The role of SGK and CFTR in acute adaptation to seawater in Fundulus heteroclitus.

Killifish are euryhaline teleosts that adapt to increased salinity by up regulating CFTR mediated Cl(-) secretion in the gill and opercular membrane. Although many studies have examined the mechanisms responsible for long term (days) adaptation to increased salinity, little is known about the mechanisms responsible for acute (hours) adaptation. Thus, studies were conducted to test the hypotheses that the acute homeostatic regulation of NaCl balance in killifish involves a translocation of CFTR to the plasma membrane and that this effect is mediated by serum-and glucocorticoid-inducible kinase (SGK1). Cell surface biotinyation and Ussing chamber studies revealed that freshwater to seawater transfer rapidly (1 hour) increased CFTR Cl(-) secretion and the abundance of CFTR in the plasma membrane of opercular membranes. Q-RT-PCR and Western blot studies demonstrated that the increase in plasma membrane CFTR was preceded by an increase in SGK1 mRNA and protein levels. Seawater rapidly (1 hr) increases cortisol and plasma tonicity, potent stimuli of SGK1 expression, yet RU486, a glucocorticoid receptor antagonist, did not block the increase in SGK1 expression. Thus, in killifish SGK1 does not appear to be regulated by the glucocorticoid receptor. Since SGK1 has been shown to increase the plasma membrane abundance of CFTR in Xenopus oocytes, these observations suggest that acute adaptation (hours) to increased salinity in killifish involves translocation of CFTR from an intracellular pool to the plasma membrane, and that this effect may be mediated by SGK1.

MRP2 and acquired tolerance to inorganic arsenic in the kidney of killifish (Fundulus heteroclitus).

We used proximal tubules isolated from the killifish, Fundulus heteroclitus, to examine the effect of environmentally relevant, sublethal levels of arsenic on the function and expression of MRP2, an ABC transporter that transports xenobiotics into urine, including arsenic-glutathione conjugates. Exposure of fish to arsenic as sodium arsenite (4-14 days) increased both MRP2 expression in the apical membrane of proximal tubules and MRP2-mediated transport activity. The level of MRP2 mRNA was not affected, suggesting a posttranslational mechanism of action. Acute exposure of proximal tubules isolated from control fish to 75-375 ppb arsenic decreased mitochondrial function (inner membrane electrical potential). However, in tubules from fish that were preexposed to arsenic (4-14 days), no such effect on mitochondrial function was observed. Thus, chronic in vivo exposure to arsenic induces mechanisms that protect proximal tubules during subsequent arsenic exposure. Upregulation of MRP2 expression and activity is one likely contributing factor.

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.

Cystic fibrosis transmembrane conductance regulator (CFTR)-mediated Cl(-) secretion across fluid-transporting epithelia is regulated, in part, by modulating the number of CFTR Cl(-) channels in the plasma membrane by adjusting CFTR endocytosis and recycling. However, the mechanisms that regulate CFTR recycling in airway epithelial cells remain unknown, at least in part, because the recycling itineraries of CFTR in these cells are incompletely understood. In a previous study, we demonstrated that CFTR undergoes trafficking in Rab11a-specific apical recycling endosomes in human airway epithelial cells. Myosin Vb is a plus-end-directed, actin-based mechanoenzyme that facilitates protein trafficking in Rab11a-specific recycling vesicles in several cell model systems. There are no published studies examining the role of myosin Vb in airway epithelial cells. Thus, the goal of this study was to determine whether myosin Vb facilitates CFTR recycling in polarized human airway epithelial cells. Endogenous CFTR formed a complex with endogenous myosin Vb and Rab11a. Silencing myosin Vb by RNA-mediated interference decreased the expression of wild-type CFTR and DeltaF508-CFTR in the apical membrane and decreased CFTR-mediated Cl(-) secretion across polarized human airway epithelial cells. A recombinant tail domain fragment of myosin Vb attenuated the plasma membrane expression of CFTR by arresting CFTR recycling. The dominant-negative effect was dependent on the ability of the myosin Vb tail fragment to interact with Rab11a. Taken together, these data indicate that myosin Vb is required for CFTR recycling in Rab11a-specific apical recycling endosomes in polarized human airway epithelial cells.

Targeting CAL as a negative regulator of DeltaF508-CFTR cell-surface expression: an RNA interference and structure-based mutagenetic approach.

PDZ domains are ubiquitous peptide-binding modules that mediate protein-protein interactions in a wide variety of intracellular trafficking and localization processes. These include the pathways that regulate the membrane trafficking and endocytic recycling of the cystic fibrosis transmembrane conductance regulator (CFTR), an epithelial chloride channel mutated in patients with cystic fibrosis. Correspondingly, a number of PDZ proteins have now been identified that directly or indirectly interact with the C terminus of CFTR. One of these is CAL, whose overexpression in heterologous cells directs the lysosomal degradation of WT-CFTR in a dose-dependent fashion and reduces the amount of CFTR found at the cell surface. Here, we show that RNA interference targeting endogenous CAL specifically increases cell-surface expression of the disease-associated DeltaF508-CFTR mutant and thus enhances transepithelial chloride currents in a polarized human patient bronchial epithelial cell line. We have reconstituted the CAL-CFTR interaction in vitro from purified components, demonstrating for the first time that the binding is direct and allowing us to characterize its components biochemically and biophysically. To test the hypothesis that inhibition of the binding site could also reverse CAL-mediated suppression of CFTR, a three-dimensional homology model of the CAL.CFTR complex was constructed and used to generate a CAL mutant whose binding pocket is correctly folded but has lost its ability to bind CFTR. Although produced at the same levels as wild-type protein, the mutant does not affect CFTR expression levels. Taken together, our data establish CAL as a candidate therapeutic target for correction of post-maturational trafficking defects in cystic fibrosis.

Role of glucocorticoid receptor in acclimation of killifish (Fundulus heteroclitus) to seawater and effects of arsenic.

Killifish are euryhaline teleosts that adapt to rapid changes in the salinity of the seawater. It is generally accepted that acclimation to seawater is mediated by cortisol activation of the glucocorticoid receptor (GR), which stimulates CFTR mRNA expression and CFTR-mediated Cl- secretion by the gill. Because there is no direct evidence in killifish that the GR stimulates CFTR gene expression, quantitative PCR studies were conducted to test the hypothesis that cortisol activation of GR upregulates CFTR mRNA expression and that this response is required for acclimation to seawater. Inhibition of the GR by RU-486 prevented killifish from acclimating to increased salinity and blocked the increase in CFTR mRNA. In contrast, inhibition of the mineralocorticoid receptor by spironolactone had no effect on acclimation to seawater. Thus acclimation to increased salinity in killifish requires signaling via the GR and includes an increase in CFTR gene expression. Because arsenic, a toxic metalloid that naturally occurs in the aquatic environment, has been shown to disrupt GR transcriptional regulation in avian and mammalian systems, studies were also conducted to determine whether arsenic disrupts cortisol-mediated activation of CFTR gene expression in this in vivo fish model and thereby blocks the ability of killifish to acclimate to increased salinity. Arsenic prevented acclimation to seawater and decreased CFTR protein abundance. However, arsenic did not disrupt the GR-induced increase in CFTR mRNA. Thus arsenic blocks acclimation to seawater in killifish by a mechanism that does not disrupt GR-mediated induction of CFTR gene expression.

The short apical membrane half-life of rescued {Delta}F508-cystic fibrosis transmembrane conductance regulator (CFTR) results from accelerated endocytosis of {Delta}F508-CFTR in polarized human airway epithelial cells.

