Cystic fibrosis transmembrane conductance regulator is required for protein kinase A activation of an outwardly rectified anion channel purified from bovine tracheal epithelia

CLIC2α Chloride Channel Orchestrates Immunomodulation of Hemocyte Phagocytosis and Bactericidal Activity in Crassostrea gigas

Chloride ion plays critical roles in modulating immunological interactions. Herein, we demonstrated that the anion channel CLIC2α mediates Cl⁻ flux to regulate hemocytes functions in the Pacific oyster (Crassostrea gigas). Specifically, during infection by Vibrio parahemolyticus, chloride influx was activated following onset of phagocytosis. Phosphorylation of Akt was stimulated by Cl⁻ ions entering host cells, further contributing to signal transduction regulating internalization of bacteria through the PI3K/Akt signaling pathway. Concomitantly, Cl⁻ entered phagosomes, promoted the acidification and maturation of phagosomes, and contributed to production of HOCl to eradicate engulfed bacteria. Finally, genomic screening reveals CLIC2α as a major Cl⁻ channel gene responsible for regulating Cl⁻ influx in oysters. Knockdown of CLIC2α predictably impeded phagosome acidification and restricted bacterial killing in oysters. In conclusion, our work has established CLIC2α as a prominent regulator of Cl⁻ influx and thus Cl⁻ function in C. gigas in bacterial infection contexts.

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Influx of chloride ions is switched on during phagocytosis in oyster hemocytes

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PI3K/Akt signaling pathway mediates chloride-dependent activation of phagocytosis

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Cl⁻ promotes phagosomal acidification and HOCl production

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CLIC2α is the principal chloride channel encoding gene within oyster genome

CFTR chloride channel in the apical compartments: spatiotemporal coupling to its interacting partners

The cystic fibrosis transmembrane conductance regulator (CFTR) is a cAMP-regulated chloride channel located primarily at the apical or luminal surfaces of epithelial cells in the airway, intestine, pancreas, kidney, sweat gland, as well as male reproductive tract, where it plays a crucial role in transepithelial fluid homeostasis. CFTR dysfunction can be detrimental and may result in life-threatening disorders. CFTR hypofunctioning because of genetic defects leads to cystic fibrosis, the most common lethal genetic disease in Caucasians, whereas CFTR hyperfunctioning resulting from various infections evokes secretory diarrhea, the leading cause of mortality in early childhood. Therefore, maintaining a dynamic balance between CFTR up-regulating processes and CFTR down-regulating processes is essential for maintaining fluid and body homeostasis. Accumulating evidence suggests that protein-protein interactions play a critical role in the fine-tuned regulation of CFTR function. A growing number of proteins have been reported to interact directly or indirectly with CFTR chloride channel, suggesting that CFTR might be coupled spatially and temporally to a wide variety of interacting partners including ion channels, receptors, transporters, scaffolding proteins, enzyme molecules, signaling molecules, and effectors. Most interactions occur primarily between the opposing terminal tails (amino or carboxyl) of CFTR protein and its binding partners, either directly or mediated through various PDZ scaffolding proteins. These dynamic interactions impact the channel function, as well as localization and processing of CFTR protein within cells. This article reviews the most recent progress and findings about the interactions between CFTR and its binding partners through PDZ scaffolding proteins, as well as the spatiotemporal regulation of CFTR-containing macromolecular signaling complexes in the apical compartments of polarized cells lining the secretory epithelia.

The H-loop in the second nucleotide-binding domain of the cystic fibrosis transmembrane conductance regulator is required for efficient chloride channel closing

The cystic fibrosis transmembrane conductance regulator (CFTR) is an ATP-binding cassette (ABC) transporter that functions as a cAMP-activated chloride channel. The recent model of CFTR gating predicts that the ATP binding to both nucleotide-binding domains (NBD1 and NBD2) of CFTR is required for the opening of the channel, while the ATP hydrolysis at NBD2 induces subsequent channel closing. In most ABC proteins, efficient hydrolysis of ATP requires the presence of the invariant histidine residue within the H-loop located in the C-terminal part of the NBD. However, the contribution of the corresponding region (H-loop) of NBD2 to the CFTR channel gating has not been examined so far. Here we report that the alanine substitution of the conserved dipeptide HR motif (HR-->AA) in the H-loop of NBD2 leads to prolonged open states of CFTR channel, indicating that the H-loop is required for efficient channel closing. On the other hand, the HR-->AA substitution lead to the substantial decrease of CFTR-mediated current density (pA/pF) in transfected HEK 293 cells, as recorded in the whole-cell patch-clamp analysis. These results suggest that the H-loop of NBD2, apart from being required for CFTR channel closing, may be involved in regulating CFTR trafficking to the cell surface.

The formation of the cAMP/protein kinase A-dependent annexin 2-S100A10 complex with cystic fibrosis conductance regulator protein (CFTR) regulates CFTR channel function

Cystic fibrosis results from mutations in the cystic fibrosis conductance regulator protein (CFTR), a cAMP/protein kinase A (PKA) and ATP-regulated Cl(-) channel. CFTR is increasingly recognized as a component of multiprotein complexes and although several inhibitory proteins to CFTR have been identified, protein complexes that stimulate CFTR function remain less well characterized. We report that annexin 2 (anx 2)-S100A10 forms a functional cAMP/PKA/calcineurin (CaN)-dependent complex with CFTR. Cell stimulation with forskolin/3-isobutyl-1-methylxanthine significantly increases the amount of anx 2-S100A10 that reciprocally coimmunoprecipitates with cell surface CFTR and calyculin A. Preinhibition with PKA or CaN inhibitors attenuates the interaction. Furthermore, we find that the acetylated peptide (STVHEILCKLSLEG, Ac1-14), but not the nonacetylated equivalent N1-14, corresponding to the S100A10 binding site on anx 2, disrupts the anx 2-S100A10/CFTR complex. Analysis of 4,4'-diisothiocyanatostilbene-2,2'-disulfonic acid (DIDS) and CFTR(inh172)-sensitive currents, taken as indication of the outwardly rectifying Cl(-) channels (ORCC) and CFTR-mediated currents, respectively, showed that Ac1-14, but not N1-14, inhibits both the cAMP/PKA-dependent ORCC and CFTR activities. CaN inhibitors (cypermethrin, cyclosporin A) discriminated between ORCC/CFTR by inhibiting the CFTR(inh172)-, but not the DIDS-sensitive currents, by >70%. Furthermore, peptide Ac1-14 inhibited acetylcholine-induced short-circuit current measured across a sheet of intact intestinal biopsy. Our data suggests that the anx 2-S100A10/CFTR complex is important for CFTR function across epithelia.

The electro-oculogram

The retinal pigment epithelium (RPE) lying distal to the retina regulates the extracellular environment and provides metabolic support to the outer retina. RPE abnormalities are closely associated with retinal death and it has been claimed several of the most important diseases causing blindness are degenerations of the RPE. Therefore, the study of the RPE is important in Ophthalmology. Although visualisation of the RPE is part of clinical investigations, there are a limited number of methods which have been used to investigate RPE function. One of the most important is a study of the current generated by the RPE. In this it is similar to other secretory epithelia. The RPE current is large and varies as retinal activity alters. It is also affected by drugs and disease. The RPE currents can be studied in cell culture, in animal experimentation but also in clinical situations. The object of this review is to summarise this work, to relate it to the molecular membrane mechanisms of the RPE and to possible mechanisms of disease states.

