A cross-species analysis of the cystic fibrosis transmembrane conductance regulator. Potential functional domains and regulatory sites

Characterizing diverse orthologues of the cystic fibrosis transmembrane conductance regulator protein for structural studies

As an ion channel, the cystic fibrosis transmembrane conductance regulator (CFTR) protein occupies a unique niche within the ABC family. Orthologues of CFTR are extant throughout the animal kingdom from sharks to platypods to sheep, where the osmoregulatory function of the protein has been applied to differing lifestyles and diverse organ systems. In humans, loss-of-function mutations to CFTR cause the disease cystic fibrosis, which is a significant health burden in populations of white European descent. Orthologue screening has proved fruitful in the pursuit of high-resolution structural data for several membrane proteins, and we have applied some of the princples developed in previous studies to the expression and purification of CFTR. We have overexpressed this protein, along with evolutionarily diverse orthologues, in Saccharomyces cerevisiae and developed a purification to isolate it in quantities sufficient for structural and functional studies.

Generation of SV40-transformed rabbit tracheal-epithelial-cell-derived blastocyst by somatic cell nuclear transfer

The prospect of developing large animal models for the study of inherited diseases, such as cystic fibrosis (CF), through somatic cell nuclear transfer (SCNT) has opened up new opportunities for enhancing our understanding of disease pathology and for identifying new therapies. Thus, the development of species-specific in vitro cell systems that will provide broader insight into organ- and cell-type-specific functions relevant to the pathology of the disease is crucial. Studies have been undertaken to establish transformed rabbit airway epithelial cell lines that display differentiated features characteristic of the primary airway epithelium. This study describes the successful establishment and characterization of two SV40-transformed rabbit tracheal epithelial cell lines. These cell lines, 5RTEo- and 9RTEo-, express the CF transmembrane conductance regulator (CFTR) gene, retain epithelial-specific differentiated morphology and show CFTR-based cAMP-dependent Cl(-) ion transport across the apical membrane of a confluent monolayer. Immunocytochemical analysis indicates the presence of airway cytokeratins and tight-junction proteins in the 9RTEo- cell line after multiple generations. However, the tight junctions appear to diminish in their efficacy in both cell lines after at  least 100 generations. Initial SCNT studies with the 9RTEo- cells have revealed that SV40-transformed rabbit airway epithelial donor cells can be used to generate blastocysts. These cell systems provide valuable models for studying the developmental and metabolic modulation of CFTR gene expression and rabbit airway epithelial cell biology.

Molecular and functional characterization of the cystic fibrosis transmembrane conductance regulator from the Australian common brushtail possum, Trichosurus vulpecula

Unlike eutherian mammals, the colon of the Australian common brushtail possum, Trichosurus vulpecula, a metatherian mammal, is incapable of electrogenic Cl(-) secretion and has elevated levels of electrogenic Na(+) absorption, while the ileum secretes HCO (3) (-) rather than Cl(-). In eutherian mammals, the cystic fibrosis transmembrane conductance regulator (CFTR) is essential for both Cl(-) and HCO (3) (-) secretion and the regulation of Na(+) absorption. Therefore, we have sequenced possum (p)CFTR, described its distribution and characterized the properties of cloned pCFTR expressed in Fischer rat thyroid (FRT) cells. pCFTR (GenBank accession No. AY916796) has a 1,478 amino acid open reading frame, which has >90% identity with CFTR from other marsupials and >80% identity with non-rodent eutherian mammals. In pCFTR, there is a high level of conservation of the transmembrane and nucleotide binding domains although, with the exception of other marsupials, there is considerable divergence from other species in the R domain. FRT cells transfected with pCFTR express mature CFTR protein which functions as a small Cl(-) channel activated by cAMP-dependent phosphorylation. In whole-cell recordings it has a linear, time and voltage-independent conductance, with a selectivity sequence P(Br) > P(Cl) > P(I) > P(HCO)(3) >> P(Gluconate). pCFTR transcript is present in a range of epithelia, including the ileum and the colon. The presence of pCFTR in the ileum and its measured HCO (3) (-) permeability suggest that it may be involved in ileal HCO (3) (-) secretion. Why the possum colon does not secrete Cl(-) and has elevated electrogenic Na(+) absorption, despite the apparent expression of CFTR, remains to be determined.

Identification and characterization of CFTR gene mutations in Indian CF patients

Cystic fibrosis (CF) is an autosomal recessive disease caused by mutations in the cystic fibrosis transmembrane conductance regulator (CFTR) gene. This study was performed on Indian CF patients (n = 50) to investigate the spectrum of mutations in the CFTR gene and their association with intragenic and extragenic marker haplotypes. We report identification of 14 previously known and eight novel mutations, namely 3986-3987delC, 876-6del4, 1792InsA, L69H, S158N, Q493L, I530L and E1329Q. The frequency of delta F508 was found to be 27%. Absolute linkage between delta F508 and the KM.19-GATT-TUB9-M470V-T854T haplotype (2-2-1-1-1) predicts a relatively recent appearance of delta F508 in Indian CF patients. Low frequency of delta F508 mutation and detection of eight novel and thirteen rare mutations reflect a heterogeneous spectrum of mutations in Indian CF patients. Failure to detect mutations in 34% of alleles indicates the possible presence of gross deletions involving one or more exons or may indicate the location of the molecular defects in either the noncoding parts of the gene or in the promoter region, which warrants analysis of those regions.

Secretory cells of the airway express molecules of the chemoreceptive cascade

Airway secretion is maintained by specialized non-ciliated epithelial cells whose phenotype varies with their topographical location. In addition, specialized epithelial cells located in the airway contain the molecular machinery of chemoreceptive elements. Our aim has been to evaluate whether the secretory cells themselves possess a chemoreceptive capability, which requires the simultaneous presence of chemosensory and secretory mechanisms. We performed immunohistochemical analysis with antibodies against the Clara-cell-specific secretory proteins, CC10 and CC26, as secretory markers. As chemoreceptive markers, we employed antibodies against alpha-gustducin and phospholipase C beta 2 (PLCbeta2), two components of the taste transduction pathway. We also attempted to characterize further the secretory cell type by using a marker of chloride secretion, cystic fibrosis transmembrane regulator (CFTR). We found alpha-gustducin localized in non-ciliated cells of the epithelium lining the trachea and bronchioles of adult rats, where it was also co-expressed with CC10 and CC26. Ultrastructural immunohistochemistry revealed alpha-gustducin in the apical cytoplasm of secretory cells, concentrated around and inside the granules. CFTR was also observed in a subpopulation of non-ciliated epithelial cells, co-localized with some alpha-gustducin- and PLCbeta2-immunoreactive cells, at all levels of the airway epithelium. We conclude that non-ciliated epithelial cells of the rat airway express components of distinct signaling mechanisms and suggest that secretory events are driven by a molecular mechanism activated by the binding of luminal substances to G-protein-coupled receptors.

Genotype-phenotype correlation and frequency of the 3199del6 cystic fibrosis mutation among I148T carriers: results from a collaborative study

PURPOSE

We expect that the mutation panel currently recommended for preconception/prenatal CF carrier screening will be modified as new information is learned regarding the phenotype associated with specific mutations and allele frequencies in various populations. One such example is the I148T mutation, originally described as a severe CF mutation. After implementation of CF population-based carrier screening, we learned that I148T exists as a complex allele with 3199del6 in patients with clinical CF, whereas asymptomatic compound heterozygotes for I148T and a second severe CF mutation were negative for 3199del6.

METHODS

We performed reflex testing for 3199del6 on 663 unrelated specimens, including I148T heterozygotes, compound heterozygotes, and a homozygous individual.

RESULTS

Less than 1% of I148T carriers were also positive for 3199del6. Excluding subjects tested because of a suspected or known CF diagnosis or positive family history, 0.6% of I148T-positive individuals were also positive for 3199del6. We identified 1 I148T homozygote and 6 unrelated compound heterozygous individuals with I148T and a second CF variant (2 of whom also carried 3199del6). In addition, one fetus with echogenic bowel and one infertile male were heterozygous for I148T (3199del6 negative).

CONCLUSIONS

Reflex testing for 3199del6 should be considered whenever I148T is identified. Reflex testing is of particular importance for any symptomatic patient or whenever one member of a couple carries a deleterious CF mutation and the other member is an I148T heterozygote. Further population data are required to determine if I148T, in the absence of 3199del6, is associated with mild or atypical CF or male infertility.

A combined analysis of the cystic fibrosis transmembrane conductance regulator: implications for structure and disease models

Over the past decade, nearly 1,000 variants have been identified in the cystic fibrosis transmembrane conductance regulator (CFTR) gene in classic and atypical cystic fibrosis (CF) patients worldwide, and an enormous wealth of information concerning the structure and function of the protein has also been accumulated. These data, if evaluated together in a sequence comparison of all currently available CFTR homologs, are likely to refine the global structure-function relationship of the protein, which will, in turn, facilitate interpretation of the identified mutations in the gene. Based on such a combined analysis, we had recently defined a "functional R domain" of the CFTR protein. First, presenting two full-length cDNA sequences (termed sCFTR-I and sCFTR-II) from the Atlantic salmon (Salmo salar) and an additional partial coding sequence from the eastern gray kangaroo (Macropus giganteus), this study went further to refine the boundaries of the two nucleotide-binding domains (NBDs) and the COOH-terminal tail (C-tail), wherein NBD1 was defined as going from P439 to G646, NBD2 as going from A1225 to E1417, and the C-tail as going from E1418 to L1480. This approach also provided further insights into the differential roles of the two halves of CFTR and highlighted several well-conserved motifs that may be involved in inter- or intramolecular interactions. Moreover, a serious concern that a certain fraction of missense mutations identified in the CFTR gene may not have functional consequences was raised. Finally, phylogenetic analysis of all the full-length CFTR amino acid sequences and an extended set of exon 13--coding nucleotide sequences reinforced the idea that the rabbit may represent a better CF model than the mouse and strengthened the assertion that a long-branch attraction artifact separates the murine rodents from the rabbit and the guinea pig, the other Glires.

Two mild cystic fibrosis-associated mutations result in severe cystic fibrosis when combined in cis and reveal a residue important for cystic fibrosis transmembrane conductance regulator processing and function

The number of complex cystic fibrosis transmembrane conductance regulator (CFTR) genotypes identified as having double-mutant alleles with two mutations inherited in cis has been growing. We investigated the structure-function relationships of a severe cystic fibrosis (CF)-associated double mutant (R347H-D979A) to evaluate the contribution of each mild mutation to the phenotype. CFTR mutants expressed in HeLa cells were analyzed for protein biosynthesis and Cl(-) channel activity. Our data show that R347H is associated with mild defective Cl(-) channel activity and that the D979A defect leads to misprocessing. The mutant R347H-D979A combines both defects for a dramatic decrease in Cl(-) current. To decipher the molecular mechanism of this phenotype, single and double mutants with different charge combinations at residues 347 and 979 were constructed as charged residues were involved in this complex genotype. These studies revealed that residue 979, located in the third cytoplasmic loop, is critical for CFTR processing and Cl(-) channel activity highlighting the role of charged residues. These results have also important implications for CF, as they show that two mutations in cis can act in concert to alter dramatically CFTR function contributing to the wide phenotypic variability of CF disease.

Definition of a "functional R domain" of the cystic fibrosis transmembrane conductance regulator

The R domain of the cystic fibrosis transmembrane conductance regulator (CFTR) was originally defined as 241 amino acids, encoded by exon 13. Such exon/intron boundaries provide a convenient way to define the R domain, but do not necessarily reflect the corresponding functional domain within CFTR. A two-domain model was later proposed based on a comparison of the R-domain sequences from 10 species. While RD1, the N-terminal third of the R domain is highly conserved, RD2, the large central region of the R domain has less rigid structural requirements. Although this two-domain model was given strong support by recent functional analysis data, the simple observation that two of the four main phosphorylation sites are excluded from RD2 clearly indicates that RD2 still does not satisfy the requirements of a "functional R domain." Nevertheless, knowledge of the CFTR structure and function accumulated over the past decade and reevaluated in the context of a comprehensive sequence comparison of 15 CFTR homologues made it possible to define such a "functional R domain," i.e., amino acids C647 to D836. This definition is validated primarily because it contains all of the important potential consensus phosphorylation sequences. In addition, it includes the highly charged motif from E822 to D836. Finally, it includes all of the deletions/insertions in this region. This definition also aids in understanding the effects of missense mutations occurring within this domain.

Biochemical evidence for ATPase activity in CFTR-enriched apical membrane vesicles from tracheal epithelium

In apical membrane vesicles from beef tracheal epithelia expressing up to 30% of the proteins as functional cystic fibrosis transmembrane conductance regulator (CFTR)-- i.e. a voltage-independent and PKA-sensitive 36Cl- flux--an ATPase activity, different from P, F0F1 and V types, was reproducibly detected. Its specific activity averaged 20 micromol Pi h(-1) mg(-1) with an apparent affinity for ATP of 530 +/- 30 microM. Its possible involvement in CFTR functions was supported by (1) the linear relationship between the ATPase activity and the magnitude of 36Cl- fluxes (turnover rate: 3 ATP hydrolyzed per CFTR per second), (2) the same rank of potency of ATP, ITP, GTP, UTP and CTP to be hydrolyzed and to open CFTR chloride channels, (3) the similar and parallel inhibition of the ATPase and CFTR Cl- fluxes by NS004 (IC50: 60 microM) and (4) the potency of anti-R domain antibodies to increase by 18% the ATPase activity.

Permeation through the CFTR chloride channel

The cystic fibrosis transmembrane conductance regulator (CFTR) protein forms a Cl(−) channel found in the plasma membranes of many epithelial cells, including those of the kidney, gut and conducting airways. Mutation of the gene encoding CFTR is the primary defect in cystic fibrosis, a disease that affects approximately 30 000 individuals in the United States alone. Alteration of CFTR function also plays an important role in the pathophysiology of secretory diarrhea and polycystic kidney disease. The basic mechanisms of permeation in this channel are not well understood. It is not known which portions of the protein contribute to forming the pore or which amino acid residues in those domains are involved in the biophysical processes of ion permeation. In this review, I will discuss (i) the present understanding of ion transport processes in the wild-type CFTR channel, (ii) the experimental approaches currently being applied to investigate the pore, and (iii) a proposed structure that takes into account the present data on mechanisms of ion selectivity in the CFTR channel and on blockade of the pore by open-channel blockers.

Sensitivity of the denaturing gradient gel electrophoresis technique in detection of known mutations and novel Asian mutations in the CFTR gene

More than 500 mutations have been identified in the CFTR gene, making it an excellent system for testing mutation scanning techniques. To assess the sensitivity of denaturing gradient gel electrophoresis (DGGE), we collected a representative group of 202 CFTR mutations. All mutations analyzed were detected by scanning methods other than the DGGE approach evaluated in this study. DGGE analysis was performed on 24 of the 27 exons and their flanking splice site sequences. After optimization, 201 of the 202 control samples produced an altered migration pattern in the region in which an alteration occurred. The remaining sample was sequenced and found not to have the reported mutation. The ability of DGGE to identify novel mutations was evaluated in three Asian CF patients with four unknown CF alleles. Three novel Asian mutations were detected-K166E, L568X, and 3121-2 A-->G (in homozygosity)-accounting for all CF alleles. These results indicate that an optimized DGGE scanning strategy is highly sensitive and specific and can detect 100% of mutations.

Anion transport in heart

Anion transport proteins in mammalian cells participate in a wide variety of cell and intracellular organelle functions, including regulation of electrical activity, pH, volume, and the transport of osmolites and metabolites, and may even play a role in the control of immunological responses, cell migration, cell proliferation, and differentiation. Although significant progress over the past decade has been achieved in understanding electrogenic and electroneutral anion transport proteins in sarcolemmal and intracellular membranes, information on the molecular nature and physiological significance of many of these proteins, especially in the heart, is incomplete. Functional and molecular studies presently suggest that four primary types of sarcolemmal anion channels are expressed in cardiac cells: channels regulated by protein kinase A (PKA), protein kinase C, and purinergic receptors (I(Cl.PKA)); channels regulated by changes in cell volume (I(Cl.vol)); channels activated by intracellular Ca(2+) (I(Cl.Ca)); and inwardly rectifying anion channels (I(Cl.ir)). In most animal species, I(Cl.PKA) is due to expression of a cardiac isoform of the epithelial cystic fibrosis transmembrane conductance regulator Cl(-) channel. New molecular candidates responsible for I(Cl.vol), I(Cl.Ca), and I(Cl.ir) (ClC-3, CLCA1, and ClC-2, respectively) have recently been identified and are presently being evaluated. Two isoforms of the band 3 anion exchange protein, originally characterized in erythrocytes, are responsible for Cl(-)/HCO(3)(-) exchange, and at least two members of a large vertebrate family of electroneutral cotransporters (ENCC1 and ENCC3) are responsible for Na(+)-dependent Cl(-) cotransport in heart. A 223-amino acid protein in the outer mitochondrial membrane of most eukaryotic cells comprises a voltage-dependent anion channel. The molecular entities responsible for other types of electroneutral anion exchange or Cl(-) conductances in intracellular membranes of the sarcoplasmic reticulum or nucleus are unknown. Evidence of cardiac expression of up to five additional members of the ClC gene family suggest a rich new variety of molecular candidates that may underlie existing or novel Cl(-) channel subtypes in sarcolemmal and intracellular membranes. The application of modern molecular biological and genetic approaches to the study of anion transport proteins during the next decade holds exciting promise for eventually revealing the actual physiological, pathophysiological, and clinical significance of these unique transport processes in cardiac and other mammalian cells.

Terminal glycosylation in cystic fibrosis

Cystic fibrosis (CF) is a common genetic disease for which the gene was identified within the last decade. Pulmonary disease predominates in this ultimately fatal disease and current therapy only slows the progression. CF transmembrane regulator (CFTR), the gene product, is an integral membrane glycoprotein that normally functions as a chloride channel in epithelial cells. The most common mutation, deltaF508, results in mislocalization and altered glycosylation of CFTR. Altered fucosylation and sialylation are hallmarks of both membrane and secreted glycoproteins in CF and the focus here is on these investigations. Oligosaccharides from CF membrane glycoproteins have the Lewis x, selectin ligand in terminal positions. In addition, two major bacterial pathogens in CF, Pseudomonas aeruginosa and Haemophilus influenzae, have binding proteins, which recognize fucose in alpha1,3 linkage and asialoglycoconjugates. We speculate that the altered terminal glycosylation of airway epithelial glycoproteins in CF contributes to the chronic infection and robust inflammatory response in the CF lung. Understanding the effects of mutant CFTR on glycosylation may provide further insight into the regulation of glycoconjugate processing as well as therapy for CF.

Activation of cAMP-dependent C1- currents in guinea-pig paneth cells without relevant evidence for CFTR expression

1. To determine whether Paneth cells exhibit functional expression of cAMP-activated Cl- currents and molecular expression of the cystic fibrosis transmembrane conductance regulator (CFTR), we applied whole-cell patch clamp and single-cell mRNA analysis by reverse transcription (RT) followed by polymerase chain reaction (PCR) amplification to single Paneth cells in crypts isolated from the guinea-pig small intestine. 2. Prominent activation of Cl- currents was consistently observed after stimulation with dibutyryl cAMP and forskolin or with vasoactive intestinal polypeptide (VIP). The cAMP-activated Cl- current was inhibited by removal of intracellular ATP or administration of an inhibitor of protein kinase A. 3. Many of the biophysical and pharmacological properties of the currents were phenotypically similar to those of the CFTR Cl- channel, such as the ohmic current-voltage relationship, the anion selectivity with a Type III sequence (Br- > Cl- > I- >> F- >= gluconate-), I--induced blockage, insensitivity to a stilbene-derivative Cl- channel blocker, and sensitivity to a carboxylate analogue Cl- channel blocker. The sensitivity of the current to glibenclamide was, however, much weaker than that reported for the CFTR Cl- channel current. In contrast to the time independence of CFTR currents, the inward component of the Paneth cell Cl- currents exhibited inactivation kinetics. 4. Expression of CFTR mRNA could not be detected by RT-PCR analysis in almost all single Paneth cells, although its expression was consistently detected at the whole-crypt level. The presence of a small number of CFTR-expressing epithelial cells, which were scattered both in villi and crypts but not at the crypt base where Paneth cells were located, was demonstrated by immunocytochemistry. 5. Taken together, it appears that guinea-pig Paneth cells functionally express cAMP-activated Cl- conductance without relevant evidence for molecular expression of CFTR. Functional expression of VIP receptors in the Paneth cells was also demonstrated.

