Cystic fibrosis: the 'bicarbonate before chloride' hypothesis

The specific effects of some mutations that cause cystic fibrosis suggest that reduced HCO(3)(-) transport is the key to understanding cystic fibrosis pathology. But there is a puzzling discrepancy between measures of CFTR-mediated chloride conductance in expression systems and the sweat chloride values of patients.

Cystic fibrosis transmembrane conductance regulator gating requires cytosolic electrolytes

Cystic fibrosis transmembrane conductance regulator (CFTR), which causes cystic fibrosis when nonfunctional, is an anion channel and a member of the ATP binding cassette superfamily. After phosphorylation, CFTR gates by binding and hydrolyzing ATP. We show that CFTR open probability (P(o)) also depends on the electrolyte concentration of the cytosol. Inside-out patches from Calu-3 cells were transiently exposed to solutions of 160 mm salt or solutions in which up to 90% of the salt was replaced by nonionic osmolytes such as sucrose. In lowered salt solutions, CFTR P(o) declined within 1 s to a stable lower value that depended on the electrolyte concentration, (K(1/2) approximately 80 mm NaCl). P(o) was rapidly restored in normal salt concentrations without regard to the electrolyte species. Reducing external electrolytes did not affect CFTR P(o). The same results were obtained when CFTR was stably phosphorylated with adenosine 5'-O-(thiotriphosphate). The decrease in P(o) resulted entirely from an increase in mean closed time. Increasing ATP levels up to 20-fold did not counteract the effect of low electrolytes. The same effect was observed for CFTR expressed in C127 cells but not for a different species of anion channel. Cytosolic electrolytes are an unsuspected, essential cofactor for CFTR gating.

Comprehensive mutation screening in a cystic fibrosis center

OBJECTIVES AND BACKGROUND

The identities of a cystic fibrosis (CF) patient's CFTR mutations can influence therapeutic strategies, but because >800 CFTR mutations exist, cost-effective, comprehensive screening requires a multistage approach. Single-strand conformation polymorphism and heteroduplex analysis (SSCP/HA) can be an important part of mutation detection, but must be calibrated within each laboratory. The sensitivity of a combined commercial-SSCP/HA approach to genotyping in a large, ethnically diverse US center CF population has not been established.

STUDY DESIGN

We screened all 27 CFTR exons in 10 human participants who had an unequivocal CF diagnosis including a positive sweat chloride test and at least 1 unknown allele after commercial testing for the 70 most common mutations by SSCP/HA. These participants were compared with 7 participants who had negative sweat tests but at least 1 other CF-like symptom meriting complete genotyping.

RESULTS

For the 10 CF participants, we detected 11 of 16 unknown alleles (69%) and all 4 of the known alleles (100%), for an overall rate of 75% inpatients not fully genotyped by conventional 70 mutation screen. For 7 participants with negative sweat tests, we confirmed 1 identified mutation in 14 alleles and detected 3 additional mutations. Mutations detected in both groups included 7 missense mutations (S13F, P67L, G98R, S492F, G970D, L1093P, N1303K) and 9 deletion, frameshift, nonsense or splicing mutations (R75X, G542X, DeltaF508, 451-458Delta8 bp, 5T, 663DeltaT, exon 13 frameshift, 1261+1G-->A and 3272-26A-->G). Three of these mutations were novel (G970D, L1093P, and 451-458Delta8 bp(1)). Thirteen other changes were detected, including the novel changes 1812-3 ins T, 4096-278 ins T, 4096-265 ins TG, and 4096-180 T-->G.

CONCLUSION

When combined with the 70 mutation Genzyme test, SSCP/HA analysis allows for detection of >95% of the mutations in an ethnically heterogeneous CF center population. We discuss 5 possible explanations that could account for the few remaining undetected mutations.

