS

Tissue segments were mounted in Ussing chambers under open circuit conditions. The basal potential difference in tissues from nine patients with cystic fibrosis was -3.6 mV (SE 0.3 mV), not different from tissues from 12 patients without cystic fibrosis of -3.6 mV (0.5 mV). Amiloride (10 microM) caused a significantly greater fall in potential difference in bronchi from patients with cystic fibrosis (83.5% (SE 2.9%)) than in those from controls (55.1% (7.1%)). Isoprenaline (100 microM) produced no significant change in bioelectric properties in non-cystic fibrosis tissues, but induced a 26.2% (6.3%) increase in potential difference in cystic fibrosis airways. The latter response was reduced by amiloride pretreatment. Mucosal chloride substituted Krebs-Henseleit solution caused no change in bioelectric properties in cystic fibrosis airways. Sodium substituted Krebs solution produced a substantial fall in potential difference similar in magnitude to that seen after amiloride. Isotropic flux measurements showed no significant differences between non-cystic fibrosis and cystic fibrosis airways. No net movement of Na+ or Cl- was detected under open circuit conditions in either group.

CONCLUSIONS

Cystic fibrosis bronchi obtained at heart-lung transplantation provide a viable source of tissue for in vitro studies of bioelectric properties. The increased response to amiloride characteristic of the upper airways in cystic fibrosis is retained in these tissues, as is the reduced chloride conductance. Although no differences in isotopic fluxes were seen between non-cystic fibrosis and cystic fibrosis tissues, heavily infected airways from patients with cystic fibrosis may not be suitable for ion flux measurements.

Sodium transport and intracellular sodium activity in cultured human nasal epithelium

Human airway epithelia are predominantly Na(+)-absorbing epithelia. To investigate the mechanisms for Na+ absorption across airway epithelia, the driving forces and paths for Na+ translocation across each membrane were examined with double-barreled Na(+)-selective microelectrodes in cultured human nasal epithelium (HNE). Under control conditions, intracellular Na+ activity (acNa) was 23 +/- 1 mM (n = 44 preparations, 393 impalements). Amiloride (10(-4) M) hyperpolarized the apical membrane and increased the fractional apical membrane resistance but did not affect acNa. Exposure to Na(+)-free luminal solution induced bioelectric responses similar to amiloride but also reduced acNa to 8 +/- 1 mM. Reduction of luminal Na+ concentration ([Na+]) in the presence of amiloride also reduced acNa without further changes in bioelectric parameters. Reduction of serosal [Na+] decreased aNac, a response blocked by bumetanide (10(-4) M). Ouabain (10(-4) M, serosal) led to a reduction in equivalent short-circuit current (Ieq) and increase in acNa. We conclude that 1) acNa is higher in HNE than in most mammalian epithelial cells, 2) the apical membrane expresses a conductive Na+ path, and 3) the basolateral membrane transports Na+ via the Na(+)-K(+)-adenosinetriphosphatase and a Na(+)-K(+)-2Cl- cotransport system.

Cystic fibrosis. 4. Abnormalities of airway epithelial function and the implications of the discovery of the cystic fibrosis gene

Details of ion transporting abnormalities in cystic fibrosis airway epithelium are now known. The central hypothesis, that excessive drying of the airway surfaces is a primary event that leads to all the manifestations of the respiratory insufficiency in cystic fibrosis, is not proved. The hypothesis is, however, consistent with the known transporting abnormalities and is strengthened by the modest clinical improvement produced by a strategy designed to correct the transporting abnormalities. The discovery of the cystic fibrosis gene, together with the presumed structure of the protein product, provides a focal point that must eventually connect the functional abnormalities with the genetic defect. The cellular function of the cystic fibrosis transmembrane regulator must now be the major target in research on cystic fibrosis. Strategies for treatment based on known cellular and molecular abnormalities are beginning to emerge but will be undoubtedly more focused once the responsibility of the cystic fibrosis transmembrane regulator is known.

Ion transport in cultured epithelia from human sweat glands: comparison of normal and cystic fibrosis tissues

1. Cultured epithelia derived from whole human sweat glands, isolated secretory coils, isolated reabsorptive ducts and whole glands from cystic fibrosis (CF) subjects have been used to examine drug sensitivity by use of short circuit current recording. 2. Short circuit current increases were observed with lysylbradykinin, carbachol and histamine in epithelia of different origins. All responses were due to stimulation of electrogenic sodium absorption, evidenced by the inhibition of these responses by amiloride. The latter also abolished the basal current. The terpenes, thapsigargin and forskolin had no effect on transport. 3. The stimulation of a sodium current by agonists was dependent upon calcium, responses being inhibited by lanthanum ions and EGTA. Further A23187 induced a sodium current. 4. Pronounced oscillations in the sodium currents were a feature of the responses, implying synchronous, regulated cell activity. 5. Forskolin produced a ten fold increase in adenylate cyclase activity. All agonists listed in 2 except forskolin caused an increase in intracellular calcium [Ca]i, [Ca]i responses in CF cells were not different from those of normal cells, except with thapsigargin where the responses were smaller. 6. It is concluded that in culture, cells develop ductal characteristics, whether derived from normal or CF glands, coils or ducts. An increase in [Ca]i followed by activation of calcium-sensitive potassium channels and apical membrane hyperpolarization may be the major mechanism for increasing sodium influx.

A novel method for culturing sweat gland epithelia: comparison of normal and cystic fibrosis tissues

A new method is described to produce epithelial sheets by direct explantation of human sweat glands onto matrigel-coated millipore filters. The method is applicable to whole glands, separated coils or ducts and to normal and CF tissues. Electrogenic transport studies show that epithelia develop sodium transporting capability, even when explants are derived from secretory coils.