Altered fluid transport across airway epithelium in cystic fibrosis

Water transport across mammalian cell membranes

This review summarizes recent progress in water-transporting mechanisms across cell membranes. Modern biophysical concepts of water transport and new measurement strategies are evaluated. A family of water-transporting proteins (water channels, aquaporins) has been identified, consisting of small hydrophobic proteins expressed widely in epithelial and nonepithelial tissues. The functional properties, genetics, and cellular distributions of these proteins are summarized. The majority of molecular-level information about water-transporting mechanisms comes from studies on CHIP28, a 28-kDa glycoprotein that forms tetramers in membranes; each monomer contains six putative helical domains surrounding a central aqueous pathway and functions independently as a water-selective channel. Only mutations in the vasopressin-sensitive water channel have been shown to cause human disease (non-X-linked congenital nephrogenic diabetes insipidus); the physiological significance of other water channels remains unproven. One mercurial-insensitive water channel has been identified, which has the unique feature of multiple overlapping transcriptional units. Systems for expression of water channel proteins are described, including Xenopus oocytes, mammalian and insect cells, and bacteria. Further work should be directed at elucidation of the role of water channels in normal physiology and disease, molecular analysis of regulatory mechanisms, and water channel structure determination at atomic resolution.

A role for CFTR in human autosomal dominant polycystic kidney disease

Human autosomal dominant polycystic kidney disease (ADPKD) is the most common lethal dominant hereditary disorder characterized by enormous renal enlargement and the development of multiple cysts originating from nephrons. We investigated the pathogenesis of cyst formation in ADPKD by using patch-clamp and immunocytochemical techniques. Adenosine 3',5'-cyclic monophosphate-activated Cl- currents are present in primary cultures of ADPKD cells and have characteristics such as a linear current-voltage relation, insensitivity to 4,4'-diisothiocyanostilbene-2,2'-disulfonic acid, sensitivity to glibenclamide and diphenylamine carboxylic acid, and an anion selectivity sequence of Br- > Cl- > I- > glutamate, all of which are identical to cystic fibrosis transmembrane conductance regulator (CFTR). With the use of CFTR antibodies raised against the regulatory and first nucleotide-binding domains, CFTR was detected in primary cultures of ADPKD cells. Similar results were obtained in vivo in cyst-lining epithelial cells in ADPKD kidneys, where staining was seen associated with the apical membrane regions. These data indicate that the CFTR Cl- channel exists in apical membranes of ADPKD cells and may play an important role in cyst formation or enlargement.

The delayed basolateral membrane hyperpolarization of the bovine retinal pigment epithelium: mechanism of generation

1. Conventional and ion-selective double-barrelled microelectrodes were used in an in vitro preparation of bovine retinal pigment epithelium (RPE)-choroid to measure the changes in membrane voltage, resistance and intracellular Cl- activity (aCli) produced by small, physiological changes in extracellular potassium concentration ([K+]o). These apical [K+]o changes approximate those produced in the extracellular (subretinal) space between the photoreceptors and the RPE following transitions between light and dark. 2. Changing apical [K+]o from 5 to 2 mM in vitro elicited membrane voltage responses with three distinct phases. The first phase was generated by an apical membrane hyperpolarization, followed by a (delayed) basolateral membrane hyperpolarization (DBMH); the third phase was an apical membrane depolarization. The present experiments focus on the membrane and cellular mechanisms that generate phase 2 of the response, the DBMH. 3. The DBMH was abolished in the presence of apical bumetanide (100 microM); this response was completely restored after bumetanide removal. 4. Reducing apical [K+]o, adding apical bumetanide (500 mM), or removing apical Cl- decreased aCli by 25 +/- 6 (n = 8), 28 +/- 1 (n = 2) and 26 +/- 5 mM (n = 3), respectively; adding 100 microM apical bumetanide decreased aCli by 12 +/- 2 mM (n = 3). Adding apical bumetanide or removing apical bath Cl- hyperpolarized the basolateral membrane and decreased the apparent basolateral membrane conductance (GB). 5. DIDS (4,4'-diisothiocyanostilbene-2,2'-disulphonic acid) blocked the RPE basolateral membrane Cl- conductance and inhibited the DBMH and the basolateral membrane hyperpolarization produced by apical bumetanide addition or by removal of apical Cl-o. The present results show that the DBMH is caused by delta[K]o-induced inhibition of the apical membrane Na(+)-K(+)-2Cl- cotransporter; the subsequent decrease in aCli generated a hyperpolarization at the basolateral membrane Cl- channel.