CFTR activation raises extracellular pH of NIH/3T3 mouse fibroblasts and C127 epithelial cells

Cystic Fibrosis (CF) is caused by mutations in the gene for CFTR, a cAMP-activated anion channel found in apical membranes of wet epithelia. Since CFTR is permeable to HCO3- and changes in extracellular fluid composition may contribute to CF lung disease, we investigated possible differences in extracellular pH (pHo) between CFTR-expressing and control cell lines. The Cytosensor Microphysiometer was used to study forskolin-stimulated extracellular acidification rates in CFTR-expressing and control mouse mammary epithelial (C127) and fibroblast (NIH/3T3) cell lines. Forskolin, which activates CFTR via raised cAMP, caused decreased extracellular acidification of CFTR-expressing NIH/3T3 and C127 cells by 15-35%. By contrast, forskolin caused increased extracellular acidification of control cells by 10-20%. Ionomycin, which may activate CFTR via PKC, also elicited this decreased extracellular acidification signal only in cells expressing CFTR. In control experiments, dideoxyforskolin had no effect on the acidification rates and osmotic stimuli were shown to equally stimulate all cell lines. These results suggest a role for CFTR in controlling pHo and complement recent evidence that HCO3- dependent epithelial secretion may be reduced in amount and altered in composition in CF.

Novel Cystic Fibrosis mutation L1093P: functional analysis and possible Native American origin

A novel mutation was detected using single-strand conformation polymorphism and heteroduplex analysis in a cystic fibrosis subject of mixed ancestry. Mutation 3410T-->C in exon 17b caused the novel missense mutation L1093P; the other chromosome has mutation N1303K. The 31-year-old subject is pancreatic insufficient, had an FEV(1) score that was 33% of normal prior to a heart/lung transplant, and sweat chloride values of 116 and 95 mM when tested at ages 1 and 11. Functional analysis using forskolin-stimulated efflux of (125)I in HEK cells transfected with an ABCC7 construct harboring the L1093P mutation confirmed that cAMP-mediated anion efflux was abnormal, but some function was preserved. Analysis of parental DNA established that N1303K was of English origin, while L1093P was of Greek, Irish or Native American (Cherokee) origin. Given the intensive screening for CF mutations in European populations, we hypothesize that L1093P is of Native American origin. Hum Mutat 15:208, 2000.

Two novel mutations in a cystic fibrosis patient of Chinese origin

Cystic fibrosis is rare in non-Caucasian populations, and in such populations little is known about the spectrum of mutations and polymorphisms in the CFTR gene. We studied a 23-year-old patient of Chinese ethnicity with sweat chloride values of 104 mM/l, pancreatic sufficiency, an FEV1 60% of normal, sputum cultures positive for Staphylococcus aureus and Burkholderia cepacia, and a history of allergic bronchopulmonary aspergillosis. Genetic screening for 31 common CFTR mutations was negative, leading us to search for unknown mutations using single-strand conformation polymorphism and heteroduplex analysis (SSCP/HA). Two novel mutations were detected. In exon 4, a deletion of 8 bp (451458, deltaGCTTCCTA) causes a frameshift and immediately creates a stop codon. In exon 16, mutation 3041G-->A causes the missense change G970D. Functional analysis using an isotopic flux assay indicated that the G970D mutation retains partial function; western blotting indicated that the protein is glycosylated. The patient is heterozygous for the common polymorphisms (2694T/G) in exon 14a and (GATT)6/7 in intron 6a, indicating that these variants arose in ancestors common to Caucasians and Chinese.

Safety and biological efficacy of an adeno-associated virus vector-cystic fibrosis transmembrane regulator (AAV-CFTR) in the cystic fibrosis maxillary sinus

OBJECTIVE

The host immune response and low vector efficiency have been key impediments to effective cystic fibrosis transmembrane regulator (CFTR) gene transfer for cystic fibrosis (CF). An adeno-associated virus vector (AAV-CFTR) was used in a phase I dose-escalation study to transfer CFTR cDNA into respiratory epithelial cells of the maxillary sinus of 10 CF patients.

STUDY DESIGN

A prospective, randomized, unblinded, dose-escalation, within-subjects, phase I clinical trial of AAV-CFTR was conducted.

PATIENTS

Ten patients with previous bilateral maxillary antrostomies were treated.

MAIN OUTCOME MEASURES

Safety, gene transfer as measured by semiquantitative polymerase chain reaction (PCR), and sinus transepithelial potential difference (TEPD) were measured.

RESULTS

The highest level of gene transfer was observed in the range of 0.1-1 AAV-CFTR vector copy per cell in biopsy specimens obtained 2 weeks after treatment. When tested, persistence was observed in one patient for 41 days and in another for 10 weeks. Dose-dependent changes in TEPD responses to pharmacologic intervention were observed following treatments. Little or no inflammatory or immune responses were observed.

