Trafficking of GFP-tagged DeltaF508-CFTR to the plasma membrane in a polarized epithelial cell line

Mimicking the host and its microenvironment in vitro for studying mucosal infections by Pseudomonas aeruginosa

Why is a healthy person protected from Pseudomonas aeruginosa infections, while individuals with cystic fibrosis or damaged epithelium are particularly susceptible to this opportunistic pathogen? To address this question, it is essential to thoroughly understand the dynamic interplay between the host microenvironment and P. aeruginosa. Therefore, using model systems that represent key aspects of human mucosal tissues in health and disease allows recreating in vivo host-pathogen interactions in a physiologically relevant manner. In this review, we discuss how factors of mucosal tissues, such as apical-basolateral polarity, junctional complexes, extracellular matrix proteins, mucus, multicellular complexity (including indigenous microbiota), and other physicochemical factors affect P. aeruginosa pathogenesis and are thus important to mimic in vitro. We highlight in vitro cell and tissue culture model systems of increasing complexity that have been used over the past 35 years to study the infectious disease process of P. aeruginosa, mainly focusing on lung models, and their respective advantages and limitations. Continued improvements of in vitro models based on our expanding knowledge of host microenvironmental factors that participate in P. aeruginosa pathogenesis will help advance fundamental understanding of pathogenic mechanisms and increase the translational potential of research findings from bench to the patient's bedside.

Pharmacological Approaches to Correcting the Ion Transport Defect in Cystic Fibrosis

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

Cl transport in complemented CF bronchial epithelial cells correlates with CFTR mRNA expression levels

Little is known about the relationship between CF transmembrane conductance regulator (CFTR) gene expression and the corresponding transport of Cl. The phenotypic characteristics of polarized DeltaF508 homozygote CF bronchial epithelial (CFBE41o-) cells were evaluated following transfection with episomal expression vector containing either full-length (6.2kb) wild type (wt) and (4.7kb) DeltaF508CFTR cDNA. Forskolin-stimulated Cl secretion in two clones expressing the full-length wild type CFTR was assessed; clone c7-6.2wt gave 13.4+/-2.5 microA/cm(2) and clone c10-6.2wt showed 41.3+/-25.3 microA/cm(2). Another clone (c4-4.7DeltaF) complemented with the DeltaF508 CFTR cDNA showed high and stable expression of vector-derived DeltaF508 CFTR mRNA and a small cAMP-stimulated Cl current (4.7+/-0.7 microA/cm(2)) indicating DeltaF508CFTR trafficking to the plasma membrane at physiological temperatures. Vector-driven CFTR mRNA levels were 5-fold (c7-6.2wt), 14-fold (c10-6.2wt), and 27-fold (c7-4.7DeltaF) higher than observed in normal bronchial epithelial cells (16HBE14o-) endogenously expressing wtCFTR. Assessment of CFTR mRNA levels and CFTR function showed that cAMP-stimulated CFTR Cl currents were 33%, 167% and 24%, respectively, of those in 16HBE14o- cells. The data suggest that transgene expression needs to be significantly higher than endogenously expressed CFTR to restore functional wtCFTR Cl transport to levels sufficient to reverse CF pathology.

The DeltaF508-CFTR mutation results in increased biofilm formation by Pseudomonas aeruginosa by increasing iron availability

Enhanced antibiotic resistance of Pseudomonas aeruginosa in the cystic fibrosis (CF) lung is thought to be due to the formation of biofilms. However, there is no information on the antibiotic resistance of P. aeruginosa biofilms grown on human airway epithelial cells or on the effects of airway cells on biofilm formation by P. aeruginosa. Thus we developed a coculture model and report that airway cells increase the resistance of P. aeruginosa to tobramycin (Tb) by >25-fold compared with P. aeruginosa grown on abiotic surfaces. Therefore, the concentration of Tb required to kill P. aeruginosa biofilms on airway cells is 10-fold higher than the concentration achievable in the lungs of CF patients. In addition, CF airway cells expressing DeltaF508-CFTR significantly enhanced P. aeruginosa biofilm formation, and DeltaF508 rescue with wild-type CFTR reduced biofilm formation. Iron (Fe) content of the airway in CF is elevated, and Fe is known to enhance P. aeruginosa growth. Thus we investigated whether enhanced biofilm formation on DeltaF508-CFTR cells was due to increased Fe release by airway cells. We found that airway cells expressing DeltaF508-CFTR released more Fe than cells rescued with WT-CFTR. Moreover, Fe chelation reduced biofilm formation on airway cells, whereas Fe supplementation enhanced biofilm formation on airway cells expressing WT-CFTR. These data demonstrate that human airway epithelial cells promote the formation of P. aeruginosa biofilms with a dramatically increased antibiotic resistance. The DeltaF508-CFTR mutation enhances biofilm formation, in part, by increasing Fe release into the apical medium.