Pharmacologic modulation of salt and water in the airway epithelium in cystic fibrosis

Cystic fibrosis (CF) is a recessive genetic disease with thickened airway secretions that result from abnormal airway epithelial ion transport, including defective cyclic AMP-mediated Cl- (liquid) secretion and excessive Na+ (liquid) absorption. These abnormalities reflect mutations in the gene coding for the cystic fibrosis transmembrane conductance regulator (CFTR) protein, which normally functions as a cyclic AMP-regulated Cl- channel. Aerosolized pharmacologic agents are being tested as novel treatment for these genetic ion transport defects. Amiloride aerosol inhibits excessive Na+ absorption, and pilot studies in adult patients with CF show improved biorheology and mucociliary clearance of airway secretions, as well as slowing of the decline in lung function. Phase III studies of amiloride in adults and adolescents are underway, and short-term safety studies in children are under way. Aerosolized uridine triphosphate (UTP) induces Cl- secretion in CF airway epithelia via non-CFTR Cl- channels. Initial safety studies suggest that acute aerosolized UTP is well tolerated, and acute studies of the effect on mucociliary clearance are underway. Pharmacotherapy that targets abnormal ion transport holds promise for the treatment of CF airway disease.

Pharmacologic treatment of abnormal ion transport in the airway epithelium in cystic fibrosis

Cystic fibrosis (CF) is a recessive genetic disease reflecting mutations in the gene coding for the CF transmembrane regulator (CFTR) protein, which normally functions as a cyclic adenosine monophosphate (cAMP)-regulated chloride (Cl-) channel. Functional abnormalities include thick airway secretions resulting from defective cAMP-mediated Cl- (liquid) secretion and a related defect, excessive sodium (Na+) (liquid) absorption. Novel pharmacologic agents are being tested as therapy for these ion transport defects. Aerosolized amiloride inhibits excessive Na+ absorption, and pilot studies in adult patients with CF show slowing of the disease-associated decline in lung function. Clinical trials of amiloride are currently underway in adults and adolescents, and short-term safety studies have been initiated in children. Aerosolized uridine triphosphate (UTP) induces Cl- (and liquid) secretion in CF airway epithelia via non-CFTR Cl- channels. Short-term aerosolized UTP is well tolerated by normal subjects and patients with CF, and pilot studies in normal subjects show that aerosolized UTP is an effective stimulator of mucociliary clearance. Pharmacotherapy that modifies airway epithelial ion transport may provide new opportunities for treatment of CF lung disease.

Regulation of human airway surface liquid

Human airways are lined with a film of liquid from 5-100 microns in depth, consisting of a periciliary sol around and a mucous gel above the cilia. Microscopical studies have shown the sol to be invariably the same depth as the length of the cilia, and we discuss possible reasons for this. The composition and sources of the airway surface liquid are also described. In addition the forces regulating its volume are analyzed. Several airway diseases are characterised by dramatic changes in the volume and composition of airway liquid. We review recent research suggesting that the accumulation of airway mucous secretions in cystic fibrosis is caused by alterations in active transport of ions and water across both the surface and gland epithelia.

The genetics of cystic fibrosis: a paradigm for uncovering new drug targets

A broad-based approach will be required for the development of new therapies for cystic fibrosis lung disease. Recently, rapid progress has been made in identifying and testing a number of gene transfer vectors, including adenoviral vectors and liposomes. Major problems, however, have been identified with respect to the efficiency of these systems. Preliminary studies suggest that small molecules (e.g. amelioride and UTP) may normalize the clearance of secretions from the cystic fibrosis lung. The concept of recombinant protein based therapy for cystic fibrosis has now been realized with the successful application of DNase in clinical trials.

Novel pore-lining residues in CFTR that govern permeation and open-channel block

The cystic fibrosis transmembrane conductance regulator (CFTR) is both a member of the ATP-binding cassette superfamily and a Cl(-)-selective ion channel. We investigated the permeation pathway of human CFTR with measurements on conduction and open-channel blockade by diphenylamine-2-carboxylic acid (DPC). We used site-directed mutagenesis and oocyte expression to locate residues in transmembrane domain (TM) 6 and TM 12 that contact DPC and control rectification and single-channel conductances. Thus, TM 12 and the previously investigated TM 6 line the CFTR pore. In each TM, residues in contact with DPC are separated by two turns of an alpha helix. The contributions of TM 6 and TM 12 to DPC block and Cl- permeation, however, are not equivalent. The resulting structural model for the conduction pathway may guide future studies of permeation in other Cl- channels and ATP-binding cassette transporters.

Cystic fibrosis gene therapy

A variety of cystic fibrosis gene therapy approaches based on viral (adenovirus, retrovirus, and adeno-associated virus) and non-viral (liposomes and receptor-mediated endocytosis) routes are currently being assessed for safety and efficacy. Of these, the trials involving liposomal and adenoviral vectors are the most advanced, as both have been shown to correct the cystic fibrosis Cl- conductance defect in vivo.