CONCLUSION

AAV-CFTR administration to the maxillary sinus results in successful, dose-dependent gene transfer to the maxillary sinus and alterations in sinus TEPD suggestive of a functional effect, with little or no cytopathic or host immune response. Further study is warranted for AAV vectors as they may prove useful for CFTR gene transfer and other in vivo gene transfer therapies. A prospective, randomized, double-blind, placebo-controlled, within-subjects, phase II clinical trial of the effect AAV-CFTR on clinical recurrence of sinusitis will determine the clinical efficacy of AAV gene therapy for CF.

Maxillary sinusitis as a surrogate model for CF gene therapy clinical trials in patients with antrostomies

BACKGROUND

Assessing the biological activity and clinical efficacy of gene therapy is critically important in cystic fibrosis (CF). It is widely accepted that clinical testing using surrogate markers including pulmonary function will be useful in assessing clinical efficacy. One problem with pulmonary surrogate markers of CF disease is the large number of patients and length of time required to demonstrate clinical efficacy. An alternative to pulmonary testing of new CF treatments is use of the maxillary sinuses as a surrogate model of CF lung disease. Using CF sinusitis as a surrogate model for testing clinical efficacy of new treatments is attractive because CF upper respiratory disease is similar to the lower respiratory disease with respect to electrophysiology and microbiology.

METHODS

Sinusitis recurrence in untreated sinuses was analyzed during a prospective, randomized, unblinded, dose-escalation, within-subjects, phase I clinical trial of the adeno-associated virus mediated cystic fibrosis transmembrane conductance regulator (AAV-CFTR) gene transfer.

RESULTS

Clinical symptoms combined with sinus endoscopy proved useful in the diagnosis of unilateral and bilateral sinusitis recurrence. Sinusitis recurred at a rate of 45% during one month of follow-up. IL-8 concentration rose in sinus fluids from affected sinuses. Bacterial cultures and increased sinus leukocytes corroborated recurrent sinusitis. Sinus CT scans were also useful in diagnosing recurrent sinusitis in this surrogate model of CF infectious exacerbations.

CONCLUSIONS

CF sinusitis as a surrogate for lung disease is particularly well-suited for phase II clinical trials of gene transfer agents, with the potential for measuring clinical efficacy in relatively small numbers of patients over relatively short periods of time.

Genomic DNA sequence of Rhesus (M. mulatta) cystic fibrosis (CFTR) gene

Cystic fibrosis is a common human genetic disease caused by mutations in CFTR, a gene that codes for a chloride channel that is regulated by phosphorylation and cytosolic nucleotides. As part of a program to discover natural animal models for human genetic diseases, we have determined the genomic sequence of CFTR in the Rhesus monkey, Macaca mulatta. The coding region of rhesus CFTR is 98.3% identical to human CFTR at the nucleotide level and 98.2% identical and 99.7% similar at the amino acid level. Partial sequences of flanking introns (5582 base pair positions analyzed) revealed 91.1% identity with human introns. Relative to rhesus intronic sequence, the human sequences had 27 insertions and 22 deletions. Primer sequences for amplification of rhesus genomic CFTR sequences are provided. The accession number is AF013753 (all 27 exons and some flanking intronic sequence).

Evidence that Calu-3 human airway cells secrete bicarbonate

The Calu-3 cell line is being investigated as a model for human submucosal gland serous cells. In a previous investigation of basal short-circuit current (Isc) in Calu-3 cells, high levels of bumetanide-insensitive basal Isc (approximately 60 microA/cm2) were measured in cells grown at an air interface. Basal Isc was reduced only 7% by bumetanide, and the largest component of basal Isc required both Cl- and HCO3- in the bathing solutions. Because Isc could be partially inhibited by basolateral 4,4'-dinitrostilbene-2,2'-disulfonic acid and because the only known apical exit pathway for anions is the cystic fibrosis transmembrane conductance regulator, which has a relatively poor conductance for HCO3-, it was concluded that most basal Isc is HCO3(-)-dependent Cl- secretion [M. Singh, M. Krouse, S. Moon, and J. J. Wine. Am. J. Physiol. 272 (Lung Cell. Mol. Physiol. 16): L690-L698, 1997]. We have now measured isotopic fluxes of 36Cl- and 22Na+ across short-circuited Calu-3 cells and found that virtually none of the basal Isc is Cl- secretion or Na+ absorption. Thus, in contrast to the earlier report, we conclude that the major component of basal Isc is HCO3- secretion. Stimulation recruits primarily Cl- secretion, as previously proposed.