The most common mutation in the cystic fibrosis transmembrane conductance regulator (CFTR) gene in individuals with cystic fibrosis, DeltaF508, causes retention of DeltaF508-CFTR in the endoplasmic reticulum and leads to the absence of CFTR Cl(-) channels in the apical plasma membrane. Rescue of DeltaF508-CFTR by reduced temperature or chemical means reveals that the DeltaF508 mutation reduces the half-life of DeltaF508-CFTR in the apical plasma membrane. Because DeltaF508-CFTR retains some Cl(-) channel activity, increased expression of DeltaF508-CFTR in the apical membrane could serve as a potential therapeutic approach for cystic fibrosis. However, little is known about the mechanisms responsible for the short apical membrane half-life of DeltaF508-CFTR in polarized human airway epithelial cells. Accordingly, the goal of this study was to determine the cellular defects in the trafficking of rescued DeltaF508-CFTR that lead to the decreased apical membrane half-life of DeltaF508-CFTR in polarized human airway epithelial cells. We report that in polarized human airway epithelial cells (CFBE41o-) the DeltaF508 mutation increased endocytosis of CFTR from the apical membrane without causing a global endocytic defect or affecting the endocytic recycling of CFTR in the Rab11a-specific apical recycling compartment.

PDZ domain interaction controls the endocytic recycling of the cystic fibrosis transmembrane conductance regulator.

The C terminus of CFTR contains a PDZ interacting domain that is required for the polarized expression of cystic fibrosis transmembrane conductance regulator (CFTR) in the apical plasma membrane of polarized epithelial cells. To elucidate the mechanism whereby the PDZ interacting domain mediates the polarized expression of CFTR, Madin-Darby canine kidney cells were stably transfected with wild type (wt-CFTR) or C-terminally truncated human CFTR (CFTR-DeltaTRL). We tested the hypothesis that the PDZ interacting domain regulates sorting of CFTR from the Golgi to the apical plasma membrane. Pulse-chase studies in combination with domain-selective cell surface biotinylation revealed that newly synthesized wt-CFTR and CFTR-DeltaTRL were targeted equally to the apical and basolateral membranes in a nonpolarized fashion. Thus, the PDZ interacting domain is not an apical sorting motif. Deletion of the PDZ interacting domain reduced the half-life of CFTR in the apical membrane from approximately 24 to approximately 13 h but had no effect on the half-life of CFTR in the basolateral membrane. Thus, the PDZ interacting domain is an apical membrane retention motif. Next, we examined the hypothesis that the PDZ interacting domain affects the apical membrane half-life of CFTR by altering its endocytosis and/or endocytic recycling. Endocytosis of wt-CFTR and CFTR-DeltaTRL did not differ. However, endocytic recycling of CFTR-DeltaTRL was decreased when compared with wt-CFTR. Thus, deletion of the PDZ interacting domain reduced the half-life of CFTR in the apical membrane by decreasing CFTR endocytic recycling. Our results identify a new role for PDZ proteins in regulating the endocytic recycling of CFTR in polarized epithelial cells.

Myosin VI regulates endocytosis of the cystic fibrosis transmembrane conductance regulator.

The cystic fibrosis transmembrane conductance regulator (CFTR) is a cyclic AMP-regulated Cl(-) channel expressed in the apical plasma membrane in fluid-transporting epithelia. Although CFTR is rapidly endocytosed from the apical membrane of polarized epithelial cells and efficiently recycled back to the plasma membrane, little is known about the molecular mechanisms regulating CFTR endocytosis and endocytic recycling. Myosin VI, an actin-dependent, minus-end directed mechanoenzyme, has been implicated in clathrin-mediated endocytosis in epithelial cells. The goal of this study was to determine whether myosin VI regulates CFTR endocytosis. Endogenous, apical membrane CFTR in polarized human airway epithelial cells (Calu-3) formed a complex with myosin VI, the myosin VI adaptor protein Disabled 2 (Dab2), and clathrin. The tail domain of myosin VI, a dominant-negative recombinant fragment, displaced endogenous myosin VI from interacting with Dab2 and CFTR and increased the expression of CFTR in the plasma membrane by reducing CFTR endocytosis. However, the myosin VI tail fragment had no effect on the recycling of endocytosed CFTR or on fluid-phase endocytosis. CFTR endocytosis was decreased by cytochalasin D, an actin-filament depolymerizing agent. Taken together, these data indicate that myosin VI and Dab2 facilitate CFTR endocytosis by a mechanism that requires actin filaments.