Increased CFTR expression and function from an optimized lentiviral vector for cystic fibrosis gene therapy

Despite significant advances in cystic fibrosis (CF) treatments, a one-time treatment for this life-shortening disease remains elusive. Stable complementation of the disease-causing mutation with a normal copy of the CF transmembrane conductance regulator (CFTR) gene fulfills that goal. Integrating lentiviral vectors are well suited for this purpose, but widespread airway transduction in humans is limited by achievable titers and delivery barriers. Since airway epithelial cells are interconnected through gap junctions, small numbers of cells expressing supraphysiologic levels of CFTR could support sufficient channel function to rescue CF phenotypes. Here, we investigated promoter choice and CFTR codon optimization (coCFTR) as strategies to regulate CFTR expression. We evaluated two promoters-phosphoglycerate kinase (PGK) and elongation factor 1-α (EF1α)-that have been safely used in clinical trials. We also compared the wild-type human CFTR sequence to three alternative coCFTR sequences generated by different algorithms. With the use of the CFTR-mediated anion current in primary human CF airway epithelia to quantify channel expression and function, we determined that EF1α produced greater currents than PGK and identified a coCFTR sequence that conferred significantly increased functional CFTR expression. Optimized promoter and CFTR sequences advance lentiviral vectors toward CF gene therapy clinical trials.

An elusive adenylate cyclase complicit in cholera is exposed

The intestinal consequences of cholera enterotoxin are caused by activation of the cystic fibrosis transmembrane conductance regulator (CFTR) anion channel through the actions of an as-yet-unknown adenylate cyclase. A new study hunts down this elusive enzyme, showing that mouse and human intestinal epithelium functionally and structurally pair adenylate cyclase isoform 6 (AC6) with CFTR. These findings provide important insights into the molecular mechanisms underlying the robust pathological activation of CFTR activity and promise new opportunities to treat cholera.

Monocyte derived macrophages from CF pigs exhibit increased inflammatory responses at birth

BACKGROUND

We sought to address whether CF macrophages have a primary functional defect as a consequence of CFTR loss and thus contribute to the onset of infection and inflammation observed in CF lung disease.

METHODS

Monocyte derived macrophages (MDMs) were prepared from newborn CF and non-CF pigs. CFTR mRNA expression was quantified by rtPCR and anion channel function was determined using whole cell patch clamp analysis. IL8 and TNFα release from MDMs in response to lipopolysaccharide stimulation was measured by ELISA.

RESULTS

CFTR was expressed in MDMs by Q-rtPCR at a lower level than in epithelial cells. MDMs exhibited functional CFTR current at the cell membrane and this current was absent in CF MDMs. CF MDMs demonstrated an exaggerated response to lipopolysaccharide stimulation.

CONCLUSIONS

In the absence of CFTR function, macrophages from newborn CF pigs exhibit an increased inflammatory response to a lipopolysaccharide challenge. This may contribute to the onset and progression of CF lung disease.

Airway acidification initiates host defense abnormalities in cystic fibrosis mice

Cystic fibrosis (CF) is caused by mutations in the gene that encodes the cystic fibrosis transmembrane conductance regulator (CFTR) anion channel. In humans and pigs, the loss of CFTR impairs respiratory host defenses, causing airway infection. But CF mice are spared. We found that in all three species, CFTR secreted bicarbonate into airway surface liquid. In humans and pigs lacking CFTR, unchecked H(+) secretion by the nongastric H(+)/K(+) adenosine triphosphatase (ATP12A) acidified airway surface liquid, which impaired airway host defenses. In contrast, mouse airways expressed little ATP12A and secreted minimal H(+); consequently, airway surface liquid in CF and non-CF mice had similar pH. Inhibiting ATP12A reversed host defense abnormalities in human and pig airways. Conversely, expressing ATP12A in CF mouse airways acidified airway surface liquid, impaired defenses, and increased airway bacteria. These findings help explain why CF mice are protected from infection and nominate ATP12A as a potential therapeutic target for CF.

Mutating the Conserved Q-loop Glutamine 1291 Selectively Disrupts Adenylate Kinase-dependent Channel Gating of the ATP-binding Cassette (ABC) Adenylate Kinase Cystic Fibrosis Transmembrane Conductance Regulator (CFTR) and Reduces Channel Function in Primary Human Airway Epithelia

The ATP-binding cassette (ABC) transporter cystic fibrosis transmembrane conductance regulator (CFTR) and two other non-membrane-bound ABC proteins, Rad50 and a structural maintenance of chromosome (SMC) protein, exhibit adenylate kinase activity in the presence of physiologic concentrations of ATP and AMP or ADP (ATP + AMP ