Dynasore, a small molecule inhibitor of dynamin, was used to probe the role of dynamin in the endocytosis of wild-type and mutant CFTR (cystic fibrosis transmembrane conductance regulator). Internalization of both wild-type and ‘temperature-corrected’ ΔF508 CFTR was markedly inhibited by a short exposure to dynasore, implicating dynamin as a key element in the endocytic internalization of both wild-type and mutant CFTR. The inhibitory effect of dynasore was readily reversible upon washout of dynasore from the growth media. Corr-4 ({2-(5-chloro-2-methoxy-phenylamino)-4′-methyl-[4,5′]-bithiazolyl-2′-yl}-phenyl-methanonone), a pharmacological corrector of ΔF508 CFTR biosynthesis, caused a marked increase in the cell surface expression of mutant CFTR. Co-incubation of ΔF508 CFTR expressing cells with Corr-4 and dynasore caused a significantly greater level of cell surface CFTR than that observed in the presence of Corr-4 alone. These results argue that inhibiting the endocytic internalization of mutant CFTR provides a novel therapeutic target for augmenting the benefits of small molecule correctors of mutant CFTR biosynthesis.

INTRODUCTION

Clathrin-coated vesicles mediate endocytosis of many transporters, receptors and ion channels, as well as soluble macromolecules and viruses. The large GTPase dynamin plays an essential role in clathrin-mediated endocytic events [13], although the precise molecular mechanism whereby dynamin leads to the fission of clathrin coated pits from the cell surface still remains unclear [35]. There are three mammalian isoforms of dynamin, of these dynamins 1 and 2 are the best characterized. Dynamin 1 is expressed almost exclusively in the central nervous system, whereas dynamin 2 is expressed ubiquitously. Overexpression of dominant-negative forms of dynamin 2 that bind poorly to GTP (e.g. dynamin K44A) inhibits endocytosis and leads to the accumulation of invaginated coated pits [1].

CF (cystic fibrosis), the most common life-threatening genetic disease in Caucasians [6], is caused by mutations in the CFTR (CF transmembrane conductance regulator; ABCC7). This anion-selective channel is required for the normal function of epithelia lining the airways and intestinal tract, as well as the ducts of the exocrine pancreas, salivary and sweat glands. The biosynthetic processing and intracellular trafficking of CFTR has been studied extensively because the most common CF-causing mutation in the CFTR gene, ΔF508, fails to fold properly in the ER (endoplasmic reticulum), with little or no ΔF508 trafficking to the cell surface [7]. Several aspects of the internalization of wild-type CFTR from the cell surface have been elucidated. CFTR is endocytosed by clathrin-coated vesicles [810], utilizing a tyrosine based endocytic motif (and possibly a dileucine motif) located in the C-terminal tail of CFTR [1113]. In addition, we have shown that the tyrosine-based motif in CFTR is recognized by the medium subunit of the AP-2 clathrin adaptor complex [14], and blocking this interaction prevents endocytosis of wild-type CFTR. In contrast with wild-type CFTR, less is known about the mechanisms whereby ΔF508 CFTR undergoes internalization. According to one model, ΔF508 undergoes accelerated endocytosis compared with wild-type CFTR [15], whereas others have argued that the endocytic rates of ΔF508 CFTR are comparable with those of wild-type CFTR [16].

To evaluate further the proteins involved in CFTR internalization, we determined the role of dynamin in both wild-type and ΔF508 CFTR endocytosis. Until recently, very few tools were available to modulate clathrin-mediated endocytosis, and although dominant-negative (K44A) dynamin has been used extensively to monitor the effects of dynamin on membrane protein internalization [1], it takes a day or more to establish an inhibitory phenotype. We therefore took advantage of a newly discovered small molecule non-competitive inhibitor of dynamin's GTPase activity, dynasore [17]. Dynasore has been shown to act as a potent, rapid and reversible inhibitor of dynamin-dependent pathways [1719]. Our results show that dynasore blocked the endocytic internalization of both wild-type and ΔF508 CFTR, leading to an increase in steady-state surface levels of these proteins. Moreover, these studies provide proof-of-concept that inhibition of ΔF508 CFTR endocytosis can probably play a role in a therapeutic intervention to treat patients with CF.

EXPERIMENTAL

Reagents

Dynasore [3-hydroxy-napthalene-2-carboxylic acid (3,4-dihydroxy-benzylidene)-hydrazide] was initially a gift from Dr Tomas Kirchhausen (Department of Cell Biology, Harvard University, Boston, MA, U.S.A.). For other experiments, dynasore, Corr-4 ({2-(5-chloro-2-methoxy-phenylamino)-4′-methyl-[4,5′]-bithiazolyl-2′-yl}-phenyl-methanonone) [20] and glycine hydrazide [21] were all obtained from the Cystic Fibrosis Foundation Therapeutics Network. Anti-HA (haemagglutinin) antibody 12CA5 was obtained from Abcam (Cambridge, MA, U.S.A.). Cy3 (indocarbocyanine) transferrin, Lipofectamine 2000™ and validated Stealth™ siRNA (small interfering RNA) against dynamin II were obtained from Invitrogen.

Cell culture and transfection

HeLa cells were obtained from the A.T.C.C. and grown at 37 °C in 5% CO2. HeLa cells stably expressing extope–CFTR and extope–ΔF508-CFTR [22] were created by transfection of pcDNA3 containing extope–CFTR constructs using Lipofectamine 2000™ according to the manufacturer's instructions, with subsequent selection using G418. In some cases, to allow maturation of extope–ΔF508 CFTR, cells were grown at 26 °C for 2 days.

[125I]-labelled transferrin uptake

[125I]transferrin (Amersham) uptake was performed as described previously by Bradbury et al. [9]. Cells were grown to confluence, and then incubated in serum-free media containing 1% BSA for 1 h, and then cooled to 17 °C. Cells were then incubated for 30 min in the presence or absence of dynasore (80 μM), and then for a further 30 min in media supplemented with [125I]transferrin. To measure transferrin endocytosis, cells were warmed rapidly back to 37 °C and incubated in the continued presence or absence of dynasore for a further 15 min at 37 °C. Cells were cooled to 4 °C and washed twice in media containing 1% BSA, three times with PBS/1% BSA and then four times with PBS. The cells were released from the culture dish by incubation in PBS containing 0.5% (w/v) trypsin at 4 °C and transferred to a microcentrifuge tube, and rotated in trypsin media for 45 min to ensure complete release of [125I]transferrin from the cell surface. Serum (10%; v/v) was added to quench the trypsin and the cells were centrifuged (10000 g for 5 min) and washed by resuspension in PBS. The supernatant, containing released surface [125I]transferrin, and the cell pellet, containing internalized transferrin, were monitored by gamma radiation (Packard). Data were plotted as c.p.m. against dynasore concentration and fitted by non-linear regression analysis using a sigmoidal dose-response algorithm (Sigmaplot 10.0). IC50 was determined from the fitted dose–response curve.

Imaging of transferrin endocytosis

Cells were cooled to 4 °C to inhibit endocytic internalization, and incubated for 30 min at 4 °C with Cy3-labelled transferrin (10 μg/ml) in the presence or absence of dynasore (80 μM). Unbound transferrin was washed away and the cells incubated in media at 37 °C in the continued presence or absence of dynasore. Following further incubation at 37 °C, cells were cooled to 4 °C and remaining surface transferrin was removed by a short acid wash (pH 3.7). Cells were fixed in 4% paraformaldehyde, and the nuclei were stained with DAPI (4′,6-diamidino-2-phenylindole).