Macromolecular complexes of cystic fibrosis transmembrane conductance regulator and its interacting partners

The cystic fibrosis transmembrane conductance regulator (CFTR) is the product of the gene mutated in patients with cystic fibrosis (CF). CFTR is a cAMP-regulated chloride channel localized primarily at the apical or luminal surfaces of epithelial cells lining the airway, gut, exocrine glands, etc., where it is responsible for transepithelial salt and water transport. CFTR chloride channel belongs to the superfamily of the ATP-binding cassette (ABC) transporters, which bind ATP and use the energy to drive the transport of a wide variety of substrates across extra- and intracellular membranes. A growing number of proteins have been reported to interact directly or indirectly with CFTR chloride channel, suggesting that CFTR might regulate the activities of other ion channels, receptors, or transporters, in addition to its role as a chloride conductor. The molecular assembly of CFTR with these interacting proteins is of great interest and importance because several human diseases are attributed to altered regulation of CFTR, among which cystic fibrosis is the most serious one. Most interactions primarily occur between the opposing terminal tails (N- or C-) of CFTR and its binding partners, either directly or mediated through various PDZ domain-containing proteins. These dynamic interactions impact the channel function as well as the localization and processing of CFTR protein within cells. This review focuses on the recent developments in defining the assembly of CFTR-containing complexes in the plasma membrane and its interacting proteins.

Localization of Cystic Fibrosis Transmembrane Conductance Regulator to Lipid Rafts of Epithelial Cells Is Required forPseudomonas aeruginosa-Induced Cellular Activation

The cystic fibrosis (CF) transmembrane conductance regulator (CFTR) protein is an epithelial cell receptor for the outer core oligosaccharide of the Pseudomonas aeruginosa LPS. Bacterial binding leads to CFTR-dependent bacterial internalization, initiation of NF-kappaB nuclear translocation, cellular desquamation, and eventual apoptosis of the infected cells, all of which are critical for innate immune resistance to infection with this pathogen. Lack of this reaction in CF patients underlies their hypersusceptibility to chronic P. aeruginosa infection. In this study we tested whether these epithelial cell responses are dependent upon the localization of CFTR to lipid rafts. Confocal microscopy showed that green fluorescent protein-tagged CFTR (GFP-CFTR) and the lipid raft marker ganglioside GM1 colocalized at sites of P. aeruginosa contact and internalization. GFP-CFTR localized to low density Triton X-100-insoluble fractions in lysates of Madin-Darby canine kidney GFP-CFTR cells, and P. aeruginosa infection increased the levels of GFP-CFTR in these fractions as determined by Western blot. Cells expressing GFP-DeltaF508-CFTR did not have rafts with detectable CFTR protein. Extraction of cell surface cholesterol via cyclodextrin treatment of the cells inhibited CFTR entry into rafts. In addition, cyclodextrin treatment of both human and canine epithelial cells inhibited cellular ingestion of P. aeruginosa, NF-kappaB nuclear translocation, and apoptosis. These results indicate that lipid raft localization of CFTR is required for signaling in response to P. aeruginosa infection. Such signaling is needed for the coordination of innate immunity to P. aeruginosa lung infection, a process that is defective in CF.

Membrane transplantation to correct integral membrane protein defects

In this report we show that the tendency of certain viruses to carry host membrane proteins in their envelopes can be harnessed for transplantation of small patches of plasma membrane, including fully functional, polytopic ion channel proteins and their regulatory binding partners. As a stringent model we tested the topologically complex epithelial ion channel CFTR. Initially an attenuated vaccinia virus was found capable of transferring CFTR in a properly folded, functional and regulatable form to CFTR negative cells. Next we generated viruslike particles (VLPs) composed of retroviral structural proteins that assemble and bud at the host cell plasma membrane. These particles were also shown to mediate functional ion channel transfer. By testing the capacity of complex membrane proteins to incorporate into viral envelopes these experiments provide new insight into the permissiveness of viral envelopment, including the ability of incorporated proteins to retain function and repair defects at the cell surface, and serve as a platform for studies of ion channel and membrane protein biochemistry.

No evidence for inhibition of ENaC through CFTR-mediated release of ATP

Both purinergic stimulation and activation of cystic fibrosis transmembrane conductance regulator (CFTR) increases Cl(-) secretion and inhibit amiloride-sensitive Na(+) transport. CFTR has been suggested to conduct adenosine 5'-triphosphate (ATP) or to control ATP release to the luminal side of epithelial tissues. Therefore, a possible mechanism on how CFTR controls the activity of epithelial Na(+) channels (ENaC) could be by release of ATP or uridine 5'-triphosphate (UTP), which would then bind to P2Y receptors and inhibit ENaC. We examined this question in native tissues from airways and colon and in Xenopus oocytes. Inhibition of amiloride-sensitive transport by both CFTR and extracellular nucleotides was observed in colon and trachea. However, nucleotides did not inhibit ENaC in Xenopus oocytes, even after coexpression of P2Y(2) receptors. Using different tools such as hexokinase, the P2Y inhibitor suramin or the Cl(-) channel blocker 4,4'-diisothiocyanatostilbene-2,2'-disulfonic acid (DIDS), we did not detect any role of a putative ATP secretion in activation of Cl(-) transport or inhibition of amiloride sensitive short circuit currents by CFTR. In addition, N(2),2'-O-dibutyrylguanosine 3',5'-cyclic monophosphate (cGMP) and protein kinase G (PKG)-dependent phosphorylation or the nucleoside diphosphate kinase (NDPK) do not seem to play a role for the inhibition of ENaC by CFTR, which, however, requires the presence of extracellular Cl(-).

Cystic fibrosis and the use of pharmacogenomics to determine surrogate endpoints for drug discovery

Cystic fibrosis (CF) is caused by a mutation in the CFTR gene, encoding a chloride channel. For the most common mutation, Delta F508, the basis of the deficit is the failure of the mutant CFTR channel protein to traffic properly to the apical plasma membrane of the affected epithelial cell. The trafficking failure results in loss of the cyclic adenosine monophosphate (cAMP)-activated chloride channel function of the CFTR protein in the plasma membrane. The lung is the principal site affecting patient morbidity and mortality in CF. The main reason is that the CF airway epithelial cells also secrete high levels of the proinflammatory cytokine interleukin (IL)-8, resulting in massive cellular inflammation, infection, tissue damage and lung destruction. The relationship between the trafficking defect, the loss of chloride channel activity, and inflammation is not known. However, gene therapy of CF lung epithelial cells with the wild-type CFTR gene can repair the chloride channel defect, as well as suppress the intrinsic hypersecretion of IL-8. Repair of both defective channels and high IL-8 secretion can also be effected by treatment with the candidate CF drug CPX, which is in clinical trials in CF patients. CPX acts by binding to the mutant CFTR protein, and helps the protein to mature and gain access to the plasma membrane. CPX also suppresses the synthesis and secretion of IL-8 from CF epithelial cells, presumably by virtue of its repair of the trafficking defect of mutant CFTR. To guide pharmacogenomic experiments we have therefore hypothesized that the genomic signature of CF epithelial cells treated with CPX should resemble the signature of the same cells repaired by gene therapy. We have developed two algorithms for identifying genes modified by repair of CFTR defects. The GRASP algorithm uses a statistical test to identify the most profoundly changing genes. The GENESAVER algorithm allows us to identify those genes whose pattern of expression changes in-phase or out-of-phase with IL-8 secretion by CF cells. For the latter algorithm we modified IL-8 secretion from CF cells by treatment with wild-type CFTR, with CPX, or by exposure to bacteria. The results have supported the hypothesis, and have provided a basis for considering the common pharmacogenomic expression signature as a surrogate endpoint for CF drug discovery. Significantly, the nature of the hypothesis, as well as the algorithm developed for this study, can be easily applied to pharmacogenomic studies with other goals.

Lung infections associated with cystic fibrosis

While originally characterized as a collection of related syndromes, cystic fibrosis (CF) is now recognized as a single disease whose diverse symptoms stem from the wide tissue distribution of the gene product that is defective in CF, the ion channel and regulator, cystic fibrosis transmembrane conductance regulator (CFTR). Defective CFTR protein impacts the function of the pancreas and alters the consistency of mucosal secretions. The latter of these effects probably plays an important role in the defective resistance of CF patients to many pathogens. As the modalities of CF research have changed over the decades from empirical histological studies to include biophysical measurements of CFTR function, the clinical management of this disease has similarly evolved to effectively address the ever-changing spectrum of CF-related infectious diseases. These factors have led to the successful management of many CF-related infections with the notable exception of chronic lung infection with the gram-negative bacterium Pseudomonas aeruginosa. The virulence of P. aeruginosa stems from multiple bacterial attributes, including antibiotic resistance, the ability to utilize quorum-sensing signals to form biofilms, the destructive potential of a multitude of its microbial toxins, and the ability to acquire a mucoid phenotype, which renders this microbe resistant to both the innate and acquired immunologic defenses of the host.

Electrolyte transport in the mammalian colon: mechanisms and implications for disease

The colonic epithelium has both absorptive and secretory functions. The transport is characterized by a net absorption of NaCl, short-chain fatty acids (SCFA), and water, allowing extrusion of a feces with very little water and salt content. In addition, the epithelium does secret mucus, bicarbonate, and KCl. Polarized distribution of transport proteins in both luminal and basolateral membranes enables efficient salt transport in both directions, probably even within an individual cell. Meanwhile, most of the participating transport proteins have been identified, and their function has been studied in detail. Absorption of NaCl is a rather steady process that is controlled by steroid hormones regulating the expression of epithelial Na(+) channels (ENaC), the Na(+)-K(+)-ATPase, and additional modulating factors such as the serum- and glucocorticoid-regulated kinase SGK. Acute regulation of absorption may occur by a Na(+) feedback mechanism and the cystic fibrosis transmembrane conductance regulator (CFTR). Cl(-) secretion in the adult colon relies on luminal CFTR, which is a cAMP-regulated Cl(-) channel and a regulator of other transport proteins. As a consequence, mutations in CFTR result in both impaired Cl(-) secretion and enhanced Na(+) absorption in the colon of cystic fibrosis (CF) patients. Ca(2+)- and cAMP-activated basolateral K(+) channels support both secretion and absorption of electrolytes and work in concert with additional regulatory proteins, which determine their functional and pharmacological profile. Knowledge of the mechanisms of electrolyte transport in the colon enables the development of new strategies for the treatment of CF and secretory diarrhea. It will also lead to a better understanding of the pathophysiological events during inflammatory bowel disease and development of colonic carcinoma.

Cystic fibrosis transmembrane conductance regulator facilitates ATP release by stimulating a separate ATP release channel for autocrine control of cell volume regulation

These studies provide evidence that cystic fibrosis transmembrane conductance regulator (CFTR) potentiates and accelerates regulatory volume decrease (RVD) following hypotonic challenge by an autocrine mechanism involving ATP release and signaling. In wild-type CFTR-expressing cells, CFTR augments constitutive ATP release and enhances ATP release stimulated by hypotonic challenge. CFTR itself does not appear to conduct ATP. Instead, ATP is released by a separate channel, whose activity is potentiated by CFTR. Blockade of ATP release by ion channel blocking drugs, gadolinium chloride (Gd(3+)) and 4,4'-diisothiocyanatostilbene-2,2'disulfonic acid (DIDS), attenuated the effects of CFTR on acceleration and potentiation of RVD. These results support a key role for extracellular ATP and autocrine and paracrine purinergic signaling in the regulation of membrane ion permeability and suggest that CFTR potentiates ATP release by stimulating a separate ATP channel to strengthen autocrine control of cell volume regulation.

Genotype-phenotype relationships in cystic fibrosis

The genotype-phenotype relationship in CF is complex despite its being a monogenic disorder. Factors that contribute to variability among individuals with the same genotype are an area of intense study. Nevertheless, certain conclusions can be derived from these studies. First, mutations in both CFTR alleles cause the CF phenotype. Homozygosity for delta F508 or compound heterozygosity for delta F508 and another severe mutation (e.g., G551D, W1282X) cause classic CF: obstructive pulmonary disease, exocrine pancreatic deficiency, male infertility, and elevated sweat chloride concentrations. Clinical variability is observed among patients with the classic form of CF, especially with regards to the severity of lung disease. Although understanding of the role of other genes and environment in the development of lung disease is incomplete, evidence that other factors are important raises the possibility that therapeutic intervention may be possible at several levels. Second, genotype correlates more closely with certain features of the CF phenotype than others. Mutations that allow partial function of CFTR are often associated with pancreatic sufficiency, occasionally identified with normal sweat gland function, and sporadically correlated with mild lung disease. Partially functioning mutants rarely prevent maldevelopment of the male reproductive tract; an exception is 3849 + 10 Kb C-->T. These observations suggest that certain tissues require different levels of CFTR function to avoid the pathologic manifestations typical of CF. The genetic cause of several disorders that clinically overlap CF can be attributed, in part, to mutations in CFTR. Finally, molecular analysis of disease-associated mutations identified through genotype-phenotype studies provides a mechanistic framework for genotype-based therapeutic approaches and pharmaceutical interventions.

Swelling-activated, cystic fibrosis transmembrane conductance regulator-augmented ATP release and Cl- conductances in murine C127 cells

1. A hypotonic challenge, but not cAMP stimulation, was found to induce release of ATP measured by the luciferin-luciferase assay from both the murine mammary carcinoma cell line C127i and C127 cells stably transfected with the cDNA for human cystic fibrosis transmembrane conductance regulator (CFTR) protein (C127/CFTR). CFTR expression augmented swelling-induced ATP release by 10-20 times under hypotonic conditions (< or = 80 % osmolality). 2. Glibenclamide failed to suppress swelling-induced ATP release from C127/CFTR cells. In contrast, whole-cell patch-clamp recordings showed that both the cAMP-activated ohmic Cl- currents and volume-sensitive outwardly rectifying (VSOR) Cl- currents were prominently suppressed by glibenclamide. 3. Gd3+ markedly blocked swelling-induced ATP release but failed to suppress both cAMP- and swelling-activated Cl- currents in the CFTR-expressing cells. Even after pretreatment and during treatment with Gd3+, VSOR Cl- currents were activated normally. 4. The continuous presence of an ATP-hydrolysing enzyme, apyrase, in the bathing solution did not prevent activation of VSOR Cl- currents in C127/CFTR cells. 5. The rate of regulatory volume decrease (RVD) in C127/CFTR cells was much faster than that in C127i cells. When apyrase was added to the bathing solution, the RVD rate was retarded in C127/CFTR cells. 6. On balance, the following conclusions can be deduced. First, swelling-induced ATP release is augmented by expression of CFTR but is not mediated by the CFTR Cl- channel. Second, swelling-induced ATP release is not mediated by the VSOR Cl- channel. Third, the released ATP facilitated the RVD process but is not involved in the activation of VSOR Cl- channels in C127/CFTR cells.

The regulation of epithelial cell cAMP- and calcium-dependent chloride channels

This chapter has focused on two types of chloride conductance found in epithelial cells. The leap from the Ussing chamber to patch-clamp studies has identified yet other conductances present which have also been electrophysiologically characterized. In the case of the swelling activated wholecell chloride current, a physiological function is apparent and a single-channel basis found, but its genetic identity remains unknown (see reviews by Frizzell and Morris, 1994; and Strange et al., 1996). The outwardly rectified chloride channel has been the subject of considerable electrophysiological interest over the past 10 years and is well characterized at the single-channel level, but its physiological function remains controversial (reviewed by Frizzell and Morris, 1994; Devidas and Guggino, 1997). Yet other conductances related to the CLC gene family also appear to be present in epithelial cells of the kidney (reviewed by Jentsch, 1996; Jentsch and Gunter, 1997) where physiological functions for some isoforms are emerging. Clearly, there remain many unknowns. Chief among these is the molecular basis of GCa2+Cl and many of other the conductances. As sequences become available it is expected that the wealth of information gained by investigation into CFTR function will provide a conceptual blueprint for similar studies in these later channel clones.

The first-nucleotide binding domain of the cystic-fibrosis transmembrane conductance regulator is important for inhibition of the epithelial Na+ channel

The cystic-fibrosis transmembrane conductance regulator (CFTR) functions as a cAMP-regulated Cl- channel and as a regulator of other membrane conductances. cAMP-dependent activation of CFTR inhibits epithelial Na+ channels (ENaC). The specificity of interaction between CFTR and ENaC was examined by coexpression of ENaC and ATP-binding cassette (ABC) proteins other than CFTR. In addition, we identified domains within CFTR that are of particular importance for the inhibition of ENaC. To that end, two-electrode voltage-clamp experiments were performed on Xenopus oocytes coexpressing ENaC together with CFTR, the multidrug resistance protein MDR1, the sulfonyl urea receptor SUR1, or the cadmium permease YCF1. Except for CFTR, none of the other ABC proteins were able to inhibit ENaC. Several truncated versions of CFTR were examined for their inhibitory effects on ENaC. In fact, it is shown that C-terminal truncated CFTR is able to inhibit ENaC on activation by intracellular cAMP. Moreover, the data also show that an intact first-nucleotide binding domain (NBF-1) is important for inhibition of ENaC. We conclude that NBF-1 of CFTR contains a CFTR-specific regulatory site that down-regulates ENaC. It is speculated that this regulatory site also is needed for CFTR-mediated interactions with other membrane proteins and that it is not present in NBF-1 of other ABC proteins.

The cystic fibrosis transmembrane conductance regulator and its function in epithelial transport

CF is a well characterized disease affecting a variety of epithelial tissues. Impaired function of the cAMP activated CFTR Cl- channel appears to be the basic defect detectable in epithelial and non-epithelial cells derived from CF patients. Apart from cAMP-dependent Cl- channels also Ca2+ and volume activated Cl- currents may be changed in the presence of CFTR mutations. This is supported by recent additional findings showing that different intracellular messengers converge on the CFTR Cl- channel. Analysis of the ion transport in CF airways and intestinal epithelium identified additional defects in Na+ transport. It became clear recently that mutations of CFTR may also affect the activity of other membrane conductances including epithelial Na+ channels, KvLQT-1 K+ channels and aquaporins (Fig. 7). Several additional, initially unexpected effects of CFTR on cellular functions, such as exocytosis, mucin secretion and regulation of the intracellular pH were reported during the past. Taken together, these results clearly indicate that CFTR not only acts as a cAMP regulated Cl- channel, but may fulfill several other cellular functions, particularly by regulating other membrane conductances. Failure in CFTR dependent regulation of these membrane conductances is likely to contribute to the defects observed in CF. Currently, no general concept is available that can explain how CFTR controls this variety of cellular functions. Further studies will have to verify whether direct protein interaction, specific effects on membrane turnover, changes of the intracellular ion concentration or additional proteins are involved in these regulatory loops. At the end of this review one cannot share the provocative and reassuring title "CFTR!" of a review written a few years ago [114]. Today one might rather finish with the statement "CFTR?".

Inhibition of volume-regulated anion channels by expression of the cystic fibrosis transmembrane conductance regulator

1. To investigate whether the cystic fibrosis transmembrane conductance regulator (CFTR) interacts with volume regulated anion channels (VRACs), we measured the volume-activated chloride current (ICl,swell) using the whole-cell patch-clamp technique in calf pulmonary artery endothelial (CPAE) cells and in COS cells transiently transfected with wild-type (WT) CFTR and the deletion mutant DeltaF508 CFTR. 2. ICl,swell was significantly reduced in CPAE cells expressing WT CFTR to 66.5 +/- 8.8 % (n = 13; mean +/- s. e.m.) of the control value (n = 11). This reduction was independent of activation of the CFTR channel. 3. Expression of DeltaF508 CFTR resulted in two groups of CPAE cells. In the first group IBMX and forskolin could activate a Cl- current. In these cells ICl,swell was reduced to 52.7 +/- 18.8 % (n = 5) of the control value (n = 21). In the second group IBMX and forskolin could not activate a current. The amplitude of ICl,swell in these cells was not significantly different from the control value (112.4 +/- 13.7 %, n = 11; 21 control cells). 4. Using the same method we showed that expression of WT CFTR in COS cells reduced ICl,swell to 62.1 +/- 11.9 % (n = 14) of the control value (n = 12) without any changes in the kinetics of the current. Non-stationary noise analysis suggested that there is no significant difference in the single channel conductance of VRAC between CFTR expressing and non-expressing COS cells. 5. We conclude that expression of WT CFTR down-regulates ICl, swell in CPAE and COS cells, suggesting an interaction between CFTR and VRAC independent of activation of CFTR.

CFTR is a conductance regulator as well as a chloride channel

CFTR Is a Conductance Regulator as well as a Chloride Channel. Physiol. Rev. 79, Suppl.: S145-S166, 1999. - Cystic fibrosis transmembrane conductance regulator (CFTR) is a member of the ATP-binding cassette (ABC) transporter gene family. Although CFTR has the structure of a transporter that transports substrates across the membrane in a nonconductive manner, CFTR also has the intrinsic ability to conduct Cl- at much higher rates, a function unique to CFTR among this family of ABC transporters. Because Cl- transport was shown to be lost in cystic fibrosis (CF) epithelia long before the cloning of the CF gene and CFTR, CFTR Cl- channel function was considered to be paramount. Another equally valid perspective of CFTR, however, derives from its membership in a family of transporters that transports a multitude of different substances from chemotherapeutic drugs, to amino acids, to glutathione conjugates, to small peptides in a nonconductive manner. Moreover, at least two members of this ABC transporter family (mdr-1, SUR) can regulate other ion channels in the membrane. More simply, ABC transporters can regulate somehow the function of other cellular proteins or cellular functions. This review focuses on a plethora of studies showing that CFTR also regulates other ion channel proteins. It is the hope of the authors that the reader will take with him or her the message that CFTR is a conductance regulator as well as a Cl- channel.

Molecular and functional characterization of a calcium-sensitive chloride channel from mouse lung

A protein (mCLCA1) has been cloned from a mouse lung cDNA library that bears strong sequence homology with the recently described bovine tracheal, Ca2+-sensitive chloride channel protein (bCLCA1), bovine lung endothelial cell adhesion molecule-1 (Lu-ECAM-1), and the human intestinal Ca2+-sensitive chloride channel protein (hCLCA1). In vitro, its 3.1-kilobase message translates into a 100-kDa protein that can be glycosylated to an approximately 125-kDa product. SDS-polyacrylamide gel electrophoresis from lysates of mCLCA1 cDNA-transfected transformed human embryonic kidney cells (HEK293) reveals proteins of 130, 125, and 90 kDa as well as a protein triplet in the 32-38 kDa size range. Western analyses with antisera raised against Lu-ECAM-1 peptides show that the N-terminal region of the predicted open reading frame is present only in the larger size proteins (i.e. 130, 125, and 90 kDa), whereas the C-terminal region of the open reading frame is observed in the 32-38 kDa size proteins, suggesting a posttranslational, proteolytic processing of a precursor protein (125/130 kDa) into 90 kDa and 32-38 kDa components similar to that reported for Lu-ECAM-1. Hydrophobicity analyses predict four transmembrane domains for the 90-kDa protein. The mCLCA1 mRNA is readily detected by Northern analysis and by in situ hybridization in the respiratory epithelia of trachea and bronchi. Transient expression of mCLCA1 in HEK293 cells was associated with an increase in whole cell Cl- current that could be activated by Ca2+ and ionomycin and inhibited by 4, 4'-diisothiocyanatostilbene-2,2'-disulfonic acid, dithiothreitol, and niflumic acid. The discovery of mCLCA1 opens the door for further investigating the possible contribution of a Ca2+-sensitive chloride conductance to the pathogenesis of cystic fibrosis.

Therapies directed at the basic defect in cystic fibrosis

There are over 600 unique mutations in the cystic fibrosis (CF) gene that can be classified in five general categories with respect to specific defect. Through basic research into the genetic and physiologic consequences of these mutations, it has become possible to design genotype-specific therapeutic strategies. New pharmaceutical agents are under development for the rescue of defective cystic fibrosis transmembrane conductance regulator mRNA or protein. Some of these compounds are undergoing study in CF patients in Phase I clinical trials. This article evaluates the current research directed at translating a basic molecular understanding of the disease into innovative new treatments.

Phosphorylation of the periplasmic binding protein in two transport systems for arginine incorporation in Escherichia coli K-12 is unrelated to the function of the transport system

In Escherichia coli K-12, the accumulation of arginine is mediated by two distinct periplasmic binding protein-dependent transport systems, one common to arginine and ornithine (AO system) and one for lysine, arginine, and ornithine (LAO system). Each of these systems includes a specific periplasmic binding protein, the AO-binding protein for the AO system and the LAO-binding protein for the LAO system. The two systems include a common inner membrane transport protein which is able to hydrolyze ATP and also phosphorylate the two periplasmic binding proteins. Previously, a mutant resistant to the toxic effects of canavanine, with low levels of transport activities and reduced levels of phosphorylation of the two periplasmic binding proteins, was isolated and characterized (R. T. F. Celis, J. Biol. Chem. 265:1787-1793, 1990). The gene encoding the transport ATPase enzyme (argK) has been cloned and sequenced. The gene possesses an open reading frame with the capacity to encode 268 amino acids (mass of 29.370 Da). The amino acid sequence of the protein includes two short sequence motifs which constitute a well-defined nucleotide-binding fold (Walker sequences A and B) present in the ATP-binding subunits of many transporters. We report here the isolation of canavanine-sensitive derivatives of the previously characterized mutant. We describe the properties of these suppressor mutations in which the transport of arginine, ornithine, and lysine has been restored. In these mutants, the phosphorylation of the AO- and LAO-binding proteins remains at a low level. This information indicates that whereas hydrolysis of ATP by the transport ATPase is an obligatory requirement for the accumulation of these amino acids in E. coli K-12, the phosphorylation of the periplasmic binding protein is not related to the function of the transport system.

What we know and what we do not know about cystic fibrosis transmembrane conductance regulator

The cystic fibrosis transmembrane conductance regulator (CFTR) is a cAMP-regulated chloride channel that resides in the apical membrane of many epithelial cells. Channel opening requires phosophorylation of serine residues in an intracellular regulatory domain by protein kinase A and as the binding and hydrolysis of ATP by intracellular nucleotide binding domains. Besides conducting the chloride ion, CFTR also regulates the function of other membrane proteins, directly or indirectly, notably the outwardly rectifying chloride channel and the epithelial sodium channel. The disease cystic fibrosis is caused by mutations in CFTR, which can result in defective protein production, defective processing and degradation in the endoplasmic reticulum, or defective channel pore properties or gating properties.

Membrane trafficking of the cystic fibrosis gene product, cystic fibrosis transmembrane conductance regulator, tagged with green fluorescent protein in madin-darby canine kidney cells

The mechanism by which cAMP stimulates cystic fibrosis transmembrane conductance regulator (CFTR)-mediated chloride (Cl-) secretion is cell type-specific. By using Madin-Darby canine kidney (MDCK) type I epithelial cells as a model, we tested the hypothesis that cAMP stimulates Cl- secretion by stimulating CFTR Cl- channel trafficking from an intracellular pool to the apical plasma membrane. To this end, we generated a green fluorescent protein (GFP)-CFTR expression vector in which GFP was linked to the N terminus of CFTR. GFP did not alter CFTR function in whole cell patch-clamp or planar lipid bilayer experiments. In stably transfected MDCK type I cells, GFP-CFTR localization was substratum-dependent. In cells grown on glass coverslips, GFP-CFTR was polarized to the basolateral membrane, whereas in cells grown on permeable supports, GFP-CFTR was polarized to the apical membrane. Quantitative confocal fluorescence microscopy and surface biotinylation experiments demonstrated that cAMP did not stimulate detectable GFP-CFTR translocation from an intracellular pool to the apical membrane or regulate GFP-CFTR endocytosis. Disruption of the microtubular cytoskeleton with colchicine did not affect cAMP-stimulated Cl- secretion or GFP-CFTR expression in the apical membrane. We conclude that cAMP stimulates CFTR-mediated Cl- secretion in MDCK type I cells by activating channels resident in the apical plasma membrane.

Purification and reconstitution of an outwardly rectified Cl- channel from tracheal epithelia

We reported the identification of three outwardly rectified Cl- channel (ORCC) candidate proteins (115, 85, and 52 kDa) from bovine tracheal epithelia. We have raised polyclonal antibodies against these isolated proteins. Incorporation into planar lipid bilayers of material partly purified from bovine tracheal apical membranes with one of these antibodies as a ligand (anti-p115) resulted in the incorporation of an ORCC identical in biophysical characteristics to one we previously described. We developed a new purification procedure to increase the yield and purity of this polypeptide. The purification scheme that gave the best results in terms of overall protein yield and purity was a combination of anion- and cation-exchange chromatography followed by immunopurification. By use of this purification scheme, 7 microg of the 115-kDa protein were purified from 20 mg of tracheal apical membrane proteins. Incorporation of this highly purified material into planar lipid bilayers revealed a DIDS-inhibitable channel with the following properties: linear conductance of 87 +/- 9 pS in symmetrical Cl- solutions, halide selectivity sequence of I- > Cl- > Br-, and lack of sensitivity to protein kinase A, Ca2+, or dithiothreitol. Using anti-Galphai antibodies to precipitate Galphai protein(s) from the partly purified preparations, we demonstrated that the loss of rectification of the ORCC was due to uncoupling of Galphai protein(s) from the ORCC protein and that the 115-kDa polypeptide is an ORCC.

Lysophosphatidylglycerol: a novel effective detergent for solubilizing and purifying the cystic fibrosis transmembrane conductance regulator

Similar to the recombinant cystic fibrosis transmembrane conductance regulator (CFTR) expressed in Sf9 insect cells, underglycosylated CFTR expressed in yeast is not effectively solubilized by a variety of commonly used detergents, requiring instead harsh alkali and SDS treatments, which would denature most proteins. Moreover, solubilized CFTR has a strong tendency to aggregate and form high-molecular-weight aggregates during subsequent purification. We report here that the mild detergent, lysophosphatidylglycerol (LPG), is a very effective detergent for solubilizing the CFTR expressed in both yeast and Sf9 insect cells. LPG solubilizes nearly 100% of the CFTR in yeast in the absence of NaCl and none in the presence of 1 M NaCl. It is also very potent in preventing aggregation of the CFTR during subsequent purification. Exploiting these characteristics, a rapid simple procedure for the purification of functional recombinant CFTR expressed in yeast has been developed. It includes selective CFTR solubilization in the presence and the absence of NaCl followed by nickel-chelate chromatography of His-tagged CFTR. The CFTR produced by this procedure is about 70% pure. Purified CFTR molecules were reconstituted into liposomes and then fused to planar lipid bilayers for single-channel recording. The reconstituted CFTR exhibits regulatory chloride channel activities with a slope conductance of 7.1 pS and a reversal potential of -32 mV. The effectiveness and simplicity of this new purification procedure for the CFTR should greatly facilitate a variety of biochemical and biophysical studies of this important protein. Furthermore, the potency of LPG in solubilizing the notoriously intractable underglycosylated CFTR suggest that this detergent may be useful for solubilizing the CFTR from other sources and for other difficult membrane proteins as well.

ClC and CFTR chloride channel gating

Chloride channels are widely expressed and play important roles in cell volume regulation, transepithelial transport, intracellular pH regulation, and membrane excitability. Most chloride channels have yet to be identified at a molecular level. The ClC gene family and the cystic fibrosis transmembrane conductance regulator (CFTR) are distinct chloride channels expressed in many cell types, and mutations in their genes are the cause of several diseases including myotonias, cystic fibrosis, and kidney stones. Because of their molecular definition and roles in disease, these channels have been studied intensively over the past several years. The focus of this review is on recent studies that have provided new insights into the mechanisms governing the opening and closing, i.e. gating, of the ClC and CFTR chloride channels.

Cloning and characterization of lung-endothelial cell adhesion molecule-1 suggest it is an endothelial chloride channel

Lung-endothelial cell adhesion molecule-1 (Lu-ECAM-1) is an endothelial cell surface molecule that mediates adhesion of metastatic melanoma cells to lung endothelium. Here we analyze the organization of the Lu-ECAM-1 protein complex, report the sequence of Lu-ECAM-1 cDNAs, and reveal a novel function of the protein. Lu-ECAM-1 immunopurified from bovine aortic endothelial cells (BAEC) consists of tightly associated glycoproteins of 90, 38, and 32 kDa, with minor components of 130 and 120 kDa. We present evidence that all of these protein species are encoded by a single open reading frame whose initial translation product is proteolytically processed to yield the other products. Correct processing in vitro was demonstrated by transfection of the longest cDNA into human embryonic kidney 293 cells; immunoblot analysis showed that the approximately 120-kDa precursor gave rise to 90- and 38-kDa products. RNA blots of BAEC mRNA detected messages in agreement with the sizes of the cDNA clones in addition to several of high molecular weight. DNA blot analysis showed that Lu-ECAM-1 is conserved throughout its length in all mammals tested, usually as a single or low copy gene. In the bovine, Lu-ECAM-1 protein is 88% identical to a calcium-dependent chloride channel described recently in tracheal epithelium, Ca-CC. Probes for Lu-ECAM-1 mRNA and protein confirmed the presence of a homolog in this tissue. We show that messages for both proteins are present in lung while only Ca-CC is present in trachea and only Lu-ECAM-1 is present in BAEC. These results suggest that endothelial cells express a chloride channel that is related to, but distinct from, that expressed in tracheal epithelium. They further suggest that an adhesion molecule can also be a chloride channel.

The cystic fibrosis transmembrane conductance regulator and ATP

A controversy in the field of cystic fibrosis (CF) research has arisen concerning the role of the cystic fibrosis transmembrane conductance regulator (CFTR) in the transport of ATP. Does the CFTR actually conduct ATP or does it regulate the conductance of ATP? Recent findings either support or reject the hypothesis that the CFTR can transport ATP. In addition, recent research from several laboratories has suggested that ATP mediates its effects after traversing the plasma membrane and reaching the extracellular surface. The current model suggests that the released ATP exerts its various influences via a purinergic receptor to regulate outwardly rectifying chloride channels and epithelial sodium channels.

Leukotriene D4 activates a chloride conductance in hepatocytes from lipopolysaccharide-treated rats

Endotoxin (LPS) can cause hepatocellular injury under several circumstances, and leukotrienes have been implicated as a contributing factor. Since ion channel activation has been associated with cytotoxicity, the aim of this study was to determine the circumstances under which LPS and/or leukotrienes activate ionic conductances in hepatocytes. LPS treatment of rats increased Cl- conductance in hepatocytes from 232+/-42 to 1236+/-134 pS/pF. Voltage dependence and inhibitor specificity of this conductance were similar to that of a swelling-activated Cl- conductance, and internal dialysis with nucleoside analogues suggested control by an inhibitory G protein. The lipoxygenase inhibitor nordihydroguaiaretic acid, the specific leukotriene D4 (LTD4) receptor antagonist MK-571, and the 5-lipoxygenase activating protein inhibitor MK-886 all significantly inhibited the conductance. Intracellular dialysis with LTD4 (1.5 microM) elevated intracellular Ca2+ from 143+/-6.5 to 388+/-114 nM within 6 min and stimulated an outwardly rectifying conductance from 642+/-159 to 1669+/-224 pS/pF (n = 9, P < 0.001). In hepatocytes prepared from untreated rats, this concentration of intracellular LTD4 neither raised intracellular Ca2+ nor activated the conductance. The LTD4 response could be induced in normal hepatocytes by culture with either conditioned medium from LPS-treated macrophages or purified TNF-alpha. In conclusion, intracellular LTD4 activates a chloride conductance in hepatocytes isolated from rats treated with LPS or primed in vitro with TNF-alpha. Changes in the hepatocellular accumulation of leukotrienes therefore mediate channel activation and may contribute to liver injury during sepsis and other inflammatory conditions.

Cystic fibrosis transmembrane conductance regulator-associated ATP and adenosine 3'-phosphate 5'-phosphosulfate channels in endoplasmic reticulum and plasma membranes

Cystic fibrosis (CF) is characterized by abnormal regulation of epithelial ion and fluid transport due to mutations in the CF transmembrane conductance regulator (CFTR), an apical membrane-localized Cl- channel, that usually prevent it from exiting the endoplasmic reticulum. Defective or absent CFTR in the epithelium is believed to disrupt fluid balance in human airways and thereby contribute to chronic respiratory inflammation. Patch-clamp of the plasma membrane and outer membrane of the nuclear envelope of nuclei isolated from CFTR-expressing Chinese hamster ovary cells revealed that CFTR is associated with a regulated ATP channel in both membrane compartments. CFTR expression was also shown to be associated with permeability to another adenine nucleotide, adenosine 3'-phosphate 5'-phosphosulfate, the universal sulfate donor in cells. These results may provide a link between the ion channel function of CFTR and abnormal glycoprotein processing observed in CF.

Expression of the cystic fibrosis phenotype in a renal amphibian epithelial cell line

Mutations in a Cl- channel (cystic fibrosis transmembrane conductance regulator or CFTR) are responsible for the cystic fibrosis (CF) phenotype. Increased Na+ transport rates are observed in CF airway epithelium, and recent studies suggest that this is due to an increase in Na+ channel open probability (Po). The Xenopus renal epithelial cell line, A6, expresses both cAMP-activated 8-picosiemen (pS) Cl- channels and amiloride-sensitive 4-pS Na+ channels, and provides a model system for examining the interactions of CFTR and epithelial Na+ channels. A6 cells express CFTR mRNA, as demonstrated by reverse transcriptase-polymerase chain reaction and partial sequence analysis. A phosphorothioate antisense oligonucleotide, complementary to the 5' end of the open reading frame of Xenopus CFTR, was used to inhibit functional expression of CFTR in A6 cells. Parallel studies utilized the corresponding sense oligonucleotide as a control. CFTR protein expression was markedly reduced in cells incubated with the antisense oligonucleotide. Incubation of A6 cells with the antisense oligonucleotide led to inhibition of forskolin-activated amiloride-insensitive short circuit current (Isc). After a 30-min exposure to 10 microM forskolin, 8-pS Cl- channel activity was detected in only 1 of 31 (3%) cell-attached patches on cells treated with antisense oligonucleotide, compared to 5 of 19 (26%) patches from control cells. A shift in the single-channel current-voltage relationship derived from antisense-treated cells was also consistent with a reduction in Cl- reabsorption. Both amiloride-sensitive Isc and Na+ channel Po were significantly increased in antisense-treated, forskolin-stimulated A6 cells, when compared with forskolin-stimulated controls. These data suggest that the regulation of Na+ channels by CFTR is not limited to respiratory epithelia and to epithelial cells in culture overexpressing CFTR and epithelial Na+ channels.

The biogenesis, traffic, and function of the cystic fibrosis transmembrane conductance regulator

The cystic fibrosis transmembrane conductance regulator (CFTR) is a cyclic AMP-activated chloride channel that is encoded by the gene that is defective in cystic fibrosis. This ion channel resides at the luminal surfaces and in endosomes of epithelial cells that line the airways, intestine, and a variety of exocrine glands. In this article we discuss current hypotheses regarding how CFTR functions as a regulated ion channel and how CF mutations lead to disease. We also evaluate the emerging notion that CFTR is a multifunctional protein that is capable of regulating epithelial physiology at several levels, including the modulation of other ion channels and the regulation of intracellular membrane traffic. Elucidating the various functions of CFTR should contribute to our understanding of the pathology in cystic fibrosis, the most common lethal genetic disorder among Caucasians.

cAMP- and swelling-activated chloride conductance in rat hepatocytes

An outwardly rectifying Cl- conductance was identified in primary isolated rat hepatocytes, and the whole cell patch-clamp technique was used to characterize its properties and mechanisms of activation. With symmetrical Cl(-)-containing solutions on both sides and adenosine 3',5'-cyclic monophosphate (cAMP; 100 microM) in the pipette solution, a large outwardly rectifying conductance (1,014 +/- 153 pS/pF, n = 20) developed in all cells within 3 min. This cAMP-activated conductance was highly anion selective and slowly inactivated at voltages > 80 mV. It was completely inhibited by the anion channel blocker 5-nitro-2-(3-phenylpropylamino) benzoic acid (200 microM, n = 6) and partially inhibited by 4,4'-diisothiocyanostilbene-2,2'-disulfonic acid (150 microM, n = 7). It displayed a halide selectivity of I- > Br- > Cl-. In the absence of cAMP, a functionally similar conductance was activated by cell swelling. Reduction of bath osmolality from 300 to 250 mosmol/kg increased membrane conductance from 64 +/- 16.4 to 487 +/- 23 pS/pF (n = 4). This swelling-activated conductance was also highly anion selective and had identical halide selectivity and blocker sensitivity as the cAMP-activated conductance. Although cell swelling was not necessary for cAMP activation, cell shrinkage with hyperosmotic bath (350 mosmol/kg), either before or after exposure to cAMP, inhibited the cAMP-activated conductance. By the determination of conductance as a function of bath osmolality in the presence and absence of cAMP, it was observed that cAMP shifted the osmotic set point for conductance activation without changing either the maximum or minimum conductance. In conclusion, both cAMP and cell swelling activate a large outwardly rectifying Cl- conductance in rat hepatocytes. Its ionic selectivity and sensitivity to channel blockers are identical to those seen for swelling-activated Cl- conductances in many cell types. The conductive properties are not those of cystic fibrosis transmembrane conductance regulator-mediated Cl- conductance. cAMP appears to activate this conductance by altering the volume set point of a swelling-activated channel.

Apical recruitment of CFTR in T-84 cells is dependent on cAMP and microtubules but not Ca2+ or microfilaments

Previous studies from this laboratory have demonstrated that chloride transport induced by forskolin, but not ionomycin, in T84 cells is highly dependent on an intact microtubular network. Using an antibody raised against a region of the R domain of CFTR, we now show by indirect immunofluorescence that forskolin causes relocation of CFTR to the apical domain of T84 cells. T84 cells grown on transparent filters were incubated with agonists and/or cytoskeletal inhibitors prior to fixation, permeabilization, and staining with the antibody. A 30 second stimulation with forskolin (10 microM) caused a twofold increase in relative fluorescence intensity at the apical surface. In contrast, a 30 second exposure to ionomycin (2 microM), had no effect on the distribution of CFTR-related fluorescence. Incubation of the cells with nocodazole (33 microM), a microtubule disrupting agent, prevented the forskolin-induced rise in CFTR fluorescence at the apical surface. However, incubation of the cells with cytochalasin D, an actin inhibitor, was without effect on forskolin-related re-distribution of CFTR-associated fluorescence. In double label experiments using antibodies against both beta-tubulin and actin, CFTR-related fluorescence was found to co-localize with the microtubule network, but not with actin filaments. These observations are consistent with the microtubule-dependent acute recruitment of CFTR to the apical plasma membrane of T84 cells in response to elevations in intracellular cAMP.

Regulation of epithelial sodium channels by the cystic fibrosis transmembrane conductance regulator

Cystic fibrosis airway epithelia exhibit enhanced Na+ reabsorption in parallel with diminished Cl- secretion. We tested the hypothesis that the cystic fibrosis transmembrane conductance regulator (CFTR) directly affects epithelial Na+ channel activity by co-incorporating into planar lipid bilayers immunopurified bovine tracheal CFTR and either heterologously expressed rat epithelial Na+ channel ( alpha,b eta,gamma-rENaC) or an immunopurified bovine renal Na+ channel protein complex. The single channel open probability (Po) of rENaC was decreased by 24% in the presence of CFTR. Protein kinase A (PKA) plus ATP activated CFTR, but did not have any effect on rENaC. CFTR also decreased the extent of elevation of the renal Na+ channel Po following PKA-mediated phosphorylation. Moreover, the presence of CFTR prohibited the inward rectification of the gating of this renal Na+ channel normally induced by PKA-mediated phosphorylation, thus down-regulating inward Na+ current. This interaction between CFTR and Na+ channels occurs independently of whether or not wild-type CFTR is conducting anions. However, the nonconductive CFTR mutant, G551D CFTR, cannot substitute for the wild-type molecule. Our results indicate that CFTR can directly down-regulate single Na+ channel activity, thus accounting, at least in part, for the observed differences in Na+ transport between normal and cystic fibrosis-affected airway epithelia.

G-protein regulation of outwardly rectified epithelial chloride channels incorporated into planar bilayer membranes

Experiments were designed to test if immunopurified outwardly rectified chloride channels (ORCCs) and the cystic fibrosis transmembrane conductance regulator (CFTR) incorporated into planar lipid bilayers are regulated by G-proteins. pertussis toxin (PTX) (100 ng/ml) + NAD (1 mM) + ATP (1 mM) treatment of ORCC and CFTR in bilayers resulted in a 2-fold increase in single channel open probability (Po) of ORCC but not of CFTR. Neither PTX, NAD, nor ATP alone affected the biophysical properties of either channel. Further, PTX conferred a linearity to the ORCC current-voltage curve, with a slope conductance of 80 +/- 3 picosiemens (pS) in the +/- 100 mV range of holding potentials. PKA-mediated phosphorylation of these PTX + NAD-treated channels further increased the Po of the linear 80-pS channels from 0.66 +/- 0.05 to >0.9, and revealed the presence of a small (16 +/- 2 pS) linear channel in the membrane. PTX treatment of a CFTR-immunodepleted protein preparation incorporated into bilayer membranes resulted in a similar increase in the Po of the larger conductance channel and restored PKA-sensitivity that was lost after CFTR immunodepletion. The addition of guanosine 5'-3-O-(thio)triphosphate (100 mum) to the cytoplasmic bathing solutions decreased the activity of the ORCC and increased its rectification at both negative and positive voltages. ADP-ribosylation of immunopurified material revealed the presence of a 41-kDa protein. These results demonstrate copurification of a channel-associated G-protein that is involved in the regulation of ORCC function.

Interaction between cystic fibrosis transmembrane conductance regulator and outwardly rectified chloride channels

We have previously described a protocol for the simultaneous isolation and reconstitution of a protein kinase A (PKA)-sensitive outwardly rectified chloride channel (ORCC) and the cystic fibrosis transmembrane conductance regulator (CFTR) from bovine tracheal epithelium. Immunoprecipitation of CFTR from this preparation prevented PKA activation of the ORCC, suggesting that CFTR regulated the ORCC and that this regulatory relationship was preserved throughout the purification procedure. We now report the purification of CFTR from bovine tracheal epithelia and the purification of a CFTR conduction mutant (G551D CFTR) from retrovirally transduced mouse L cells using a combination of alkali stripping, Triton-X extraction, and immunoaffinity chromatography. Immunopurified CFTR proteins were reconstituted in the absence and presence of ORCC. To test the hypothesis that only functional CFTR can support activation of ORCC by PKA and ATP, we used an inhibitory anti-CFTR505-511 peptide antibody or G551D CFTR. When anti-CFTR505-511 peptide antibodies were present prior to the addition of PKA and ATP, activation of both the ORCC and CFTR was prevented. If the antibody was added after activation of the ORCC and CFTR Cl- channels by PKA and ATP, only the CFTR Cl- channel was inhibited. When ORCC and G551D CFTR were co-incorporated into planar bilayers, only the ORCC was recorded and this channel could not be further activated by the addition of PKA and ATP. Thus, functional CFTR is required for activation of the ORCC by PKA and ATP. We also tested the hypothesis that PKA activation of ORCC was dependent on the extracellular presence of ATP. We added ATP on the presumed extracellular side of the lipid bilayer under conditions where it was not possible to activate the ORCC, i.e. in the presence of inhibitory anti-CFTR505-511 antibody or G551D CFTR. In both cases the ORCC regained PKA sensitivity. Moreover, the addition of hexokinase + glucose to the extracellular side prevented activation of the ORCCs by PKA and ATP in the presence of CFTR. These experiments confirm that both the presence of CFTR as well as the presence of ATP on the extracellular side is required for activation of the ORCC by PKA and ATP.

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.

Two cystic fibrosis transmembrane conductance regulator mutations have different effects on both pulmonary phenotype and regulation of outwardly rectified chloride currents

Cystic fibrosis (CF), a disorder of electrolyte transport manifest in the lungs, pancreas, sweat duct, and vas deferens, is caused by mutations in the CF transmembrane conductance regulator (CFTR). The CFTR protein has been shown to function as a cAMP-activated chloride channel and also regulates a separate protein, the outwardly rectifying chloride channel (ORCC). To determine the consequence of disease-producing mutations upon these functions, mutant CFTR was transiently expressed in Xenopus oocytes and in human airway epithelial cells lacking functional CFTR. Both G551D, a mutation that causes severe lung disease, and A455E, a mutation associated with mild lung disease, altered but did not abolish CFTR's function as a chloride channel in Xenopus oocytes. Airway epithelial cells transfected with CFTR bearing either A455E or G551D had levels of chloride conductance significantly greater than those of mock-transfected and lower than those of wild-type CFTR-transfected cells, as measured by chloride efflux. A combination of channel blockers and analysis of current-voltage relationships were used to dissect the contribution of CFTR and the ORCC to whole cell currents of transfected cells. While CFTR bearing either mutation could function as a chloride channel, only CFTR bearing A455E retained the function of regulating the ORCC. These results indicate that CF mutations can affect CFTR functions differently and suggest that severity of pulmonary disease may be more closely associated with the regulatory rather than chloride channel function of CFTR.