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).

A divergent CFTR homologue: highly regulated salt transport in the euryhaline teleost F. heteroclitus

The killifish, Fundulus heteroclitus, is a euryhaline teleost fish capable of adapting rapidly to transfer from freshwater (FW) to four times seawater (SW). To investigate osmoregulation at a molecular level, a 5.7-kilobase cDNA homologous to human cystic fibrosis transmembrane conductance regulator (hCFTR) was isolated from a gill cDNA library from SW-adapted killifish. This cDNA encodes a protein product (kfCFTR) that is 59% identical to hCFTR, the most divergent form of CFTR characterized to date. Expression of kfCFTR in Xenopus oocytes generated adenosine 3',5'-cyclic monophosphate-activated, Cl(-)-selective currents similar to those generated by hCFTR. In SW-adapted killifish, kfCFTR was expressed at high levels in the gill, opercular epithelium, and intestine. After abrupt exposure of FW-adapted killifish to SW, kfCFTR expression in the gill increased severalfold, suggesting a role for kfCFTR in salinity adaptation. Under similar conditions, plasma Na+ levels rose significantly after 8 h and then fell, although it is not known whether these changes are directly responsible for the changes in kfCFTR expression. The killifish provides a unique opportunity to understand teleost osmoregulation and the role of CFTR.

Cystic fibrosis transmembrane conductance regulator (CFTR) anion binding as a probe of the pore

We compared the effects of mutations in transmembrane segments (TMs) TM1, TM5, and TM6 on the conduction and activation properties of the cystic fibrosis transmembrane conductance regulator (CFTR) to determine which functional property was most sensitive to mutations and, thereby, to develop a criterion for measuring the importance of a particular residue or TM for anion conduction or activation. Anion substitution studies provided strong evidence for the binding of permeant anions in the pore. Anion binding was highly sensitive to point mutations in TM5 and TM6. Permeability ratios, in contrast, were relatively unaffected by the same mutations, so that anion binding emerged as the conduction property most sensitive to structural changes in CFTR. The relative insensitivity of permeability ratios to CFTR mutations was in accord with the notion that anion-water interactions are important determinants of permeability selectivity. By the criterion of anion binding, TM5 and TM6 were judged to be likely to contribute to the structure of the anion-selective pore, whereas TM1 was judged to be less important. Mutations in TM5 and TM6 also dramatically reduced the sensitivity of CFTR to activation by 3-isobutyl 1-methyl xanthine (IBMX), as expected if these TMs are intimately involved in the physical process that opens and closes the channel.

Disease-associated mutations in cytoplasmic loops 1 and 2 of cystic fibrosis transmembrane conductance regulator impede processing or opening of the channel

Since little is known about the contribution to function of the N-terminal cytoplasmic loops (CL1, residues 139-194; CL2, residues 242-307) of cystic fibrosis transmembrane conductance regulator (CFTR), all nine point mutations identified in CLs 1 and 2 from patients with cystic fibrosis were reconstructed in the expression vector pcDNA3-CFTR and expressed transiently in COS-1 and HEK-293 cells and stably in Chinese hamster ovary (CHO) cells. Four amino acid substitutions retarded production of mature, fully glycosylated CFTR, suggesting that misprocessing of the channel causes the disease symptoms in the affected patients. Protein maturation could not be promoted by cell culture conditions of reduced temperature (26 degrees C). When properly processed mutants were evaluated for functional defects by the iodide efflux method, the G178R- and E193K-CFTR-expressing cell lines showed impaired anion translocation activities. Patch-clamp studies of single channels revealed that E193K variants had a significantly decreased open probability, which resulted from an increase in the mean closed time of the channels. This contrasted with a previous study of disease-associated point mutations in CL3 that mainly affected the mean open time. None of the maturation-competent CL 1 and 2 mutants had altered conductance. Thus, the N-terminal CLs appear not to contribute to the anion translocation pathway of CFTR; rather, mutations in CL1 can impede transition to the open state. Interestingly, the ability of the non-hydrolyzable ATP analogue adenylyl imidodiphosphate (AMP-PNP) to lock the channel into open bursts was abolished by the I148T and G178R amino acid substitutions.

Cytoplasmic loop three of cystic fibrosis transmembrane conductance regulator contributes to regulation of chloride channel activity

To examine the contribution of the large cytoplasmic loops of the cystic fibrosis transmembrane conductance regulator (CFTR) to channel activity, the three point-mutations (S945L, H949Y, G970R) were characterized that have been detected in the third cytoplasmic loop (CL3, residues 933-990) in patients with cystic fibrosis. Chinese hamster ovary cell lines stably expressing wild-type CFTR or mutant G970R-CFTR yielded polypeptides with apparent masses of 170 kDa as the major products, whereas the major products of mutants S945L-CFTR and H949Y-CFTR had apparent masses of 150 kDa. The 150-kDa forms of CFTR were sensitive to endoglycosidase H digestion, indicating that these mutations interfered with maturation of the protein. Increased levels of mature CFTR (170 kDa) could be obtained for mutant H949Y when cells were grown at a lower temperature (26 degrees C) or incubated in the presence of 10% glycerol. For all mutants, the open probability (P0) of the CFTR channels was significantly altered. S945L-CFTR and G970R-CFTR showed a severe reduction in the P0, whereas the H949Y mutation doubled the P0 relative to wild-type. The changes in P0 predominantly resulted from an alteration of the mean burst durations which suggests that CL3 is involved in obtaining and/or maintaining stability of the open state. In addition, mutants S945L and G970R had current-voltage relationships that were not completely linear over the range +/-80 mV, but showed slight outward rectification. The fact that CL3 mutations can have subtle effects on channel conductance indicates that this region may be physically close to the inner mouth of the pore.

Function of Xenopus cystic fibrosis transmembrane conductance regulator (CFTR) Cl channels and use of human-Xenopus chimeras to investigate the pore properties of CFTR

To explore the relationship between structure and function in the cystic fibrosis transmembrane conductance regulator (CFTR) Cl- channel, we studied Xenopus CFTR. We found that the anion permeability sequence of cAMP-activated Cl- currents in the apical membrane of Xenopus A6 epithelia differed from that of cAMP-activated Cl- currents in human epithelia expressing CFTR. To understand the molecular basis for this difference and to learn whether CFTR from another species would have properties similar to human CFTR, we assembled a full-length Xenopus CFTR cDNA from A6 cells. Expression of Xenopus CFTR in HeLa cells generated cAMP-activated whole-cell currents and cAMP-dependent protein kinase-activated single channels that resembled those of human CFTR with the exception that the anion permeability sequence was different (Br- = I- > Cl- in Xenopus CFTR and Br- = Cl- > I- in human). In addition, the single-channel conductance of Xenopus CFTR was increased. To investigate protein regions that account for these differences, we constructed chimeric proteins by replacing either the first or second membrane-spanning domain of human CFTR with the equivalent region of Xenopus CFTR (hX1-6 and hX7-12, respectively) and examined their function in HeLa cells. We found that the anion permeability sequence (Br- = I- > Cl-) and single-channel conductance of hX1-6 resembled that of Xenopus CFTR expressed in HeLa cells, whereas hX7-12 had properties like those of human CFTR. However, the gating of hX1-6 showed a flickery behavior. The altered gating of hX1-6 was attributed to residues in the first extracellular loop of Xenopus CFTR because mutation of residues in that region to the corresponding residues of human CFTR produced gating behavior similar to that of human CFTR. These data suggest that sequence differences in the first membrane-spanning domains are responsible for the differences in the permeation properties of human and Xenopus CFTR and that the first extracellular loop influences channel gating.

ABC transporters in Archaea: two genes encoding homologs of the nucleotide-binding components in the methanogen Methanosarcina mazei S-6

Two genes, 5'-orfD-orfF-3', were found in the genome of the archaeon Methanosarcina mazei S-6 that encode the deduced proteins, OrfD and OrfF, with structural motifs typical of the nucleotide-binding components of the ABC-transporter systems of Bacteria and Eukarya. These motifs, and other similarities of OrfD and OrfF with bacterial and eukaryal counterparts, indicate that the two archaeal molecules belong to the ATP-binding cassette (ABC)-transporter family.

Distribution of cAMP-activated chloride current and CFTR mRNA in the guinea pig heart

Guinea pig ventricular myocytes exhibit a Cl(-)-selective current regulated by the cAMP-dependent pathway. We have investigated the distribution of cAMP-activated Cl- channel current density and cystic fibrosis transmembrane-conductance regulator (CFTR) mRNA in three regions of the guinea pig heart: the atrium, and the epicardium and endocardium of the free wall of the left ventricle. The regional differences in the Cl- current density were investigated in enzymatically isolated myocytes using the whole-cell patch-clamp technique. Forskolin (1 mumol/L) activated Cl(-)-selective currents in all ventricular myocytes and 21% of atrial myocytes examined. The conductance density, estimated as the outward chord conductance normalized to cell capacitance, was greatest in epicardial myocytes (79.8 +/- 8.4 pS/pF, n = 21) and significantly lower in endocardial (59.8 +/- 9.5 pS/pF, n = 22) and atrial (10.9 +/- 5.0 pS/pF, n = 38) myocytes. The regional differences in CFTR mRNA expression levels were investigated by competitive reverse-transcribed polymerase chain reaction. The regional distribution of the mRNA levels was similar to that of the Cl- conductance density, ie, highest in the epicardium (23230 +/- 1840 molecules/microgram total RNA, n = 3), significantly lower in endocardium (10610 +/- 780 molecules/microgram total RNA, n = 3), and lowest in atrium (1450 +/- 290 molecules/microgram total RNA, n = 3). The data indicate that regional differences in CFTR mRNA expression in the guinea pig heart are responsible, at least in part, for the regional differences in cAMP-activated Cl- current density.

