Decreased beta-adrenergic stimulation of glycoprotein secretion in CF mice submandibular glands: reversal by the methylxanthine, IBMX

Pharmacological Approaches to Correcting the Ion Transport Defect in Cystic Fibrosis

Cystic fibrosis (CF) is a lethal genetic disease caused by a mutation in a membrane protein, the cystic fibrosis transmembrane conductance regulator (CFTR), which mainly (but not exclusively) functions as a chloride channel. The main clinical symptoms are chronic obstructive lung disease, which is responsible for most of the morbidity and mortality associated with CF, and pancreatic insufficiency. About 1000 mutations of the gene coding for CFTR are currently known; the most common of these, present in the great majority of the patients (Delta508) results in the deletion of a phenylalanine at position 508. In this mutation, the aberrant CFTR is not transported to the membrane but degraded in the ubiquitin-proteasome pathway. The aim of this review is to give an overview of the pharmacologic strategies currently used in attempts to overcome the ion transport defect in CF. One strategy to develop pharmacologic treatment for CF is to inhibit the breakdown of DeltaF508-CFTR by interfering with the chaperones involved in the folding of CFTR. At least in in vitro systems, this can be accomplished by sodium phenylbutyrate, or S-nitrosoglutathione (GSNO), and also by genistein or benzo[c]quinolizinium compounds. It is also possible to stimulate CFTR or its mutated forms, when present in the plasma membrane, using xanthines, genistein, and various other compounds, such as benzamidizoles and benzoxazoles, benzo[c]quinolizinium compounds or phenantrolines. Experimental results are not always unambiguous, and adverse effects have been incompletely tested. Some clinical tests have been done on sodium phenyl butyrate, GSNO and genistein, mostly in respect to other diseases, and the results demonstrate that these drugs are reasonably well tolerated. Their efficiency in the treatment of CF has not yet been demonstrated, however. An alternative strategy is to compensate for the defective chloride transport by CFTR by stimulation of other chloride channels. This can be done via purinergic receptors. A phase I study using a stable uridine triphosphate analog has recently been completed. A second alternative strategy is to attempt to maintain hydration of the airway mucus by inhibiting Na(+) uptake by the epithelial Na(+) channel using amiloride or stable analogs of amiloride. Clinical tests so far have been inconclusive. A number of other suggestions are currently being explored. The minority of patients with CF who have a stop mutation may benefit from treatment with gentamicin. The difficulties in finding a pharmacologic treatment for CF may be due to the fact that CFTR has additional functions besides chloride transport, and interfering with CFTR biosynthesis or activation implies interference with central cellular processes, which may have undesirable adverse effects.

Responses of well-differentiated airway epithelial cell cultures from healthy donors and patients with cystic fibrosis to Burkholderia cenocepacia infection

Well-differentiated cultures established from airway epithelia of patients with cystic fibrosis (CF cultures) exhibited goblet cell hyperplasia, increased secretion of mucus, and higher basal levels of interleukin-8 than similarly cultured cells from healthy donors. Upon apical infection with low doses (10(4) to 10(5) CFU) of Burkholderia cenocepacia isolate BC7, the two cultures gave different responses. While normal cultures trapped the added bacteria in the mucus layer, killed and/or inhibited bacterial replication, and prevented bacterial invasion of the cells, CF cultures failed to kill and/or supported the growth of bacteria, leading to invasion of underlying epithelial cells, compromised transepithelial permeability, and cell damage. Depletion of the surface mucus layer prior to bacterial infection rendered the normal cultures susceptible to bacterial invasion, but the invading bacteria were mainly confined to vacuoles within the cells and appeared to be nonviable. In contrast, bacteria that invaded cells in CF cultures were found free in the cytoplasm surrounded by intermediate filaments and also between cells. Cultured CF airway epithelium was therefore more susceptible to infection than normal epithelium. This mimics CF tissue in vivo and illustrates differences in the way epithelia in CF patients and normal subjects handle bacterial infection. In addition, we found that the CF and normal cell cultures responded differently not only to isolate BC7 but also to isolates of other B. cepacia complex species. We therefore conclude that this cell culture model is suitable for investigation of B. cepacia complex pathogenesis in CF patients.