Correction of cAMP-stimulated fluid secretion in cystic fibrosis airway epithelia: efficiency of adenovirus-mediated gene transfer in vitro

Adenovirus vectors are a promising vehicle to deliver cystic fibrosis transmembrane conductance regulator (CFTR) cDNA to airway epithelia. However, the value of adenovirus vectors will depend on the efficiency with which the vector can correct the defective fluid transport that is though to underlie the pathogenesis of the disease. To address the efficiency of gene transfer, we applied adenovirus vectors expressing CFTR (Ad2/CFTR-1) or beta-galactosidase to the mucosal surface of primary cultures of airway epithelial cells grown as polarized epithelial monolayers on permeable filter supports. These conditions provide a model that reproduces the physiology of the airways in vivo. We found that after adding 1 moi Ad2/CFTR-1 to the mucosal surface, cAMP agonists stimulated fluid secretion that was within the range observed in epithelia from normal subjects. When we measured electrolyte transport, we found that as little as 0.1 moi partially restored cAMP-stimulated Cl- secretion, and at 10 moi Cl- secretion was in the normal range. A related vector encoding beta-galactosidase generated activity in approximately 20% of cells at an moi of 1 and 90% of cells at an moi of 10. These data suggest that Ad2/CFTR-1 is very efficient at restoring normal fluid and electrolyte transport to CF airway epithelia. Thus, they suggest that relatively low input doses could be used for gene transfer to CF airway epithelia.

Defective fluid transport by cystic fibrosis airway epithelia

Cystic fibrosis (CF) airway epithelia exhibit defective transepithelial electrolyte transport: cAMP-stimulated Cl- secretion is abolished because of the loss of apical membrane cystic fibrosis transmembrane conductance regulator (CFTR) Cl- channels, and amiloride-sensitive Na+ absorption is increased two- to threefold because of increased amiloride-sensitive apical Na+ permeability. These abnormalities are thought to alter respiratory tract fluid, thereby contributing to airway disease, the major source of mortality in this genetic disease. However, the underlying hypothesis, that fluid transport is abnormal in CF airway epithelia, has not been tested. Most conjecture about fluid transport is based on measurements of Na+ and Cl- transport performed under short circuit conditions in Ussing chambers. But such studies differ from in vivo conditions in that transepithelial voltage and mucosal fluid composition are held constant. Therefore, we measured fluid transport and mucosal electrolyte composition in primary cultures of CF airway epithelia without holding transepithelial voltage and ion concentration gradients at zero. In normal epithelia, cAMP agonists plus amiloride stimulated NaCl and fluid secretion. In CF epithelia, cAMP agonists failed to stimulate fluid or electrolyte secretion, changes consistent with the loss of CFTR Cl- channels. But in striking contrast to predictions based on Ussing chamber studies, CF epithelia absorbed fluid at a rate no greater than normal epithelia. Moreover, amiloride, which inhibits Na+ channels, failed to inhibit fluid absorption by CF epithelia. These results have important implications for understanding the pathogenesis of CF airway disease and for the design and evaluation of therapy.

Epigenesis: the missing beat in biotechnology?

The range of human phenotypes/diseases for which our burgeoning bio-molecular data base is sufficient to provide understanding, diagnosis, and therapy is small. Only 2 percent of our total disease load is related to monogenic causality, and even here the final phenotype is modulated by many factors. Monogenic logic cannot, moreover, be applied to the 98 percent of our most important sources of premature disability and death. This article provides an analysis of the limits of genetic thinking in biotechnology and describes the outline for another approach to understanding complex cellular/physiological systems. In this outline, rules governing physiological regulation and cellular and higher levels of organization are located not in the genome, but in interactive epigenetic networks which themselves organize genomic response to environmental signaling.

Modulation of the ionic milieu of the airway in health and disease

Airway surface liquid (ASL) is an integral part of lung defense mechanisms. Ion transport by airway epithelia regulates the volume and composition of this fluid. A better understanding of the mechanisms of ion transport will enable the development of new therapies for airway diseases associated with defects in these mechanisms. A useful model of a disease with abnormal airway epithelial ion transport is cystic fibrosis (CF), a distinct genetic syndrome of altered lung defense mechanisms characterized by chronic bacterial infection and a steady decline in lung function. Traditional therapies for CF include antibacterial drugs and augmentation of clearance of secretions, but investigators are now studying pharmacological approaches to target the more basic defect of the disease, i.e. abnormal sodium and chloride ion transport. Early treatment in childhood, prior to lung damage, might prevent or at least retard the decline in pulmonary function that remains the hallmark of CF. Ion transport dysfunction may also contribute to other airway diseases such as asthma and chronic bronchitis. Pharmacological intervention at this level may prove beneficial in these common lung diseases as well.