Calcium-stimulated Cl- secretion in Calu-3 human airway cells requires CFTR

Human airway serous cells secrete antibiotic-rich fluid, but, in cystic fibrosis (CF), Cl(-)-dependent fluid secretion is impaired by defects in CF transmembrane conductance regulator (CFTR) Cl- channels. Typically, CF disrupts adenosine 3',5'-cyclic monophosphate (cAMP)-mediated Cl- secretion but spares Ca(2+)-mediated secretion. However, in CF airway glands, Ca(2+)-mediated secretion is also greatly reduced. To determine the basis of Ca(2+)-mediated Cl- secretion in serous cells, we used thapsigargin to elevate intracellular Ca2+ concentration ([Ca2+]i) in Calu-3 cells, an airway cell line bearing some similarities to serous cells. Cells were cultured using conventional and air interface methods. Short-circuit current (Isc) and transepithelial conductance (Gte) were measured in confluent cell layers. Thapsigargin stimulated large, sustained changes (delta) in Isc and Gte, whereas forskolin stimulated variable and smaller increases. delta Isc was decreased by basolateral bumetanide, quinidine, barium, or diphenylamine-2-carboxylate (DPAC) but was unaffected by high apical concentrations of 4,4'-diisothiocyanostilbene-2,2'-disulfonic acid (DIDS), 4,4'-dinitrostilbene-2,2'-disulfonic acid, and calixarene. Isc was measured after permeabilizing the basolateral membrane and establishing transmembrane ion gradients. Unstimulated apical membranes displayed high Cl- conductance (GCl) that was decreased by DPAC but not by DIDS. Apical GCl could be increased by elevating intracellular cAMP concentration but not [Ca2+]i. We conclude that CFTR channels are the exclusive GCl pathway in the apical membrane and display approximately 60% of maximum conductance at rest. Thus elevated [Ca2+]i increases K+ conductance to force Cl- through open CFTR channels. We hypothesize that loss of CFTR channels causes diminution of cholinergically mediated gland secretions in CF.

Disruption of monolayer integrity enables activation of a cystic fibrosis "bypass" channel in human airway epithelia

Cystic fibrosis (CF) is a genetic disease characterized by marked reduction in Cl- conductance across many epithelia. Two kinds of Cl- channels have been associated with CF. One channel, termed the cystic fibrosis transmembrane conductance regulator (CFTR), is directly coded by the CF gene. The other channel is an outwardly rectifying depolarization induced Cl- channel (ORDIC) that is distinguished from other outwardly rectifying chloride channels (ORCCs) because its activity is induced most reliably by patch excision and depolarization. An issue in current CF research is whether ORDIC channels are indirectly activated by CFTR to contribute a significant portion of apical membrane Cl- conductance in airway cells. We now show that ORDIC channels are readily activated in patches excised and depolarized from isolated cells, but are rarer or refractory to activation in patches from the apical membranes of confluent human airway epithelia. These findings have important implications for proposed therapies that would bypass the CFTR conductance by activating ORDIC channels.

Most basal I(SC) in Calu-3 human airway cells is bicarbonate-dependent Cl- secretion