Electron microscopy

Cells were grown to ∼80% confluence, then incubated in the presence of vehicle (DMSO) or dynasore at 37 °C for 30 min. Cells were scraped into PBS, pelleted by centrifugation, and fixed in 0.3% PBS-buffered glutaraldehyde for 1 h, followed by 3% glutaraldehyde overnight. Specimens were washed in Sørensens phosphate buffer, and post-fixed with 1% osmium tetraoxide. Specimens were then dehydrated through a series of increasingly concentrated ethanol washes, embedded in epon resin and cured at 70 °C for 72 h. Specimen blocks were cut into 70 nm sections using a Leica LIC-6 ultramicrotome, then sections were contrast stained with uranyl acetate and lead citrate prior to viewing by TEM (transmission electron microscopy). Micrographs were digitally acquired using a JEOL JEM-1230 TEM at 100× magnification at the Core Electron Microscopy Center of Chicago Medical School.

CFTR immunofluorescence

HeLa human cervical carcinoma cells were grown on polylysine-coated coverslips. For labelling of extope–CFTR constructs at the cell surface of intact cells, cells were washed three times with PBS/0.5% BSA and precooled for 10 min on ice in the presence or absence of dynasore followed by labelling of cell surface extope–CFTR for 30 min on ice with anti-HA monoclonal antibody (12CA5, 5 μg/ml). To determine steady-state surface CFTR, cells were washed and fixed for 10 min in 4% paraformaldehyde, followed by incubation in PBS/1% BSA containing Cy3-conjugated goat anti-mouse secondary antibodies. Quantification of surface signal was evaluated by determining pixel intensity for individual cell ROI (region of interest) (40–75 cells) with background subtraction. To determine endocytic internalization of extope–CFTR, cells incubated with 12CA5 antibodies were washed three times with ice-cold PBS, followed by media at 37 °C, and further incubated at 37 °C for the indicated times in the continued presence or absence of dynasore. To remove remaining surface-bound antibody, cells were washed twice for 30 s with 4 °C PBS, pH 3.7, followed by a wash with PBS pH 7.4. Cells were fixed with 4% paraformaldehyde, permeabilized with 0.1% saponin in PBS, and blocked with PBS containing 1% BSA and 5% normal goat serum. Secondary antibody was Cy3-conjugated goat anti-mouse antibody.

siRNA transfection

For efficient knockdown of dynamin II, cells were transfected twice with siRNA directed against dynamin II, on days 1 and 3. On day 5, cells were processed for immunofluorescence microscopy to evaluate CFTR endocytosis, or for immunoblot analysis to confirm dynamin II knockdown. Control cells were treated with a scrambled dynamin II siRNA construct.

Cell surface biotinylation

Cell surface biotinylation and calculation of CFTR internalization was performed as previously described [23]. Cell-surface proteins were biotinylated with NHS-SS-biotin [sulfosuccinimidyl-2-(biotinamido)ethyl-1,3-dithiopropionate] (1 mg/ml in PBS) twice for 15 min on ice. Following four washes with ice-cold PBS, non-reacted NHS-SS-biotin was quenched with 0.1% BSA in PBS. Internalization of biotinylated CFTR was initiated by the addition of prewarmed (37 °C) growth medium and then transferring the dishes to 37 °C, followed by arrest of internalization with ice-cold medium at the indicated time points. Biotin molecules remaining at the cell surface were removed by reducing their disulfide bonds with the reducing agent MESNA (2-mercaptoethanesulfonic acid). Cells were treated three times for 30 min with 50 mM MESNA in 100 mM NaCl, 50 mM Tris/HCl, 1 mM MgCl2 and 0.1 mM CaCl2, pH 8.6, at 4 °C. The efficiency of biotin stripping (93.3±2.9%, n=3) was evaluated by measuring the relative amount of biotinylated CFTR both before and after the reduction of the disulfide bonds on cells kept at 4 °C. Cells were lysed in buffer [50 mM Tris/HCl, pH 7.4, containing 150 mM NaCl, 10 mM NH4MoO4 and 0.1% (v/v) Nonidet P40] and subject to precipitation using streptavidin–Sepharose; the resultant precipitate was subject to Western blot analysis using antibodies directed against CFTR.

RESULTS

Dynasore inhibits the endocytic uptake of transferrin

We confirmed in the present study that dynasore was able to rapidly and reversibly inhibit dynamin-dependent endocytic internalization by monitoring the uptake of transferrin, a well described model for dynamin and clathrin-dependent endocytosis [24]. Inhibition of transferrin uptake by dynasore was dose-dependent and showed an IC50 of 15 μM±0.9 μM (means±S.D. for n=3 (Figure 1), which was similar to that reported previously for the in vitro inhibition of purified dynamin [17]. In order to visualize the dynasore-dependent inhibition of transferrin endocytosis, Cy3-labelled transferrin was bound to its receptor at 17 °C, followed by warming of cells back to 37 °C. Receptor bound transferrin was first observed in peripheral early endosomes (5 min) and then in perinuclear recycling late endosomes (15 min) (Figure 2A), consistent with previously published data [17]. The endocytosis and trafficking of transferrin was strongly inhibited in cells preincubated with dynasore. The block of transferrin uptake was similar to that observed upon overexpression of dominant-negative dynamin (K44A) (results not shown). The decrease in transferrin uptake was not due to a decrease in the ability of transferrin to bind its receptor, since the amount of bound transferrin at 17 °C was the same for cells in the presence or absence of dynasore (Figure 2B). The effect of dynasore on transferrin endocytosis was rapidly and completely reversible (Figure 2C), since accumulation of transferrin was observed in perinuclear endosomes 15 min after the removal of dynasore from the incubation medium.

Dynasore blocks dynamin-dependent internalization of transferrin in a dose-dependent manner

Figure 1
Dynasore blocks dynamin-dependent internalization of transferrin in a dose-dependent manner

HeLa cells were incubated in serum-free media for 30 min at 37 °C with increasing concentrations of dynasore, and then in the same media containing [125I]transferrin. Transferrin uptake was measured for 15 min at 37 °C. Internalized marker is expressed as c.p.m. per mg of protein (means±S.D.; n=3). Data were fitted by non-linear regression analysis using a sigmoidal dose-response algorithm. R2=0.986 for fit.

Figure 1
Dynasore blocks dynamin-dependent internalization of transferrin in a dose-dependent manner

HeLa cells were incubated in serum-free media for 30 min at 37 °C with increasing concentrations of dynasore, and then in the same media containing [125I]transferrin. Transferrin uptake was measured for 15 min at 37 °C. Internalized marker is expressed as c.p.m. per mg of protein (means±S.D.; n=3). Data were fitted by non-linear regression analysis using a sigmoidal dose-response algorithm. R2=0.986 for fit.

Visualization of dynasore-mediated inhibition of transferrin internalization

Figure 2
Visualization of dynasore-mediated inhibition of transferrin internalization

(A) HeLa cells were incubated overnight in Dulbecco's modified Eagle's medium supplemented with 10% foetal calf serum and allowed to reach 50% confluence. Prior to transferrin addition, cells were incubated for 30 min at 37 °C in the presence of dynasore (80 μM) or 0.8% DMSO only (vehicle), and the cells were incubated in the same media containing Cy3-conjugated transferrin (10 μg/ml; red) for 5 min at 17 °C. After three washes to remove unbound transferrin, cells were transferred back to 37 °C and incubated for the indicated times in the continued presence of dynasore or DMSO. Transferrin remaining at the cell surface was removed by an acid wash prior to fixation. The nuclei were stained with DAPI (blue). (B) HeLa cells were treated as above, except that following exposure of the cells to Cy3-conjugated transferrin for 5 min at 17 °C, unbound transferrin was removed by washing at 17 °C, and the cells were fixed and imaged without further manipulation. The nuclei were stained with DAPI. (C) HeLa cells were first treated for 30 min with either DMSO vehicle or dynasore, followed by treatment in the continuous presence of Cy3–transferrin for 10 min at 37 °C: treatment in the presence of DMSO (left panel), treatment in the presence of dynasore (middle panel) or 20 min after dynasore washout (right panel).