The role of regulated CFTR trafficking in epithelial secretion

The focus of this review is the regulated trafficking of the cystic fibrosis transmembrane conductance regulator (CFTR) in distal compartments of the protein secretory pathway and the question of how changes in CFTR cellular distribution may impact on the functions of polarized epithelial cells. We summarize data concerning the cellular localization and activity of CFTR and attempt to synthesize often conflicting results from functional studies of regulated endocytosis and exocytosis in CFTR-expressing cells. In some instances, findings that are inconsistent with regulated CFTR trafficking may result from the use of overexpression systems or nonphysiological experimental conditions. Nevertheless, judging from data on other transporters, an appropriate cellular context is necessary to support regulated CFTR trafficking, even in epithelial cells. The discovery that disease mutations can influence CFTR trafficking in distal secretory and recycling compartments provides support for the concept that regulated CFTR recycling contributes to normal epithelial function, including the control of apical CFTR channel density and epithelial protein secretion. Finally, we propose molecular mechanisms for regulated CFTR endocytosis and exocytosis that are based on CFTR interactions with other proteins, particularly those whose primary function is membrane trafficking. These models provide testable hypotheses that may lead to elucidation of CFTR trafficking mechanisms and permit their experimental manipulation in polarized epithelial cells.

Ion channels in secretory granules of the pancreas and their role in exocytosis and release of secretory proteins

Regulated secretion in exocrine and neuroendocrine cells occurs through exocytosis of secretory granules and the subsequent release of stored small molecules and proteins. The introduction of biophysical techniques with high temporal and spatial resolution, and the identification of Ca(2+)-dependent and -independent "docking" and "fusion" proteins, has greatly enhanced our understanding of exocytosis. The cloning of families of ion channel proteins, including intracellular ion channels, has also revived interest in the role of secretory granule ion channels in exocytotic secretion. Thus secretory granules of pancreatic acinar cell express a ClC-2 Cl(-) channel, a HCO-permeable member of the CLCA Ca(2+)-dependent anion channel family, and a KCNQ1 K(+) channel. Evidence suggests that these channels may facilitate the release of digestive enzymes and/or prevent exocytosed granules from collapsing during "kiss and run" recycling. In pancreatic beta-cells, a granular ClC-3 Cl(-) channel provides a shunt pathway for a vacuolar-type H(+)-ATPase. Acidification "primes" the granules for Ca(2+)-dependent exocytosis and release of insulin. In summary, secretory granules are equipped with specific sets of ion channels, which modulate regulated exocytosis and the release of macromolecules. These channels could represent excellent targets for therapeutic interventions to control exocytotic secretion in relevant diseases, such as pancreatitis, cystic fibrosis, or diabetes mellitus.

Pharmacological treatment of the ion transport defect in cystic fibrosis

Cystic fibrosis (CF) is a lethal monogenetic disease characterised by impaired water and ion transport over epithelia. The lung pathology is fatal and causes death in 95% of CF patients. The genetic basis of the disease is a mutation in the cystic fibrosis transmembrane conductance regulator (CFTR), a cAMP-regulated chloride channel. The most common mutation, DeltaF508, results in a protein that cannot properly be folded in the endoplasmic reticulum, is destroyed and hence does not reach the apical cell membrane. This paper will discuss those pharmacological approaches that are directed at correcting the defect in ion transport. At present, no clinically effective drug is available, although research has defined areas in which progress might be made. These are the following: (1) the drug 4-phenylbutyrate (4PBA) increases the expression of DeltaF508-CFTR in the cell membrane, probably by breaking the association between DeltaF508-CFTR and a chaperone; (2) a number of xanthines, in particular 8-cyclopentyl-1, 3-dipropylxanthine (CPX), are effective in activating CFTR, presumably by direct binding and also possibly by correcting the trafficking defect; (3) the isoflavone genistein can activate both wild-type and mutant CFTR, probably through direct binding to the channel; (4) purinergic agonists (ATP and UTP) can stimulate chloride secretion via a Ca(2+)-dependent chloride channel and in this way compensate for the defect in CFTR, but stable analogues will be required before this type of treatment has clinical significance; (5) treatment with inhaled amiloride may correct the excessive absorption of Na(+) ions and water by airway epithelial cells that appears connected to the defect in CFTR; although clinical tests have not been very successful so far, amiloride analogues with a longer half-life may give better results. The role of CFTR in bicarbonate secretion has not yet been established with certainty, but correction of the defect in bicarbonate secretion may be important in clinical treatment of the disease. Currently, major efforts are directed at developing a pharmacological treatment of the ion transport defect in CF, but much basic research remains to be done, in particular, with regard to the mechanism by which defective CFTR is removed in the endoplasmic reticulum by the ubiquitin-proteasome pathway, which is a central pathway in protein production and of significance for several other diseases apart from CF.