Serous cells secrete antibiotic-rich fluid, but secretion is impaired in cystic fibrosis. We are investigating Calu-3 cells as a serous cell model. Basal short-circuit current (I[SC]) in Calu-3 cells grown at air interface had a basal I(SC) approximately six times larger than submerged cultures (69 +/- 22 vs. 11 +/- 10 microA/cm2). Basal I(SC) in either condition was reduced only 7 +/- 5% by bumetanide and was unaffected by apical amiloride, 4,4'-diisothiocyanostilbene-2,2'-disulfonic acid, 4,4'-dinitrostilbene-2,2'-disulfonic acid (DNDS), or calixarene but was reduced 77 +/- 18% by N-phenylanthranilic acid. Three transport mechanisms accounted for almost all basal I(SC). The largest component is HCO3(-)-dependent Cl- secretion. Replacement of Krebs-Henseleit solution with N-2-hydroxyethylpiperazine-N'-2-ethanesulfonic acid-buffered solution and changing gassing from 95% O2-5% CO2 to air reduced the basal I(SC) by 61 +/- 10%. Acetazolamide decreased basal I(SC) by 33 +/- 6%, whereas acetazolamide + basolateral DNDS eliminated 42-58% of the bumetanide-insensitive basal I(SC). Neither DNDS nor acetazolamide had any effect when applied in HCO3(-)-free solution. Apical phlorizin, a blocker of Na+-glucose cotransport, eliminated one-half of the remaining I(SC). Cl- replacement with gluconate eliminated all I(SC) except the phlorizin-sensitive component. Unlike basal I(SC), 80 +/- 24% of stimulated I(SC) was inhibited by bumetanide. Thus basal and stimulated secretions are mediated by different mechanisms.

Cell-volume regulation: P-glycoprotein--a cautionary tale

P-glycoprotein turns out not to be 'VSOAC', a known channel activated by cell swelling; it does seem to influence cell-volume recovery after swelling, but the physiological importance of this effect is presently unclear.

Swelling and Ca2+-activated anion conductances in C127 epithelial cells expressing WT and delta F508-CFTR

CFTR is a chloride channel that is required for fluid secretion and salt absorption in many exocrine epithelia. Mutations in CFTR cause cystic fibrosis. CFTR expression influences some ion channels, but the range of channels influenced, the mechanism of the interaction and the significance for cystic fibrosis are not known. Possible interactions between CFTR and other ion channels were studied in C127 mouse mammary epithelial cell lines stably transfected with CFTR, delta F508-CFTR, or vector. Cell lines were compared quantitatively using an 125I efflux assay and qualitatively using whole-cell patch-clamp recording. As expected, 125I efflux was significantly increased by forskolin only in the CFTR line, and forskolin-stimulated whole-cell currents were time- and voltage independent. All three lines responded to hypotonic challenge with large 125I efflux responses of equivalent magnitude, and whole-cell currents were outwardly rectified and inactivated at positive voltages. Unexpectedly, basal 125I efflux was significantly smaller in the delta F508-CFTR cell line than in either the CFTR or control cell lines (P < 0.0001), and the magnitude of the efflux response to ionomycin was largest in the vector cell line and smallest in the cell line expressing delta F508-CFTR (P < 0.01). Whole-cell responses to ionomycin had a linear instantaneous I-V relation and activated at depolarizing voltages. Forskolin responses showed simple summation with responses to ionomycin or hypotonic challenge. Thus, we found no evidence for interactions between CFTR and the channels responsible for swelling-mediated responses. Differences were found in basal and ionomycin-stimulated efflux, but these may arise from variations in the clonally selected cell lines that are unrelated to CFTR expression.

Delta F508-CFTR channels: kinetics, activation by forskolin, and potentiation by xanthines

Trafficking, activation, and kinetics of delta F508-cystic fibrosis transmembrane conductance regulator (CFTR) and CFTR were compared in stably transduced C127I mouse mammary epithelial cells. Western blots detected a small amount of fully glycosylated delta F508-CFTR Efflux of 125I was stimulated by forskolin with the same mean effective concentration (EC50; approximately 0.5 microM) for CFTR and delta F508-CFTR cells, but the maximum response was reduced more than fivefold and its latency increased approximately threefold in delta F508-CFTR cells. In delta F508-CFTR cells, 3-isobutyl-1-methylxanthine (IBMX; EC50 = 1.45 microM) and 8-cyclopentyl-1,3-dipropylxanthine (CPX; EC50 = 58 microM) increased the peak forskolin-stimulated efflux rate approximately 2.5-fold and decreased the time to peak. A sevenfold increase in intracellular adenosine 3',5'-cyclic monophosphate (cAMP) levels accompanied potentiation of forskolin-induced 125I efflux by IBMX but not by CPX. Elevation of intracellular cAMP increased linear voltage-independent whole cell currents 30-fold in CFTR and 4-fold in delta F508-CFTR cells; the response rate in delta F508-CFTR cells was much slower. Single-channel currents were detected in 57 of 68 cell-attached patches from forskolin-prestimulated CFTR cells vs. 6 of 35 patches in delta F508-CFTR cells. Mean number of active channels per patch was 4.1 for CFTR [open probability (Po) = 0.34] and 0.2 for delta F508-CFTR (Po = 0.11). The lower Po of delta F508-CFTR resulted from an approximately threefold longer mean interburst interval. We estimate that forskolin-stimulated chloride conductance of delta F508-CFTR C127I cells is < 5% of CFTR cells. CPX is approximately 25-fold more potent than IBMX in potentiating delta F508-CFTR and may operate by a mechanism other than elevation of cAMP.