Functional role of conserved transmembrane segment 1 residues in human sodium-dependent vitamin C transporters

Sodium-dependent vitamin C transporters, SVCT1 and SVCT2, are the only two known proteins for the uptake of ascorbate, the active form of vitamin C. Little structural information is available for SVCTs, although a transport activity increase from pH 5.5 to 7.5 suggests a functional role of one or more conserved histidines (p K a approximately 6.5). Confocal fluorescence microscopy and uptake kinetic analyses were used here to characterize cells transfected with mutants of EGFP-tagged hSVCTs. Mutating any of the four conserved histidine residues (His51, 147, 210, or 354) in hSVCT1 to alanine did not affect the apical membrane localization in polarized MDCK cells. His51Ala (in putative transmembrane segment 1, TM1) was the only mutation that resulted in a significant loss of ascorbate transport and an increase in apparent Km with no significant effect on Vmax. The corresponding mutation in hSVCT2, His109Ala, also led to a loss of transport activity. Among eight other mutations of His51 in hSVCT1, significant sodium-dependent ascorbate transport activity was only observed with asparagine or tyrosine replacement. Thus, our results suggest that uncharged His51, directly or indirectly, contributes to substrate binding through the hydrogen bond. His51 cannot account for the observed pH dependence as neutral amino acid substitutions failed to abolish the pH-dependent activity increase. The importance of TM1 is further strengthened by the comparable loss of sodium-dependent ascorbate transport activity upon the mutation of adjacent conserved Gln50 and the apparent change in substrate specificity in the hSVCT1-His51Gln mutation, which showed a specific increase in sodium-independent dehydroascorbate transport.

Expression and maturation of Sendai virus vector-derived CFTR protein: functional and biochemical evidence using a GFP-CFTR fusion protein

Sendai virus (SeV) vector has been shown to efficiently transduce airway epithelial cells. As a precursor to the potential use of this vector for cystic fibrosis (CF) gene therapy, the correct maturation of the SeV vector-derived CF transmembrane conductance regulator (CFTR) protein was examined using biochemical and functional analyses. We constructed a recombinant SeV vector, based on the fusion (F) gene-deleted non-transmissible SeV vector, carrying the GFP-CFTR gene in which the N terminus of CFTR was fused to green fluorescence protein (GFP). This vector was recovered and propagated to high titers in the packaging cell line. Western blotting using an anti-GFP antibody detected both the fully glycosylated (mature) and the core-glycosylated (immature) proteins, indicating that SeV vector-derived GFP-CFTR was similar to endogenous CFTR. We also confirmed the functional channel activity of GFP-CFTR in an iodide efflux assay. The efficient expression of GFP-CFTR, and its apical surface localization, were observed in both MDCK cells in vitro, and in the nasal epithelium of mice in vivo. We concluded that recombinant SeV vector, a cytoplasmically maintained RNA vector, is able to direct production of a correctly localized, mature form of CFTR, suggesting the value of this vector for studies of CF gene therapy.

Establishment and characterization of a novel polarized MDCK epithelial cellular model for CFTR studies

F508del is the most common mutation in the cystic fibrosis transmembrane conductance regulator (CFTR) gene that is responsible for the genetic disease Cystic Fibrosis (CF). It results in a major failure of CFTR to traffic to the apical membrane of epithelial cells, where it should function as a chloride (Cl-) channel. Most studies on localization, processing and cellular trafficking of wild-type (wt) and F508del-CFTR have been performed in non-epithelial cells. Notwithstanding, polarized epithelial cells possess distinctly organized and regulated membrane trafficking pathways. We have used Madin-Darby canine kidney (MDCK) type II cells (proximal tubular cells which do not express endogenous CFTR) to generate novel epithelial, polarized cellular models stably expressing wt- or F508del-CFTR through transduction with recombinant lentiviral vectors. Characterization of these cell lines shows that wt-CFTR is correctly processed and apically localized, producing a cAMP-activated Cl- conductance. In contrast, F508del-CFTR is mostly detected in itsimmature form, localized intracellularly and producing only residual Cl- conductance. These novel cell lines constitute bona fide models and significantly improved resources to investigate the molecular mechanisms of polarized membrane traffic of wt- and F508del-CFTR in the same cellular background. They are also useful to identify/validate novel therapeutic compounds for CF.

The delta e13 isoform of the calcitonin receptor forms a six-transmembrane domain receptor with dominant-negative effects on receptor surface expression and signaling

The CTRdelta e13 splice variant of the rabbit calcitonin receptor, which lacks the 14 amino acids of the seventh transmembrane domain (TMD) that are encoded by exon 13, is poorly expressed on the cell surface, fails to mobilize intracellular calcium or activate Erk, and inhibits the cell surface expression of the full-length C1a isoform. Nuclear magnetic resonance- and fluorescence-activated cell sorter-based experiments showed that the residual seventh TMD of CTRdelta e13 fails to partition into the lipid bilayer, resulting in an extracellular C terminus. Truncating the receptor after residue 397 to delete the cytoplasmic tail resulted in reduced cell surface expression and an inability to mobilize intracellular calcium or activate Erk, but the truncated receptor did not inhibit C1a cell surface expression. In contrast, when the receptor was truncated after residue 374 to eliminate the entire seventh TMD domain and the C-terminal domain, the resulting receptor reduced the cell surface expression of C1a in a manner similar to that of CTRdelta e13. Thus, normal cell surface expression, mobilization of intracellular calcium, and Erk activation requires the cytoplasmic C-terminal tail of the CTR, whereas the absence of the seventh TMD in the transmembrane helical bundle causes the dominant-negative effect on the surface expression of C1a.

Novel adenoviral vectors coding for GFP-tagged wtCFTR and deltaF508-CFTR: characterization of expression and electrophysiological properties in A549 cells

E1/E3-deleted adenoviral vectors expressing an N-terminal green fluorescent protein (GFP) reporter gene fused to either wtCFTR (H5.040CMVEGFP-wtCFTR) or deltaF508-CFTR (H5.040CMVEGFP-deltaF508CFTR) were generated. To characterize the expression and activity, A549 cells were infected with vectors expressing GFP-tagged and non-tagged forms of CFTR and deltaF508CFTR. CFTR activity was assayed in cell-attached and excised patches. For H5.040CMVEGFP-wtCFTR, forskolin-dependent outward current was observed in cell-attached patches from 56 of 67 GFP-positive cells. Single-channel conductances, open probability, mean open and mean closed time values for GFP-CFTR and CFTR were not significantly different. After excision, GFP-CFTR activity required ATP and exhibited a linear I-V relationship. For H5.040CMVEGFP-deltaF508CFTR, media were supplemented with 5 mM butyrate 16 h after infection. Forskolin-dependent outward current was observed in cell-attached patches from 21 of 30 butyrate-treated GFP-positive cells and 0 of 8 GFP-positive cells without butyrate. Single-channel conductances, open probability, mean open and mean closed time values for GFP-deltaF508CFTR and deltaF508CFTR were not significantly different. However, the increase in open probability with genistein was significantly smaller for GFP-deltaF508CFTR than for deltaF508CFTR. In excised patches, GFP-deltaF508CFTR activity required ATP and exhibited a linear I-V relationship. Despite the consistent detection of GFP-CFTR and GFP-deltaF508CFTR channels in the plasma membrane by patch clamping, GFP fluorescence was observed only in intracellular regions and was not altered by butyrate. The data show that high levels of functional GFP-tagged CFTR channels can be expressed with these adenoviral vector constructs.

Expression of the unconventional myosin Myo1c alters sodium transport in M1 collecting duct cells

Epithelial cells rely on proper targeting of cellular components to perform their physiological function. This dynamic process utilizes the cytoskeleton and involves movement of vesicles to and from the plasma membrane, thus traversing the actin cortical cytoskeleton. Studies support both direct interaction of actin with channels and an indirect mechanism whereby actin may serve as a track in the final delivery of the channel to the plasma membrane. Actin-dependent processes are often mediated via a member of the myosin family of proteins. Myosin I family members have been implicated in multiple cellular events occurring at the plasma membrane. In these studies, we investigated the function of the unconventional myosin I Myo1c in the M1 mouse collecting duct cell line. Myo1c was observed to be concentrated at or near the plasma membrane, often in discrete membrane domains. To address the possible role of Myo1c in channel regulation, we expressed a truncated Myo1c, lacking ATP and actin domains, in M1 cells and compared electrophysiological responses to control M1 cells, M1 cells expressing the empty vector, and M1 cells expressing the full-length Myo1c construct. Interestingly, cells expressing the Myo1c constructs had modulated antidiuretic hormone (ADH)-stimulated short-circuit current and showed little inhibition of short-circuit current with amiloride addition. Evaluation of enhanced green fluorescent protein-Myo1c constructs supports the importance of the IQ region in targeting the Myo1c to its respective cellular domain. These data are consistent with Myo1c participating in the regulation of the Na+ channel after ADH stimulation.

TGFbeta down-regulation of the CFTR: a means to limit epithelial chloride secretion

Transforming growth factor beta (TGFbeta) is a multifunctional cytokine with effects on many cell types. We recently showed that in addition to epithelial barrier enhancing properties, TGFbeta causes diminished cAMP-driven chloride secretion in colonic epithelia, in a manner that is p38 MAPK-dependent. In this study, we sought to further delineate the mechanism behind TGFbeta diminution of chloride secretion. Using colonic and kidney epithelial cell lines, we found that exposure to TGFbeta causes dramatic changes in the expression and localization of the apical membrane chloride channel, cystic fibrosis transmembrane conductance regulator (CFTR). In TGFbeta-treated colonic epithelia (T84 and HT-29), CFTR mRNA was significantly reduced 2-24 h post-cytokine exposure. At a time consistent with decreased colonic epithelial secretory responses (16 h), TGFbeta treatment caused diminished intracellular CFTR protein expression (confocal microscopy) and reduced channel expression in the apical membrane during stimulated chloride secretion (biotinylation assay). In comparison, polarized kidney epithelia (MDCK) treated with TGFbeta displayed similarly reduced secretory responses to cAMP stimulating agents; however, a perinuclear accumulation of CFTR was observed, contrasting the diffuse cytoplasmic CFTR expression of control cells. Our data indicate that TGFbeta has profound effects on the expression and subcellular localization of an important channel involved in cAMP-driven chloride secretion, and thus suggest TGFbeta represents a key regulator of fluid movement.

Toward the pharmacogenomics of cystic fibrosis – an update

Cystic fibrosis (CF) is the most common autosomal recessive disorder in Caucasians, with a frequency of ∼ 1 in 3000 live births. The mutated gene is a defective chloride channel in epithelial cells, named cystic fibrosis transmembrane conductance regulator (CFTR). Several different protocols for the scanning of the entire gene have aided molecular diagnosis and improved our understanding of the disorder’s pathophysiology, but also showed the disease’s complexity. Therefore, CF phenotype remains difficult to predict from CFTR mutation data alone: several studies have suggested that additional genes could modulate its clinical outcome. Gene replacement therapy is still far from being used in patients with CF, mostly due to the difficulties with targeting the appropriate cells. In this review, we summarize recent advances, both in the pharmacological and gene therapy field, aimed for the treatment of the disease.

Endoplasmic reticulum degradation impedes olfactory G-protein coupled receptor functional expression

BACKGROUND

Research on olfactory G-protein coupled receptors (GPCRs) has been severely impeded by poor functional expression in heterologous systems. Previously, we demonstrated that inefficient olfactory receptor (OR) expression at the plasma membrane is attributable, in part, to degradation of endoplasmic reticulum (ER)-retained ORs by the ubiquitin-proteasome system and sequestration of ORs in ER aggregates that are degraded by autophagy. Thus, experiments were performed to test the hypothesis that attenuation of ER degradation improves OR functional expression in heterologous cells.

RESULTS

To develop means to increase the functional expression of ORs, we devised an approach to measure activation of the mOREG OR (Unigene # Mm.196680; Olfr73) through coupling to an olfactory cyclic nucleotide-gated cation channel (CNG). This system, which utilizes signal transduction machinery coupled to OR activation in native olfactory sensory neurons, was used to demonstrate that degradation, both by the ubiquitin-proteasome system and autophagy, limits mOREG functional expression. The stimulatory effects of proteasome and autophagy inhibitors on mOREG function required export from the ER and trafficking through the biosynthetic pathway.

CONCLUSIONS

These findings demonstrate that poor functional expression of mOREG in heterologous cells is improved by blocking proteolysis. Inhibition of ER degradation may improve the function of other ORs and assist future efforts to elucidate the molecular basis of odor discrimination.

Inhibition of ENaC by intracellular Cl- in an MDCK clone with high ENaC expression

We examined the effects of intracellular Cl- concentration ([Cl-]i) on the epithelial Na channel (ENaC) in a line of Madin-Darby canine kidney (MDCK) cells (FL-MDCK) with a high rate of Na+ transport produced by stable retroviral transfection with rENaC subunits (Morris RG and Schafer JA. J Gen Physiol 120: 71-85, 2002). Treatment with cAMP (100 microM 8-cpt-cAMP plus 100 microM IBMX) stimulated ENaC-mediated Na+ absorption as well as Cl- secretion via cystic fibrosis transmembrane conductance regulator, which was characterized in alpha-toxin-permeabilized monolayers to have the anion selectivity sequence NO3- > Br- > Cl- > I-. With the use of FL-MDCK monolayers in which the basolateral membrane was permeabilized by nystatin, the ENaC conductance of the apical membrane [determined from the amiloride-sensitive short-circuit current (AS-Isc) driven by an apical-to-basolateral Na+ concentration gradient] was progressively inhibited by increasing the [Cl-] in the basolateral solution (and hence in the cytosol), but it was insensitive to the [Cl-] in the apical solution. This inhibitory effect of [Cl-]i occurred regardless of the presence or absence of net Cl- transport. However, from fluorometric measurements using the Cl(-)-sensitive dye 6-methoxy-N-(3-sulfopropyl)-quinolinium in intact FL-MDCK monolayers on permeable supports, cAMP, which activates both Na+ absorption and Cl- secretion, produced a decrease of [Cl-]i from 76 +/- 14 to 36 +/- 8 mM (P = 0.03). Thus it might be expected that activation of Cl- secretion by cAMP would lead to stimulation rather than inhibition of ENaC. In the nystatin-treated monolayers, an increase in [Cl-]i from 15 to 145 mM decreased AS-Isc from 24.5 +/- 1.0 to 10.2 +/- 1.6 microA/cm2. This inhibition of ENaC could be attributed to nearly proportional decreases in the density of ENaC in the apical membrane from 1.91 +/- 0.16 to 1.32 +/- 0.17 fmol/cm2 and in the intrinsic channel activity (the average current per ENaC subunit) from 13.3 +/- 1.2 to 8.2 +/- 1.4 microA/fmol.

Polarized expression of members of the solute carrier SLC19A gene family of water-soluble multivitamin transporters: implications for physiological function

Humans lack biochemical pathways for the synthesis of the micro-nutrients thiamine and folate. Cellular requirements are met through membrane transport activity, which is mediated by proteins of the SLC19A gene family. By using live-cell confocal imaging methods to resolve the localization of all SLC19A family members, we show that the two human thiamine transporters are differentially targeted in polarized cells, establishing a vectorial transport system. Such polarization decreases functional redundancy between transporter isoforms and allows for independent regulation of thiamine import and export pathways in cells.

Subcellular redistribution of the renal betaine transporter during hypertonic stress

The betaine transporter (BGT1) protects cells in the hypertonic renal inner medulla by mediating uptake and accumulation of the osmolyte betaine. Transcriptional regulation plays an essential role in upregulation of BGT1 transport when renal cells are exposed to hypertonic medium for 24 h. Posttranscriptional regulation of the BGT1 protein is largely unexplored. We have investigated the distribution of BGT1 protein in live cells after transfection with BGT1 tagged with enhanced green fluorescent protein (EGFP). Fusion of EGFP to the NH2 terminus of BGT1 produced a fusion protein (EGFP-BGT) with transport properties identical to normal BGT1, as determined by ion dependence, inhibitor sensitivity, and apparent Km for GABA. Confocal microscopy of EGFP-BGT fluorescence in transfected Madin-Darby canine kidney (MDCK) cells showed that hypertonic stress for 24 h induced a shift in subcellular distribution from cytoplasm to plasma membrane. This was confirmed by colocalization with anti-BGT1 antibody staining. In fibroblasts, transfected EGFP-BGT caused increased transport in response to hypertonic stress. The activation of transport was not accompanied by increased expression of EGFP-BGT, as determined by Western blotting. Membrane insertion of EGFP-BGT protein in MDCK cells began within 2-3 h after onset of hypertonic stress and was blocked by cycloheximide. We conclude that posttranscriptional regulation of BGT1 is essential for adaptation to hypertonic stress and that insertion of BGT1 protein to the plasma membrane may require accessory proteins.

Diffusional mobility of the cystic fibrosis transmembrane conductance regulator mutant, delta F508-CFTR, in the endoplasmic reticulum measured by photobleaching of GFP-CFTR chimeras

Mutations in the cystic fibrosis transmembrane conductance regulator protein (CFTR) cause cystic fibrosis. The most common disease-causing mutation, DeltaF508, is retained in the endoplasmic reticulum (ER) and is unable to function as a plasma membrane chloride channel. To investigate whether the ER retention of DeltaF508-CFTR is caused by immobilization and/or aggregation, we have measured the diffusional mobility of green fluorescent protein (GFP) chimeras of wild type (wt)-CFTR and DeltaF508-CFTR by fluorescence recovery after photobleaching. GFP-labeled DeltaF508-CFTR was localized in the ER and wt-CFTR in the plasma membrane and intracellular membranes in transfected COS7 and Chinese hamster ovary K1 cells. Both chimeras localized to the ER after brefeldin A treatment. Spot photobleaching showed that CFTR diffusion (diffusion coefficient approximately 10(-9) cm(2)/s) was not significantly slowed by the DeltaF508 mutation and that nearly all wt-CFTR and DeltaF508-CFTR diffused throughout the ER without restriction. Stabilization of molecular chaperone interactions by ATP depletion produced remarkable DeltaF508-CFTR immobilization ( approximately 50%) and slowed diffusion (6.5 x 10(-10) cm(2)/s) but had little effect on wt-CFTR. Fluorescence depletion experiments revealed that the immobilized DeltaF508-CFTR in ATP-depleted cells remained in an ER pattern. The mobility of wt-CFTR and DeltaF508-CFTR was reduced by maneuvers that alter CFTR processing or interactions with molecular chaperones, including tunicamycin, geldanamycin, and lactacystin. Photobleaching of the fluorescent ER lipid diOC(4)(3) showed that neither ER restructuring nor fragmentation during these maneuvers was responsible for the slowing and immobilization of CFTR. These results suggest that (a) the ER retention of DeltaF508-CFTR is not due to restricted ER mobility, (b) the majority of DeltaF508-CFTR is not aggregated or bound to slowly moving membrane proteins, and (c) DeltaF508-CFTR may interact to a greater extent with molecular chaperones than does wt-CFTR.