Figure 2
Visualization of dynasore-mediated inhibition of transferrin internalization

(A) HeLa cells were incubated overnight in Dulbecco's modified Eagle's medium supplemented with 10% foetal calf serum and allowed to reach 50% confluence. Prior to transferrin addition, cells were incubated for 30 min at 37 °C in the presence of dynasore (80 μM) or 0.8% DMSO only (vehicle), and the cells were incubated in the same media containing Cy3-conjugated transferrin (10 μg/ml; red) for 5 min at 17 °C. After three washes to remove unbound transferrin, cells were transferred back to 37 °C and incubated for the indicated times in the continued presence of dynasore or DMSO. Transferrin remaining at the cell surface was removed by an acid wash prior to fixation. The nuclei were stained with DAPI (blue). (B) HeLa cells were treated as above, except that following exposure of the cells to Cy3-conjugated transferrin for 5 min at 17 °C, unbound transferrin was removed by washing at 17 °C, and the cells were fixed and imaged without further manipulation. The nuclei were stained with DAPI. (C) HeLa cells were first treated for 30 min with either DMSO vehicle or dynasore, followed by treatment in the continuous presence of Cy3–transferrin for 10 min at 37 °C: treatment in the presence of DMSO (left panel), treatment in the presence of dynasore (middle panel) or 20 min after dynasore washout (right panel).

Dynasore stabilizes clathrin-coated pits

To correlate the functional and fluorescence imaging results with structural data, we used transmission electron microscopy to image the plasma membranes of cells treated with dynasore. Representative images from control cells (Figure 3A) show sparse coated pits at various stages of invagination, as well as internalized vesicles. In contrast, cells treated with dynasore (Figure 3B) show a larger number of coated vesicles at later stages of invagination, still linked to the plasma membrane by either wide or long narrow necks.

Dynasore blocks clathrin-coated pit scission

Figure 3
Dynasore blocks clathrin-coated pit scission

Representative electron micrographs of membrane invaginations coated with a bristle-like cytosolic layer characteristic of clathrin coats in the absence (A) or presence (B) of dynasore. Cells were exposed to DMSO or dynasore for 30 min prior to fixation and processing for transmission electron microscopy. The scale bar is 100 nm.

Figure 3
Dynasore blocks clathrin-coated pit scission

Representative electron micrographs of membrane invaginations coated with a bristle-like cytosolic layer characteristic of clathrin coats in the absence (A) or presence (B) of dynasore. Cells were exposed to DMSO or dynasore for 30 min prior to fixation and processing for transmission electron microscopy. The scale bar is 100 nm.

Dynasore inhibits the endocytic uptake of wild-type CFTR

To evaluate the role of dynamin on CFTR endocytosis, we used a combination of immunofluorescence microscopy and biochemical assays. Direct imaging of CFTR internalization was performed using an HA-extope-tagged CFTR construct [22]. The internalization of cell surface CFTR was followed for 3 h. Under control conditions, CFTR was observed in discrete punctate vesicles deep within the cell, indicating an endocytic retrieval of CFTR from the cell surface (Figure 4A, control). When cells were incubated with dynasore, no intracellular CFTR signal was observed, consistent with the inhibition of CFTR internalization by dynasore (Figure 4A, dynasore). As with the binding of transferrin to its receptor, dynasore had no impact on the ability of anti-HA antibodies to bind to the extracellular epitope on CFTR (Figure 4B), which showed a similar level of binding in the presence or absence of dynasore. Kinetic analysis of CFTR endocytosis was performed using a cell surface biotinylation assay [23]. Under control conditions, CFTR was rapidly and efficiently removed from the cell surface, with 50% of plasma membrane CFTR internalized within 10 min, consistent with previous observations [13] (Figure 4D; filled circles). Pretreatment of cells with dynasore, followed by incubation in the continued presence of dynasore, blocked the endocytosis of CFTR. During a 15 min incubation, only 4–5% of plasma membrane CFTR was endocytosed in the presence of dynasore (Figure 4D; open circles). Washout of dynasore from the culture medium resulted in restoration of CFTR endocytosis, with kinetics similar to those seen under control conditions (Figure 4D; filled triangles). We compared the effects of acute dynasore exposure with siRNA for dynamin II on its ability to inhibit CFTR endocytosis. In contrast with the rapid onset of dynasore's effects, siRNA-mediated knockdown of dynamin II required 4–5 days following transfection to achieve maximal effect (results not shown). Greater than 90% knockdown of dynamin II was achieved, with no effect on the expression levels of actin controls (Figure 5A). As anticipated, inhibition of dynamin-dependent endocytosis by siRNA knockdown led to a marked increase in steady-state levels of surface CFTR (Figure 5B). Thus both protein knockdown and enzymatic inhibition of dynamin II inhibited CFTR endocytosis, allowing it to accumulate on the cell surface.

Dynasore causes a rapid and reversible inhibition of CFTR endocytosis

Figure 4
Dynasore causes a rapid and reversible inhibition of CFTR endocytosis

(A) HeLa cells were incubated overnight in Dulbecco's modified Eagle's medium supplemented with 10% foetal bovine serum and allowed to reach 50% confluence. Cells were incubated for 30 min at 37 °C in the presence of dynasore or DMSO (control). Cells were incubated with anti-HA antibodies (12CA5) for 30 min at 4 °C before cells were washed to remove unbound antibody. Antibody-labelled cells were transferred back to 37 °C and incubated in the presence or absence of dynasore for a further 3 h. 12CA5 anti-HA antibody remaining at the cell surface was removed by acid wash prior to fixation. Remaining anti-HA antibody was detected using Cy3-conjugated secondary antibodies (red). The nuclei were stained with DAPI (blue). (B) Cells were exposed to anti-HA antibodies in the presence or absence of dynasore. Following washes to remove unbound antibody, primary antibody binding was determined by the addition of a Cy3-conjugated secondary antibody prior to fixation of the cells. In this way, only cell surface CFTR was identified. (C) Endocytosis of CFTR was determined by biotinylation with NHS-SS-biotin (1 mg/ml) on ice. Endocytosis of cell surface proteins was accomplished by shifting the temperature to 37 °C for the indicated time points. Biotin molecules remaining at the cell surface were stripped with 10 mM MESNA and, following lysis, biotinylated proteins were precipitated with streptavidin–Sepharose and resolved by SDS/PAGE prior to Western blot analysis using anti-CFTR antibodies (M3A7) as previously published by us [13,23]. (D) Quantification of the signal associated with internalized CFTR from (C). Cells expressing wild-type CFTR were preincubated with DMSO control (●) or dynasore (○) for 30 min and then in the continued presence of DMSO or dynasore to determine CFTR endocytosis at the time points indicated. For washout, cells were exposed to dynasore for 30 min followed by dynasore washout prior to analysis of CFTR endocytosis at the time points indicated (▼).

Figure 4
Dynasore causes a rapid and reversible inhibition of CFTR endocytosis

(A) HeLa cells were incubated overnight in Dulbecco's modified Eagle's medium supplemented with 10% foetal bovine serum and allowed to reach 50% confluence. Cells were incubated for 30 min at 37 °C in the presence of dynasore or DMSO (control). Cells were incubated with anti-HA antibodies (12CA5) for 30 min at 4 °C before cells were washed to remove unbound antibody. Antibody-labelled cells were transferred back to 37 °C and incubated in the presence or absence of dynasore for a further 3 h. 12CA5 anti-HA antibody remaining at the cell surface was removed by acid wash prior to fixation. Remaining anti-HA antibody was detected using Cy3-conjugated secondary antibodies (red). The nuclei were stained with DAPI (blue). (B) Cells were exposed to anti-HA antibodies in the presence or absence of dynasore. Following washes to remove unbound antibody, primary antibody binding was determined by the addition of a Cy3-conjugated secondary antibody prior to fixation of the cells. In this way, only cell surface CFTR was identified. (C) Endocytosis of CFTR was determined by biotinylation with NHS-SS-biotin (1 mg/ml) on ice. Endocytosis of cell surface proteins was accomplished by shifting the temperature to 37 °C for the indicated time points. Biotin molecules remaining at the cell surface were stripped with 10 mM MESNA and, following lysis, biotinylated proteins were precipitated with streptavidin–Sepharose and resolved by SDS/PAGE prior to Western blot analysis using anti-CFTR antibodies (M3A7) as previously published by us [13,23]. (D) Quantification of the signal associated with internalized CFTR from (C). Cells expressing wild-type CFTR were preincubated with DMSO control (●) or dynasore (○) for 30 min and then in the continued presence of DMSO or dynasore to determine CFTR endocytosis at the time points indicated. For washout, cells were exposed to dynasore for 30 min followed by dynasore washout prior to analysis of CFTR endocytosis at the time points indicated (▼).

siRNA knock-down of dynamin inhibits CFTR endocytosis

Figure 5
siRNA knock-down of dynamin inhibits CFTR endocytosis

Cells were treated with a pool of three siRNA constructs against dynamin II. Cells were either lysed and subject to immunoblot analysis for dynamin and actin control (A), or subject to live cell staining for extope-tagged CFTR (B). Primary antibody binding was detected by Cy3-conjugated secondary antibodies, and average fluorescence intensity for control, and siRNA-treated cells were determined under identical conditions. Quantification was performed using image analysis subroutines of Simple PCI® (ver 6.5); Compix Inc, Pittsburgh PA. Results show percentage increase in cell surface CFTR above control for 40–70 cells in each of three replicate experiments.

Figure 5
siRNA knock-down of dynamin inhibits CFTR endocytosis

Cells were treated with a pool of three siRNA constructs against dynamin II. Cells were either lysed and subject to immunoblot analysis for dynamin and actin control (A), or subject to live cell staining for extope-tagged CFTR (B). Primary antibody binding was detected by Cy3-conjugated secondary antibodies, and average fluorescence intensity for control, and siRNA-treated cells were determined under identical conditions. Quantification was performed using image analysis subroutines of Simple PCI® (ver 6.5); Compix Inc, Pittsburgh PA. Results show percentage increase in cell surface CFTR above control for 40–70 cells in each of three replicate experiments.

Dynasore inhibits the endocytic uptake of ΔF508 CFTR

The effects of dynasore (and hence dynamin) on ΔF508 CFTR endocytosis were measured by determining steady-state surface levels of ΔF508 CFTR in the presence or absence of dynasore. Cells expressing ΔF508 CFTR were grown for 48 h at 26 °C to allow ‘temperature correction’ of ΔF508 folding and trafficking to the cell surface. Following exposure of cells to media containing either dynasore or DMSO vehicle, cells were warmed back to 37 °C in the continued presence or absence of dynasore for a further 3 h. Remaining cell surface ΔF508 CFTR was then determined by cell surface biotinylation. As expected, cells grown at 26 °C showed the presence of ΔF508 CFTR at the cell surface (Figure 6). Upon rewarming the cells back to 37 °C, there was a marked loss of ΔF508 CFTR from the cell surface, consistent with the previously reported cell surface instability of ΔF508 CFTR. When dynasore was present during the last 30 min incubation at 26 °C, and during the subsequent incubation at 37 °C, the loss of ΔF508 CFTR from the plasma membrane was markedly attenuated, implicating dynamin in the cell surface instability of ‘temperature corrected’ ΔF508 CFTR at 37 °C. Washout of dynasore prior to incubation of ‘temperature corrected’ cells at 37 °C led to a loss of ΔF508 CFTR from the cell surface, similar to that observed in cells never exposed to dynasore.

Dynasore prevents the removal of temperature-corrected ΔF508 CFTR from the cell surface

Figure 6
Dynasore prevents the removal of temperature-corrected ΔF508 CFTR from the cell surface

Cells expressing extope–ΔF508 CFTR were incubated at 26 °C for 48 h to allow cell surface expression of the mutant protein. Plasma membrane CFTR was tagged using a cleavable cell surface biotinylating agent on ice prior to rewarming the cells to 37 °C and incubating for a further 3 h at 37 °C in the presence or absence of dynasore. To monitor reversibility of dynasore, temperature-corrected cells were incubated with dynasore for 30 min at 37 °C, followed by dynasore washout and continued incubation at 37 °C for a further 3 h at 37 °C. Cell surface biotinylated proteins were isolated with streptavidin beads, and CFTR was detected by immunoblot analysis.

Figure 6
Dynasore prevents the removal of temperature-corrected ΔF508 CFTR from the cell surface

Cells expressing extope–ΔF508 CFTR were incubated at 26 °C for 48 h to allow cell surface expression of the mutant protein. Plasma membrane CFTR was tagged using a cleavable cell surface biotinylating agent on ice prior to rewarming the cells to 37 °C and incubating for a further 3 h at 37 °C in the presence or absence of dynasore. To monitor reversibility of dynasore, temperature-corrected cells were incubated with dynasore for 30 min at 37 °C, followed by dynasore washout and continued incubation at 37 °C for a further 3 h at 37 °C. Cell surface biotinylated proteins were isolated with streptavidin beads, and CFTR was detected by immunoblot analysis.

Dynasore augments the effects of pharmacological correctors of ΔF508 trafficking

Since ‘temperature correction’ is not a therapeutically viable approach to correcting the CF defect, we determined whether dynasore would augment the effects of pharmacological correctors of CFTR trafficking. Cells expressing extope–ΔF508 CFTR were incubated overnight in the presence or absence of Corr-4 (5 μM) at 37 °C. A third set of cells were incubated overnight with both Corr-4 and dynasore. Cells were cooled to 4 °C and exposed to anti-HA antibodies to label cell surface CFTR. Cells were washed, fixed and incubated with Cy3-goat anti-mouse secondary antibodies, and signal intensity was determined by pixel analysis of ROI corresponding to 40–75 cells (Simple PCI Imaging Software). Cells grown at 26 °C for 48 h were used as a positive control for surface expression of extope–ΔF508 CFTR. As expected, cells grown at 26 °C showed a significantly greater cell surface level of ΔF508 CFTR compared with cells grown at 37 °C (Figure 7). Cells grown at 37 °C in the presence of Corr-4 also showed a markedly greater level of surface ΔF508 CFTR, consistent with previously reported data [20]. Under our experimental conditions, ‘temperature correction’ was more efficacious at increasing cell surface expression of ΔF508 CFTR than was Corr-4, with Corr-4 giving approximately 55% correction of that observed with reduced temperature. Significantly, dynasore was able to increase further the level of cell surface ΔF508 CFTR when added to cells incubated with Corr-4 compared with cells grown in Corr-4 alone (Figure 7) (P=0.011, by Student's t test). The combination of Corr-4 and dynasore resulted in ΔF508 CFTR cell surface levels that were 68% of that observed with ‘temperature correction’. To determine if dynasore alone was able to increase the steady state level of surface ΔF508 CFTR, cells were incubated overnight at 37 °C in the continued presence of dynasore. A small (∼9% of ‘temperature corrected’) but significant amount of ΔF508 CFTR was observed at the cell surface following dynasore incubation, suggesting that ΔF508 CFTR, that escaped ER quality control at 37 °C and was trafficked to the cell surface, was able to be retained in the plasma membrane following dynasore treatment. The combination of dynasore and Corr-4 was synergistic rather than additive in correction of ΔF508 CFTR cell surface stability.