Disease-associated mutations in the fourth cytoplasmic loop of cystic fibrosis transmembrane conductance regulator compromise biosynthetic processing and chloride channel activity

A cluster of 18 point mutations in exon 17b of the cystic fibrosis transmembrane conductance regulator (CFTR) gene has been detected in patients with cystic fibrosis. These mutations cause single amino acid substitutions in the most C-terminal cytoplasmic loop (CL4, residues 1035-1102) of the CFTR chloride channel. Heterologous expression of the mutants showed that 12 produced only core-glycosylated CFTR, which was retained in the endoplasmic reticulum; the other six mutants matured and reached the cell surface. In some cases substitution of one member of pairs of adjacent residues resulted in misprocessing, whereas the other did not. Thus, the secondary structure of CL4 may contribute crucially to the proper folding of the entire CFTR molecule. Cyclic AMP-stimulated iodide efflux was not detected from cells expressing the misprocessed variants but was from the other six, indicating that their mutations cause relatively subtle channel defects. Consistent with this, these latter mutations generally are present in patients who are pancreatic-sufficient, while the processing mutants are mostly from patients who are pancreatic-insufficient. Single-channel patch-clamp analysis demonstrated that the processed mutants had the same ohmic conductance as wild-type CFTR, but a lower open probability, generally due to an increase in channel mean closed time and a reduction in mean open time. This suggests that mutations in CL4 do not affect pore properties of CFTR, but disrupt the mechanism of channel gating.

Multiple proteolytic systems, including the proteasome, contribute to CFTR processing

The molecular components of the quality control system that rapidly degrades abnormal membrane and secretory proteins have not been identified. The cystic fibrosis transmembrane conductance regulator (CFTR) is an integral membrane protein to which this quality control is stringently applied; approximately 75% of the wild-type precursor and 100% of the delta F508 CFTR variant found in most CF patients are rapidly degraded before exiting from the ER. We now show that this ER degradation is sensitive to inhibitors of the cytosolic proteasome, including lactacystin and certain peptide aldehydes. One of the latter compounds, MG-132, also completely blocks the ATP-dependent conversion of the wild-type precursor to the native folded form that enables escape from degradation. Hence, CFTR and presumably other intrinsic membrane proteins are substrates for proteasomal degradation during their maturation within the ER.

Animal studies of cystic fibrosis

Cystic fibrosis is the most common life-threatening autosomal recessive genetic disorder in Caucasian populations. It is a disease primarily of epithelial tissues, including the airway, pancreatic duct, intestine, genital tract and sweat glands. The affected gene was cloned and characterized in 1989. In the absence of an identified natural animal model of the disease, a major effort has been made to develop transgenic cystic fibrosis mice, by disrupting the gene in these laboratory animals. Such mice show many, but not all, of the symptoms of cystic fibrosis. In this article, the major past and present contributions of other animal systems to our understanding of cystic fibrosis are examined and their potential for future studies of this disease are discussed. It is intended to give the reader a broad overview of the field, exploring the usefulness of animal studies, rather than dealing more fully with specific aspects of cystic fibrosis.

Rabbit pancreatic acini express CFTR as a cAMP-activated chloride efflux pathway

Cystic fibrosis transmembrane conductance regulator (CFTR) is responsible for adenosine 3',5'-cyclic monophosphate (cAMP)-activated chloride transport in epithelial cells. Isolated rabbit pancreatic acini possess a cAMP-activated chloride efflux mechanism distinct from zymogen granule secretion. To determine whether CFTR is expressed in acini, we used polymerase chain reaction (PCR) to amplify a 480-base pair (bp) sequence from reverse-transcribed rabbit acinar RNA. The PCR product was consistent with a 480-bp band amplified in T84 cells, and its sequence was > 90% homologous to human CFTR. CFTR antibody M3A7 recognized a 180- and a 160-kDa protein from acinar membranes consistent with bands seen in Chinese hamster ovary (CHO) cells transfected with CFTR. To determine if CFTR was responsible for the cAMP-activated chloride efflux previously demonstrated in pancreatic acini, we incubated acinar cells for 20 h with 1.75 microM CFTR antisense or sense oligodeoxynucleotide. Chloride efflux, in response to 8-bromoadenosine 3',5'-cyclic monophosphate and phorbol ester but not to calcium ionophore, was selectively inhibited by CFTR antisense oligodeoxynucleotide. Antisense oligodeoxynucleotide did not inhibit acinar amylase secretion. These findings indicate that isolated pancreatic acini can be used for future studies of CFTR expression and function.

Homozygosity for a novel splice site mutation (2790-2 A--->G) preceding exon 15 of the CFTR gene in a cystic fibrosis patient of North-East Italian descent

We have been screening a cohort of 225 chromosomes from cystic fibrosis patients for mutations in the cystic fibrosis transmembrane regulator gene using a combination of DGGE,RNA-SSCP and DNA sequencing. A novel splice site mutation was detected by multiplex DGGE in a homozygous patient. Restriction-site generating PCR (RG-PCR) analysis demonstrated that both parents carried the same mutation. The molecular haplotype was the same. All the known ancestors came from the same (Veneto) region, and no consanguinity was documented up to the sixth generation.