Clinical and pharmacological experience with LJP-394

LJP-394 is a synthetic biological with immunomodulatory functions. Composed of four double-stranded oligodeoxynucleotides attached to a central branched platform, the drug acts as an anti-"anti-ds-DNA" B-cell toleragen by rendering specific B-lymphocytes unresponsive to immunogen so they do not produce autoantibodies. Extensive animal studies and Phase II clinical trials suggested that the effects of LJP-394 are effective and safe when used as a weekly dose of 100 mg intravenously. Analysis of a multicentre, international Phase II/III clinical trial showed that patients with lupus nephritis and high affinity IgG antibodies to LJP-394 have clinical benefits. This includes increased time to renal flares, reduced number of renal flares, time to institution of high-dose corticosteroids and/or cyclophosphamide and lower anti-ds-DNA levels. A definitive trial is in progress. LJP-394 appears to be free of serious adverse reactions. Though promising, the role of LJP-394 in patients with active, organ-threatening lupus is still not known.

A cyclic nucleotide PDE5 inhibitor corrects defective mucin secretion in submandibular cells containing antibody directed against the cystic fibrosis transmembrane conductance regulator protein

A selective cyclic nucleotide PDE5 inhibitor corrected the defective mucin secretion response to the beta-agonist isoproterenol in submandibular acinar cells inhibited by antibody directed against the cystic fibrosis transmembrane conductance regulator. The PDE5 inhibitor was as effective as cpt-cyclic AMP or a selective PDE4 inhibitor. However, the PDE5 inhibitor had no effect on basal or isoproterenol-stimulated cyclic AMP levels and did not stimulate mucin secretion. The results showing, for the first time, correction of the CFTR mucin secretion defect by a PDE5 inhibitor, which may involve cyclic GMP, will have a major impact in development of a rational drug treatment for cystic fibrosis.

Intracellular CFTR: localization and function

Intracellular CFTR: Localization and Function. Physiol. Rev. 79, Suppl.: S175-S191, 1999. - There is considerable evidence that CFTR can function as a chloride-selective anion channel. Moreover, this function has been localized to the apical membrane of chloride secretory epithelial cells. However, because cystic fibrosis transmembrane conductance regulator (CFTR) is an integral membrane protein, it will also be present, to some degree, in a variety of other membrane compartments (including endoplasmic reticulum, Golgi stacks, endosomes, and lysosomes). An incomplete understanding of the molecular mechanisms by which alterations in an apical membrane chloride conductance could give rise to the various clinical manifestations of cystic fibrosis has prompted the suggestion that CFTR may also play a role in the normal function of certain intracellular compartments. A variety of intracellular functions have been attributed to CFTR, including regulation of membrane vesicle trafficking and fusion, acidification of organelles, and transport of small anions. This paper aims to review the evidence for localization of CFTR in intracellular organelles and the potential physiological consequences of that localization.

Pathophysiology of gene-targeted mouse models for cystic fibrosis

Pathophysiology of Gene-Targeted Mouse Models for Cystic Fibrosis. Physiol. Rev. 79, Suppl.: S193-S214, 1999. - Mutations in the gene causing the fatal disease cystic fibrosis (CF) result in abnormal transport of several ions across a number of epithelial tissues. In just 3 years after this gene was cloned, the first CF mouse models were generated. The CF mouse models generated to date have provided a wealth of information on the pathophysiology of the disease in a variety of organs. Heterogeneity of disease in the mouse models is due to the variety of gene-targeting strategies used in the generation of the CF mouse models as well as the diversity of the murine genetic background. This paper reviews the pathophysiology in the tissues and organs (gastrointestinal, airway, hepatobiliary, pancreas, reproductive, and salivary tissue) involved in the disease in the various CF mouse models. Marked similarities to and differences from the human disease have been observed in the various murine models. Some of the CF mouse models accurately reflect the ion-transport abnormalities and disease phenotype seen in human CF patients, especially in gastrointestinal tissue. However, alterations in airway ion transport, which lead to the devastating lung disease in CF patients, appear to be largely absent in the CF mouse models. Reasons for these unexpected findings are discussed. This paper also reviews pharmacotherapeutic and gene therapeutic studies in the various mouse models.