Dynasore augments the actions of Corr-4 on increasing cell surface ΔF508-CFTR

Figure 7
Dynasore augments the actions of Corr-4 on increasing cell surface ΔF508-CFTR

Cells expressing extope–ΔF508 CFTR were incubated for 36 h under the conditions shown. Cells expressing extope–ΔF508 CFTR were grown for 36 h at 26 °C as a positive control (set as 100%). Cells were cooled to 4 °C and incubated with anti-HA antibodies, followed by Cy3-conjugated secondary antibodies. Following fixation in 4% paraformaldehyde, cells were subject to indirect immunofluorescence microscopy. The histogram shows the level of cell surface CFTR, determined as the integrated fluorescence associated with 60–70 cells for each condition. *P<0.05, significant difference between cells grown in the presence of Corr-4 (5 μM) alone compared with cells grown in the presence of both Corr-4 and dynasore; **P<0.05, significant difference between cells grown in the presence of dynasore alone and control (means±SEM, n=4). Quantification was performed using image analysis subroutines of Simple PCI®.

Figure 7
Dynasore augments the actions of Corr-4 on increasing cell surface ΔF508-CFTR

Cells expressing extope–ΔF508 CFTR were incubated for 36 h under the conditions shown. Cells expressing extope–ΔF508 CFTR were grown for 36 h at 26 °C as a positive control (set as 100%). Cells were cooled to 4 °C and incubated with anti-HA antibodies, followed by Cy3-conjugated secondary antibodies. Following fixation in 4% paraformaldehyde, cells were subject to indirect immunofluorescence microscopy. The histogram shows the level of cell surface CFTR, determined as the integrated fluorescence associated with 60–70 cells for each condition. *P<0.05, significant difference between cells grown in the presence of Corr-4 (5 μM) alone compared with cells grown in the presence of both Corr-4 and dynasore; **P<0.05, significant difference between cells grown in the presence of dynasore alone and control (means±SEM, n=4). Quantification was performed using image analysis subroutines of Simple PCI®.

Effects of dynasore on forskolin stimulated short-circuit current

Given the ability of dynasore to increase steady-state surface levels of CFTR in heterologous expression systems, we anticipated that incubation of polarized epithelial cells natively expressing CFTR with dynasore would lead to an increase in cell surface CFTR and a corresponding increase in the Isc (short-circuit current) response to forskolin activated chloride secretion. T84 cells, grown on permeable supports, were incubated with dynasore for 36 h at 37 °C prior to performing cell surface biotinylation and detection of CFTR. T84 cells grown in the presence of dynasore showed a 17% increase in steady-state levels of surface CFTR (Figure 8). To determine whether the observed increase in cell surface CFTR corresponded to an increase in forskolin stimulated Isc, T84 cells grown in the absence or presence of dynasore were mounted in Ussing chambers in the continued presence or absence of dynasore. As expected, in the absence of dynasore, forskolin caused a marked and rapid increase in Isc, consistent with activation of CFTR-mediated chloride secretory pathways (Figure 9A). Addition of DCEBIO (5,6-dichloro-1-ethyl-1,3-dihydro-2H-benzimidazol-2-one), to increase the activity of basolateral potassium channels, further enhanced Isc by creating an increased driving force for apical chloride exit. Subsequent addition of glycine hydrazide (100 μM) to the chambers caused a very rapid inhibition of Isc, consistent with the blockade of CFTR chloride channels (Figure 9A). T84 cells grown for 24 h in the presence of dynasore, and mounted in Ussing chambers in the continued presence of dynasore, displayed a marked reduction in transepithelial resistance; 156±12 Ωcm2 for dynasore treated cells (n=6, means±S.D.) compared with 872±60 Ωcm2 for control cells (n=6, means±S.D.). Activation of transepithelial chloride secretion by sequential additions of forskolin and DCEBIO caused a modest increase in Isc that was markedly smaller than that observed in the absence of dynasore (Figure 9B). What forskolin/DCEBIO stimulated Isc was achieved, was still attributable to CFTR, since the current was still sensitive to the CFTR blocker glycine hydrazide. As with the effects of dynasore on endocytosis, washout of dynasore completely reversed the observed actions of dynasore on forskolin-stimulated Isc and transepithelial resistance. Thus, no differences between control and dynasore washout were observed for maximal forskolin stimulated Isc or transepithelial resistance (results not shown).

Effect of dynasore on cell surface CFTR in polarized epithelial cells

Figure 8
Effect of dynasore on cell surface CFTR in polarized epithelial cells

Polarized T84 cell monolayers were cultured at 37 °C in the presence or absence of dynasore. Plasma membrane CFTR was determined by surface biotinylation followed by immunoblot analysis (A). (B) Combined results for three separate experiments; *P<0.05, n=3 by Student's t test.

Figure 8
Effect of dynasore on cell surface CFTR in polarized epithelial cells

Polarized T84 cell monolayers were cultured at 37 °C in the presence or absence of dynasore. Plasma membrane CFTR was determined by surface biotinylation followed by immunoblot analysis (A). (B) Combined results for three separate experiments; *P<0.05, n=3 by Student's t test.

Effect of dynasore on CFTR-mediated chloride secretion

Figure 9
Effect of dynasore on CFTR-mediated chloride secretion

Polarized T84 cell monolayers were cultured at 37 °C in the absence (A) or presence (B) of dynasore. Cells were mounted in Ussing chambers and baseline values of Isc and Rt were determined. Forskolin (Fsk; 2 μM) was added to both apical and basal chambers, and Isc and Rt were monitored until a new steady state was achieved. DCEBIO (60 μM) was added at the indicated time, and again Isc and Rt values were determined. Glycine hydrazide (Gly-H 101; 100 μM) was added to the apical chamber at the indicated time to block CFTR channel activity. Representative traces are shown.

Figure 9
Effect of dynasore on CFTR-mediated chloride secretion

Polarized T84 cell monolayers were cultured at 37 °C in the absence (A) or presence (B) of dynasore. Cells were mounted in Ussing chambers and baseline values of Isc and Rt were determined. Forskolin (Fsk; 2 μM) was added to both apical and basal chambers, and Isc and Rt were monitored until a new steady state was achieved. DCEBIO (60 μM) was added at the indicated time, and again Isc and Rt values were determined. Glycine hydrazide (Gly-H 101; 100 μM) was added to the apical chamber at the indicated time to block CFTR channel activity. Representative traces are shown.

DISCUSSION

Because altered trafficking of CFTR is responsible for most CF disease, understanding the mechanisms of CFTR trafficking is of therapeutic as well as theoretical importance. Increasing the number of CFTR molecules at the cell surface, either by increasing biosynthetic insertion or by inhibiting endocytic retrieval (or a combination thereof) has clear implications for the identification of pharmacological targets, since ΔF508 CFTR is singularly unstable in the plasma membrane [16,25,26]. Several researchers, including ourselves, have begun to investigate the trafficking machinery regulating the endocytosis of CFTR. Thus it was discovered that direct binding of the Y1424DSI sequence in the C-terminal tail of CFTR to the medium chain subunit of the AP-2 clathrin adaptor complex targets CFTR to clathrin-coated pits [13,14]. In addition, Rab GTPases such as Rab5 and Rab11 have been implicated in the endocytic traffic of CFTR [22,27], with Rab11b being specifically involved in CFTR trafficking in polarized epithelial cells [28]. Despite the importance of being able to manipulate the endocytic trafficking machinery, the ability to do so in a specific, rapid and reversible fashion has until recently been unattainable. For example, studies showing the involvement of AP-2 adaptor proteins and Rab proteins in CFTR trafficking have relied on the expression of dominant-negative or siRNA constructs [14,22,28]. Although such approaches engender the appropriate specificity, it takes many hours (up to 48 h) to see an effect, and the effects are not readily reversible. In contrast, techniques such as cytosolic acidification or potassium depletion to inhibit clathrin-mediated endocytosis are relatively rapid and reversible, but their specificity solely for clathrin-mediated endocytosis is poor. Moreover, the degree to which such protocols inhibit clathrin-mediated endocytosis is variable, nor do they work in all cell types. In contrast, dynasore provides a selective, rapid and reversible inhibitor of the dynamin components of clathrin mediated endocytosis. Previous studies by Cheng et al. [29] using overexpression of dominant-negative (K44A) dynamin have shown an increase in steady-state levels of plasma membrane CFTR, although it was not determined if this was due to an inhibition in CFTR endocytosis. It is, however, likely that such is the case, as our present studies using siRNA-mediated knockdown of dynamin also led to an increase in surface CFTR levels.

Pharmacological correction of the trafficking defect of ΔF508 CFTR is a major focus of both academic and pharmaceutical laboratories. However, given the instability of ΔF508 CFTR at the cell surface, approaches aimed at maintaining therapeutic levels of mutant CFTR at the cell surface are likely to be an important adjunct in the treatment of patients with CF. To this end, our present studies had two aims, (a) to determine the extent to which dynamin played a role in the endocytosis of wild-type and ΔF508 CFTR and (b) to determine if inhibition of CFTR endocytosis would lead to an increase in cell surface CFTR for both wild-type and ΔF508 CFTR, using dynasore as a proof-of-concept for future therapeutic interventions.

We observed that dynasore led to an increase in the number of partially formed coated pits that remained linked to the plasma membrane by either long narrow or wide necks (Figure 3). This is consistent with the results of Macia et al. [17], who showed a 34% increase in the number of AP-2-containing vesicles at the cell surface in the presence of dynasore compared with cells incubated in the absence of dynasore. We observed that dynasore caused a marked inhibition in the endocytosis of wild-type CFTR in a rapid yet reversible manner. This is consistent with our previous observations [9,13] and those of others [10,22,30], identifying the clathrin-mediated pathway as being the major mechanism for retrieval of CFTR from the cell surface. Cells expressing ΔF508 CFTR grown at 37 °C showed little cell surface CFTR. Growth at 26 °C facilitated the trafficking of ΔF508 CFTR to the cell surface, but upon returning the cells to 37 °C, mutant CFTR was rapidly lost from the plasma membrane. Preincubation of temperature-corrected ΔF508 CFTR-expressing cells with dynasore prior to the 27 °C→37 °C temperature shift attenuated the loss of CFTR from the cell surface. Our results also, for the first time, formally invoke dynamin in the endocytic internalization of ΔF508 CFTR from the plasma membrane.

Several small-molecule compounds have been found to be efficacious in facilitating the trafficking of ΔF508 CFTR to the cell surface at 37 °C [20,31]. Indeed, our results confirm that Corr-4 is able to significantly increase the surface levels of ΔF508 CFTR at 37 °C compared with cells grown in the absence of Corr-4. A significant result of our present study is that dynasore too was able to elicit a small increase the amount of ΔF508 CFTR at the cell surface, although in contrast with Corr-4, which probably works on the biosynthetic pathway for ΔF508, dynasore is able to retain at the plasma membrane the small amount of ΔF508 CFTR that is able to escape ER quality control [32]. Moreover, the combination of Corr-4 and dynasore acted together in a synergistic manner to increase significantly the steady-state levels of ΔF508 CFTR at the cell surface. Although our assays focused on endocytic events, it is known that dynamin also participates in exocytosis from the Golgi [33], although different splice variants of dynamin-2 appear to be required for endocytic compared with exocytic events. It is thus possible that the small but significant effect observed for the actions of dynasore on ΔF508 steady-state levels at the cell surface may be an underestimation due to the inhibition of CFTR export from the Golgi.

Since we were able to demonstrate a marked increase in cell surface CFTR in heterologous expression systems in the presence of dynasore, we anticipated that such an increase would also be seen in natively expressing cells and be reflected in an increase in either the rate of onset and/or the magnitude of CFTR-mediated chloride currents. Indeed, growth of the human colonic epithelial cell line T84 in the presence of dynasore led to a small but significant increase in cell surface CFTR. Intriguingly, such increases in surface CFTR protein were not reflected in the electrical properties of the T84 cells, where dynasore led to a decrease in tight junction integrity and a decrease in the level of forskolin-stimulated chloride currents. Recent evidence has shown that the maintenance and integrity of epithelial junctional complexes is very much dependent upon the continued endocytosis and recycling of junctional proteins, even in apparently stable confluent epithelial monolayers [34], and that endocytosis of junctional proteins such as occludins, claudins and ZO (zona occludens)-1 in T84 cells is dependent upon clathrin-mediated endocytosis [35]. Indeed, it has recently been shown that there is a direct coupling between endocytic efficiency and the stabilization of junctions and intercellular adhesion in polarized MDCK (Madin–Darby canine kidney) cells [36]. Thus inhibition of endocytosis by dynasore (and hence recycling) is likely to lead to a loss of junctional integrity and a loss of transepithelial electrical resistance, as we observed. Dynasore functions as a non-competitive inhibitor of dynamin's GTPase activity by interfering with the hydrolysis of GTP rather than preventing the binding of GTP [17]. Manavalan et al. [37] noted sequence similarities between the NBDs (nucleotide-binding domains) of CFTR and GTP-binding proteins. Moreover, Carson and Welsh [38] have argued that the terminal glutamine in the LSGGQ motif of CFTR's NBDs corresponds to a highly conserved residue in GTP-binding proteins that directly catalyse the hydrolysis of GTP. It is therefore possible that dynasore recognizes a surface on CFTR's NBDs that corresponds with GTP-interacting surfaces on GTPases or a surface adjacent to the GTP-interacting surface. Dynasore could thus prevent ATP hydrolysis in CFTR by blocking the release of ADP and Pi from the NBDs in a similar fashion to blocking the release of GDP and Pi in dynamin. A direct evaluation of dynasore on CFTR gating would require extensive additional studies beyond the scope of the present manuscript. As with endocytic events, the effects of dynasore on Isc and transepithelial resistance were also completely reversible. Such data is consistent with the notion that dynasore is acting as a specific enzyme inhibitor rather than a cellular toxin. Moreover, these observations are consistent with those of Macia et al. [17], where dynasore treatment led to inhibition of cell migration; an inhibition that was completely reversed upon dynasore washout.

In summary, we have shown that both wild-type and ΔF508 CFTR are dependent upon dynamin for their endocytic removal from the cell surface. Moreover, the cell surface stability of ΔF508 CFTR following either temperature correction or pharmacological correction is significantly enhanced by preventing the endocytic retrieval of ΔF508. Intriguingly, dynasore alone was able to modestly increase ΔF508 CFTR levels at the cell surface, suggesting that some mutant CFTR was able to escape ER quality control and be ‘trapped’ at the cell surface. Since normalization of apical membrane chloride conductance is the goal of pharmacological therapy for patients with CF, maintaining normal (or possibly elevated) levels of ΔF508 at the cell surface has clear therapeutic implications. Although dynasore itself is not a viable pharmacological agent for the treatment of CF, our studies nonetheless provide a strong proof-of-concept that inhibiting CFTR internalization can be an important adjunct to the armamentarium of CF clinical care. Identifying small molecule compounds that are selective for inhibiting CFTR endocytosis thus becomes an important area of investigation.

We thank Dr Tomas Kirchhausen for the initial gift of dynasore. We thank the Cystic Fibrosis Foundation Therapeutics Network for subsequent provision of dynasore and Corr-4. We thank Matt Green for performing the short circuit current experiments.

Abbreviations

     
  • CF

    cystic fibrosis

  •  
  • CFTR

    CF transmembrane conductance regulator

  •  
  • Corr-4

    {2-(5-chloro-2-methoxy-phenylamino)-4′-methyl-[4,5′]-bithiazolyl-2′-yl}-phenyl-methanonone

  •  
  • Cy3

    indocarbocyanine

  •  
  • DAPI

    4′,6-diamidino-2-phenylindole

  •  
  • DCEBIO

    5,6-dichloro-1-ethyl-1,3-dihydro-2H-benzimidazol-2-one

  •  
  • ER

    endoplasmic reticulum

  •  
  • HA

    haemagglutinin

  •  
  • Isc

    short-circuit current

  •  
  • MESNA

    2-mercaptoethanesulfonic acid

  •  
  • NBD

    nucleotide-binding domain

  •  
  • NHS-SS-biotin

    sulfosuccinimidyl-2-(biotinamido)ethyl-1,3-dithiopropionate

  •  
  • ROI

    region of interest

  •  
  • Rt

    electrical resistance of the tissue

  •  
  • siRNA

    small interfering RNA

  •  
  • TEM

    transmission electron microscopy

AUTHOR CONTRIBUTION

Andrew Young performed all uptake studies and imaging. Martina Gentzsch generated and characterized the extope CFTR construct. Cynthia Abban and Patricio Meneses generated the electron microscopy data. Robert Bridges generated the electrophysiological data. Ying Jia performed cell culture and generated dynamin constructs. Neil Bradbury oversaw all aspects of the present study.

FUNDING

This study was funded by grants from the Cystic Fibrosis Foundation [grant numbers BRADBU05GO (to N. A. B.) and GENTZS507GO (to M. G.).

References

References
1
Damke
H.
Baba
T.
Warnock
D. E.
Schmid
S. L.
Induction of mutant dynamin specifically blocks endocytic coated vesicle formation
J. Cell. Biol.
1994
, vol. 
127
 (pg. 
915
-
934
)
2
Praefcke
G. J.
McMahon
H. T.
The dynamin superfamily: universal membrane tubulation and fission molecules?
Nat. Rev. Mol. Cell Biol.
2004
, vol. 
5
 (pg. 
133
-
147
)
3
Kirchhausen
T.
Cell biology. Boa constrictor or rattlesnake?
Nature
1999
, vol. 
398
 (pg. 
470
-
471
)
4
Zhang
P.
Hinshaw
J. E.
Three-dimensional reconstruction of dynamin in the constricted state
Nat. Cell Biol.
2001
, vol. 
3
 (pg. 
922
-
926
)
5
Marks
B.
Stowell
M. H.
Vallis
Y.
Mills
I. G.
Gibson
A.
Hopkins
C. R.
McMahon
H. T.
GTPase activity of dynamin and resulting conformation change are essential for endocytosis
Nature
2001
, vol. 
410
 (pg. 
231
-
235
)
6
Welsh
M.
Ramsey
B.
Scriver
C.
Beaudet
A.
Valle
D.
Cystic fibrosis
The Metabolic and Molecular Basis of Inherited Disease
2001
New York
McGraw-Hill
(pg. 
5121
-
5188
)
7
Cheng
S. H.
Gregory
R. J.
Marshall
J.
Paul
S.
Souza
D. W.
White
G. A.
O'Riordan
C. R.
Smith
A. E.
Defective intracellular transport and processing of CFTR is the molecular basis of most cystic fibrosis
Cell
1990
, vol. 
63
 (pg. 
827
-
834
)
8
Ameen
N.
Silvis
M.
Bradbury
N. A.
Endocytic trafficking of CFTR in health and disease
J. Cyst. Fibros.
2007
, vol. 
6
 (pg. 
1
-
14
)
9
Bradbury
N. A.
Clark
J. A.
Watkins
S. C.
Widnell
C. C.
Smith
H. S. T.
Bridges
R. J.
Characterization of the internalization pathways for the cystic fibrosis transmembrane conductance regulator
Am. J. Physiol.
1999
, vol. 
276
 (pg. 
L659
-
L668
)
10
Lukacs
G. L.
Segal
G.
Kartner
N.
Grinstein
S.
Zhang
F.
Constitutive internalization of cystic fibrosis transmembrane conductance regulator occurs via clathrin-dependent endocytosis and is regulated by protein phosphorylation
Biochem. J.
1997
, vol. 
328
 (pg. 
353
-
361
)
11
Hu
W.
Howard
M.
Lukacs
G. L.
Multiple endocytic signals in the C-terminal tail of the cystic fibrosis transmembrane conductance regulator
Biochem. J.
2001
, vol. 
354
 (pg. 
561
-
572
)
12
Prince
L. S.
Peter
K.
Hatton
S. R.
Zaliauskiene
L.
Cotlin
L. F.
Clancy
J. P.
Marchase
R. B.
Collawn
J. F.
Efficient endocytosis of the cystic fibrosis transmembrane conductance regulator requires a tyrosine-based signal
J. Biol. Chem.
1999
, vol. 
274
 (pg. 
3602
-
3609
)
13
Weixel
K. M.
Bradbury
N. A.
The carboxyl terminus of the cystic fibrosis transmembrane conductance regulator binds to AP-2 clathrin adaptors
J. Biol. Chem.
2000
, vol. 
275
 (pg. 
3655
-
3660
)
14
Weixel
K. M.
Bradbury
N. A.
Mu 2 binding directs the cystic fibrosis transmembrane conductance regulator to the clathrin-mediated endocytic pathway
J. Biol. Chem.
2001
, vol. 
276
 (pg. 
46251
-
46259
)
15
Swiatecka-Urban
A.
Brown
A.
Moreau-Marquis
S.
Renuka
J.
Coutermarsh
B.
Barnaby
R.
Karlson
K. H.
Flotte
T. R.
Fukuda
M.
Langford
G. M.
Stanton
B. A.
The short apical membrane half-life of rescued ΔF508-cystic fibrosis transmembrane conductance regulator (CFTR) results from accelerated endocytosis of ΔF508-CFTR in polarized human airway epithelial cells
J. Biol. Chem.
2005
, vol. 
280
 (pg. 
36762
-
36772
)
16
Sharma
M.
Pampinella
F.
Nemes
C.
Benharouga
M.
So
J.
Du
K.
Bache
K. G.
Papsin
B.
Zerangue
N.
Stenmark
H.
Lukacs
G. L.
Misfolding diverts CFTR from recycling to degradation: quality control at early endosomes
J. Cell Biol.
2004
, vol. 
164
 (pg. 
923
-
933
)
17
Macia
E.
Erlich
M.
Massoi
R.
Boucrot
E.
Brunner
C.
Kirchhausen
T.
Dynasore, a cell-permeable inhibitor of dynamin
Dev. Cell
2006
, vol. 
10
 (pg. 
839
-
850
)
18
Abban
C. Y.
Bradbury
N. A.
Meneses
P. I.
HPV16 and BPV1 infection can be blocked by the dynamin inhibitor dynasore
Am. J. Ther.
2008
, vol. 
15
 (pg. 
304
-
311
)
19
Lu
W.
Ma
H.
Sheng
Z. H.
Mochida
S.
Dynamin and activity regulate synaptic vesicle recycling in sympathetic neurons
J. Biol. Chem.
2009
, vol. 
284
 (pg. 
1930
-
1937
)
20
Pedemonte
N.
Lukacs
G. L.
Du
K.
Caci
E.
Zegarra-Moran
O.
Galietta
L. J.
Verkman
A. S.
Small-molecule correctors of defective ΔF508-CFTR cellular processing identified by high-throughput screening
J. Clin. Invest.
2005
, vol. 
115
 (pg. 
2564
-
2571
)
21
Muanprasat
C.
Sonawane
N. D.
Salinas
D.
Taddei
A.
Galietta
L. J.
Verkman
A. S.
Discovery of glycine hydrazide pore-occluding CFTR inhibitors: mechanism, structure-activity analysis, and in vivo efficacy
J. Gen. Physiol.
2004
, vol. 
124
 (pg. 
125
-
137
)
22
Gentzsch
M.
Chang
X. B.
Cui
L.
Wu
Y.
Ozols
V. V.
Choudhury
A.
Pagano
R. E.
Riordan
J. R.
Endocytic trafficking routes of wild type and ΔF508 cystic fibrosis transmembrane conductance regulator
Mol. Biol. Cell
2004
, vol. 
15
 (pg. 
2684
-
2696
)
23
Weixel
K. M.
Bradbury
N. A.
Skach
W. R.
Analysis of CFTR endocytosis by cell surface biotinylation
Cystic Fibrosis Methods and Protocols
2002
Totowa
Humana Press
(pg. 
323
-
340
)
24
Hanover
J.
Willingham
M.
Pastan
I.
Kinetics of transit of transferrin and epidermal growth factor through clathrin-coated membranes
Cell
1984
, vol. 
39
 (pg. 
283
-
293
)
25
Heda
G. D.
Tanwani
M.
Marino
C. R.
The ΔF508 mutation shortens the biochemical half-life of plasma membrane CFTR in polarized epithelial cells
Am. J. Physiol.
2001
, vol. 
280
 (pg. 
C166
-
C174
)
26
Varga
K.
Goldstein
R. F.
Jurkuvenaite
A.
Chen
L.
Matalon
S.
Sorscher
E. J.
Bebok
Z.
Collawn
J. F.
Enhanced cell-surface stability of rescued ΔF508 cystic fibrosis transmembrane conductance regulator (CFTR) by pharmacological chaperones
Biochem. J.
2008
, vol. 
410
 (pg. 
555
-
564
)
27
Swiatecka-Urban
A.
Talebian
L.
Kanno
E.
Moreau-Marquis
S.
Coutermarsh
B.
Hansen
K.
Karlson
K. H.
Barnaby
R.
Cheney
R. E.
Langford
G. M.
, et al. 
Myosin Vb is required for trafficking of the cystic fibrosis transmembrane conductance regulator in Rab11a-specific apical recycling endosomes in polarized human airway epithelial cells
J. Biol. Chem.
2007
, vol. 
282
 (pg. 
23725
-
23736
)
28
Silvis
M. S.
Bertrand
C.
Ameen
N.
Golin-Bisello
F.
Butterworth
M.
Frizzell
R. A.
Bradbury
N. A.
Rab11b regulates apical recycling of CFTR in polarized intestinal epithelial cells
Mol. Biol. Cell
2009
, vol. 
20
 (pg. 
2337
-
2350
)
29
Cheng
J.
Wang
H.
Guggino
W. B.
Modulation of mature cystic fibrosis transmembrane regulator protein by the PDZ domain protein CAL
J. Biol. Chem.
2004
, vol. 
279
 (pg. 
1892
-
1898
)
30
Peter
K.
Varga
K.
Bebok
Z.
McNicholas-Bevensee
C. M.
Schwiebert
L.
Sorscher
E. J.
Schwiebert
E. M.
Collawn
J. F.
Ablation of internalization signals in the carboxyl-terminal tail of the cystic fibrosis transmembrane conductance regulator enhances cell surface expression
J. Biol. Chem.
2002
, vol. 
277
 (pg. 
49952
-
49957
)
31
Van Goor
F.
Straley
K. S.
Cao
D.
Gonzalez
J.
Hadida
S.
Hazlewood
A.
Joubran
J.
Knapp
T.
Makings
L. R.
Miller
M.
, et al. 
Rescue of ΔF508-CFTR trafficking and gating in human cystic fibrosis airway primary cultures by small molecules
Am. J. Physiol.
2006
, vol. 
290
 (pg. 
L1117
-
L1130
)
32
Schultz
B. D.
Frizzell
R. A.
Bridges
R. J.
Rescue of dysfunctional ΔF508-CFTR chloride channel activity by IBMX
J. Membr. Biol.
1999
, vol. 
170
 (pg. 
51
-
66
)
33
Liu
Y. W.
Surka
M. C.
Schroeter
T.
Lukiyanchuk
V.
Schmid
S. L.
Isoform and splice-variant specific functions of dynamin-2 revealed by analysis of conditional knock-out cells
Mol. Biol. Cell.
2008
, vol. 
19
 (pg. 
5347
-
5359
)
34
Nagafuchi
A.
Ishihara
S.
Tsukita
S.
The roles of catenins in the cadherin-mediated cell adhesion: functional analysis of E-cadherin-α catenin fusion molecules
J. Cell. Biol.
1994
, vol. 
127
 (pg. 
235
-
245
)
35
Ivanov
A. I.
Nusrat
A.
Parkos
C. A.
Endocytosis of epithelial apical junctional proteins by a clathrin-mediated pathway into a unique storage compartment
Mol. Biol. Cell
2004
, vol. 
15
 (pg. 
176
-
188
)
36
de Beco
S.
Gueudry
C.
Amblard
F.
Coscoy
S.
Endocytosis is required for E-cadherin redistribution at mature adherens junctions
Proc. Natl. Acad. Sci. U.S.A.
2009
, vol. 
106
 (pg. 
7010
-
7015.
)
37
Manavalan
P.
Dearborn
D. G.
McPherson
J. M.
Smith
A. E.
Sequence homologies between nucleotide binding regions of CFTR and G-proteins suggest structural and functional similarities
FEBS Lett.
1995
, vol. 
366
 (pg. 
87
-
91
)
38
Carson
M. R.
Welsh
M. J.
Structural and functional similarities between the nucleotide-binding domains of CFTR and GTP-binding proteins
Biophys. J.
1995
, vol. 
69
 (pg. 
2443
-
2448
)