Doxorubicin selection for MDR1/P-glycoprotein reduces swelling-activated K+ and Cl- currents in MES-SA cells

To test the hypothesis that P-glycoprotein enhances swelling currents through regulation of volume-sensitive Cl- channels [recently termed VSOAC (volume-sensitive osmolyte and anion channel)], a human uterine sarcoma cell line (MES-SA) and its doxorubicin-selected counterpart (Dx5) were studied. P-glycoprotein mRNA and protein levels were detected only in Dx5 cells. However, whole cell patch-clamp experiments showed that swollen Dx5 cells (n = 5) produced smaller VSOAC currents than MES-SA cells (n = 4; 106 +/- 26 pA/pF vs. 232 +/- 76 pA/pF at 90 mV). In radioisotopic efflux experiments, both swelling-activated 125I (Cl-) currents (n = 15) and 86Rb (K+) currents (n = 8) were found to be two-to fourfold smaller in the Dx5 (high P-glycoprotein) cells. Inhibitors of P-glycoprotein showed no specificity for the doxorubicin-selected cells (Dx5). Dideoxyforskolin (100 microM) blocked swelling-activated 125I efflux equally in both cell lines, whereas 100 microM verapamil had no effect. Thus, in this cell line, selection for P-glycoprotein expression is associated with reduced swelling currents. These findings suggest that P-glycoprotein expression does not directly facilitate VSOAC.

Failure of the cystic fibrosis transmembrane conductance regulator to conduct ATP

The cystic fibrosis transmembrane conductance regulator (CFTR) is a chloride ion channel regulated by protein kinase A and adenosine triphosphate (ATP). Loss of CFTR-mediated chloride ion conductance from the apical plasma membrane of epithelial cells is a primary physiological lesion in cystic fibrosis. CFTR has also been suggested to function an an ATP channel, although the size of the ATP anion is much larger than the estimated size of the CFTR pore. ATP was not conducted through CFTR in intact organs, polarized human lung cell lines, stably transfected mammalian cell lines, or planar lipid bilayers reconstituted with CFTR protein. These findings suggest that ATP permeation through the CFTR is unlikely to contribute to the normal function of CFTR or to the pathogenesis of cystic fibrosis.

Glycerol reverses the misfolding phenotype of the most common cystic fibrosis mutation

The common delta F508 mutation in the cystic fibrosis transmembrane conductance regulator (CFTR) interferes with the biosynthetic folding of nascent CFTR polypeptides, leading to their retention and rapid degradation in an intracellular compartment proximal to the Golgi apparatus. Neither the pathway by which wild-type CFTR folds nor the mechanism by which the Phe508 deletion interferes with this process is well understood. We have investigated the effect of glycerol, a polyhydric alcohol known to stabilize protein conformation, on the folding of CFTR and delta F508 in vivo. Incubation of transient and stable delta F508 transfectants with 10% glycerol induced a significant accumulation of delta F508 protein bearing complex N-linked oligosaccharides, indicative of their transit to a compartment distal to the endoplasmic reticulum (ER). This accumulation was accompanied by an increase in mean whole cell cAMP activated chloride conductance, suggesting that the glycerol-rescued delta F508 polypeptides form functional plasma membrane CFTR channels. These effects were dose- and time-dependent and fully reversible. Glycerol treatment also stabilized immature (core-glycosylated) delta F508 and CFTR molecules that are normally degraded rapidly. These effects of glycerol were not due to a general disruption of ER quality control processes but appeared to correlate with the degree of temperature sensitivity of specific CFTR mutations. These data suggest a model in which glycerol serves to stabilize an otherwise unstable intermediate in CFTR biosynthesis, maintaining it in a conformation that is competent for folding and subsequent release from the ER quality control apparatus.