Regulation of CFTR channel gating

The debilitating symptoms of cystic fibrosis stem from the reduced Cl- permeability of epithelial cells owing to mutations in the gene encoding the cystic fibrosis transmembrane conductance regulator (CFTR) Cl- channel. In cells with normal CFTR channels, receptor-mediated activation of cyclic-AMP-dependent protein kinase causes phosphorylation of several serines in the regulatory domain of CFTR, permitting channel opening and closing via cycles involving ATP hydrolysis. Cellular phosphatases rapidly dephosphorylate the channels, inactivating them. Here we discuss recent advances in our understanding of this complex mechanism for regulating channel gating.

A two-domain model for the R domain of the cystic fibrosis transmembrane conductance regulator based on sequence similarities

CFTR belongs to a group of proteins sharing the structural motif of six transmembrane helices and a nucleotide binding domain. Unique to CFTR is the R domain, a charged cytoplasmic domain. Comparison of R domain sequences from ten species revealed that the N-terminal third is highly conserved, while the C-terminal two-thirds is poorly conserved. The R domain shows no strong sequence similarity to known proteins; however, 14 viral pol proteins show limited similarity to fragments of the R domain. Analysis revealed a relationship between the N- and C-terminal fragments of the R domain and two discontinuous fragments of the pol protein. These observations support a two-domain model for the R domain.

Detection of 98.5% of the mutations in 200 Belgian cystic fibrosis alleles by reverse dot-blot and sequencing of the complete coding region and exon/intron junctions of the CFTR gene

We have previously shown that about 85% of the mutations in 194 Belgian cystic fibrosis alleles could be detected by a reverse dot-blot assay. In the present study, 50 Belgian chromosomes were analyzed for mutations in the cystic fibrosis transmembrane conductance regulator gene by means of direct solid phase automatic sequencing of PCR products of individual exons. Twenty-six disease mutations and 14 polymorphisms were found. Twelve of these mutations and 3 polymorphisms were not described before. With the exception of one mutant allele carrying two mutations, these mutations were the only mutations found in the complete coding region and their exon/intron boundaries. The total sensitivity of mutant CF alleles that could be identified was 98.5%. Given the heterogeneity of these mutations, most of them very rare, CFTR mutation screening still remains rather complex in our population, and population screening, whether desirable or not, does not appear to be technically feasible with the methods currently available.

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.

CFTR!

Cystic fibrosis (CF) is a fatal genetic disease primarily affecting Caucasians, although cases have been reported from other ethnic groups. CF has a complex etiology, but it is chiefly a disease of electrolyte transport and is characterized by defects in fluid secretion by several epithelia, including the sweat duct, exocrine pancreas, and the pulmonary airways. The link between CF and a defect in cAMP-mediated Cl- transport in secretory epithelia was established in the early 1980s. Since then, numerous electrophysiological studies have focused on the characterization and regulation of individual Cl- channels underlying the macroscopic Cl- currents of secretory epithelia in the airways, sweat ducts, and gut. In this review the results of these studies in the light of current knowledge of the function of the CF gene product, the CF transmembrane conductance regulator (CFTR) protein, will be analyzed. The CFTR protein is a member of a family of ATP-binding proteins that act as unidirectional solute pumps. These proteins are membrane spanning, are found in both prokaryotic and eukaryotic cells, and have two ATP-binding domains. The family includes the p-glycoproteins that are involved with the expression of multidrug resistance in certain tumor cells. The majority of CF chromosomes (70%) have a single codon deletion that translates to a missing phenylalanine residue at position 508 (delta F508) of the protein. Unique for this family of proteins, the CFTR protein possesses an additional highly charged domain (the R domain) containing several consensus polypeptide sequences for kinase phosphorylation. Although CFTR bears structural resemblance to this family of ATP-dependent pumps, overexpression of the protein in a variety of different cell types is associated with the appearence of a cAMP-sensitive Cl- channel. We critically examine current information concerning the structure-function relationships of the CFTR protein obtained from both electrophysiological and biochemical approaches. We also summarize recent evidence suggesting that the CFTR protein may act as a pump and a channel, a hypothesis in keeping with the multifaceted nature of the disease.

A cystic fibrosis allele encoding missense mutations in both nucleotide binding folds of the cystic fibrosis transmembrane conductance regulator

German cystic fibrosis (CF) chromosomes were screened for molecular lesions in exon 20 of the cystic fibrosis transmembrane conductance regulator (CFTR) gene by chemical cleavage of mismatch. An 3884G-to-A transition was detected in two patients which leads to an exchange of a serine by an asparagine in the Walker motif A of the second nucleotide binding fold. The affected serine residue is evolutionarily strongly conserved among the pro- and eukaryotic members of the protein superfamily of traffic ATPases. The two S1251N alleles were linked to the benign missense mutation F508C which is located in another conserved region of CFTR, the center region of the first nucleotide binding fold. Both patients with the complex allele F508C-S1251N are carrying delta F508 on the other CF chromosome and are suffering from severe pulmonary and gastrointestinal CF disease. Although F508C has been classified as a neutral sequence variation because of its discovery in healthy delta F508 gene carriers, it may nevertheless influence CFTR dysfunction caused by the S1251N mutation.