Actions of adenosine A1 and A2 receptor antagonists on CFTR antibody-inhibited beta-adrenergic mucin secretion response

1. The cystic fibrosis gene protein, the cystic fibrosis transmembrane conductance regulator (CFTR) acts as a chloride channel and is a key regulator of mucin secretion. The mechanism by which 3-isobutyl-1-methylxanthine (IBMX) corrects the defect in CFTR mediated beta-adrenergic stimulation of mucin secretion has not been determined. The present study has investigated the actions of adenosine A1 and A2 receptor antagonists to determine whether ability to stimulate mucin secretion correlates with correction of CFTR antibody inhibited beta-adrenergic response and whether excessive cyclic AMP rise is required. 2. CFTR antibodies were introduced into living rat submandibular acini by hypotonic swelling. Following recovery, mucin secretion in response to isoproterenol was measured. 3. The adenosine A1 receptor antagonist, 8 cyclopentyltheophylline (CPT) was a less potent stimulator of mucin secretion than was the A2 receptor antagonist dimethylpropargylxanthine (DMPX). A concentration of CPT close to the Ki for A1 receptor antagonism (10 nM) did not stimulate mucin secretion. 4. DMPX, although a potent stimulator of mucin secretion, did not correct CFTR antibody inhibited mucin secretion. 5. CPT corrected defective CFTR antibody inhibited mucin secretion at a high (1 mM) concentration, suggesting a mechanism other than adenosine receptor antagonism. 6. DMPX potentiated the isoproterenol induced cyclic AMP rise, whereas CPT did not. 7. Correction of the defective CFTR mucin secretion response did not correlate with ability to stimulate mucin secretion and did not require potentiation of beta-adrenergic induced increases in cyclic AMP. This affords real promise for the development of a selective drug treatment for cystic fibrosis.

A delta F508 mutation in mouse cystic fibrosis transmembrane conductance regulator results in a temperature-sensitive processing defect in vivo

The most prevalent mutation (delta F508) in cystic fibrosis patients inhibits maturation and transfer to the plasma membrane of the mutant cystic fibrosis transmembrane conductance regulator (CFTR). We have analyzed the properties of a delta F508 CFTR mouse model, which we described recently. We show that the mRNA levels of mutant CFTR are normal in all tissues examined. Therefore the reduced mRNA levels reported in two similar models may be related to their intronic transcription units. Maturation of mutant CFTR was greatly reduced in freshly excised oviduct, compared with normal. Accumulation of mutant CFTR antigen in the apical region of jejunum crypt enterocytes was not observed, in contrast to normal mice. In cultured gallbladder epithelial cells from delta F508 mice, CFTR chloride channel activity could be detected at only two percent of the normal frequency. However, in mutant cells that were grown at reduced temperature the channel frequency increased to over sixteen percent of the normal level at that temperature. The biophysical characteristics of the mutant channel were not significantly different from normal. In homozygous delta F508 mice we did not observe a significant effect of genetic background on the level of residual chloride channel activity, as determined by the size of the forskolin response in Ussing chamber experiments. Our data show that like its human homologue, mouse delta F508-CFTR is a temperature sensitive processing mutant. The delta F508 mouse is therefore a valid in vivo model of human delta F508-CFTR. It may help us to elucidate the processing pathways of complex membrane proteins. Moreover, it may facilitate the discovery of new approaches towards therapy of cystic fibrosis.

Intracellular free Ca2+ dynamic changes to histamine are reduced in cystic fibrosis human tracheal gland cells

This study documents a difference between cystic fibrosis human (CF-HTG) and normal human (HTG) tracheal gland cells: the ability of histamine to induce an increase of intracellular free calcium concentration [Ca2+]i was abnormally reduced in CF-HTG cells. The magnitude of the [Ca2+]i peak rise in response to histamine is smaller in CF-HTG cells than in HTG cells, and the percentage of CF-HTG cells that increase [Ca2+]i is decreased compared with HTG cells. In contrast to histamine, the human neutrophil elastase (HNE) stimulation of both CF-HTG and HTG cells generated [Ca2+]i asynchronous oscillations and the magnitude of the peak [Ca2+]i response as well as the percentage of responding cells were similar for both groups. By videomicroscopy observations, the secretory response (exocytosis of secretion granules) of CF-HTG cells occurred with HNE, but not with histamine, thus suggesting that [Ca2+]i asynchronous oscillations may be linked to the exocytosis process in human tracheal gland cells.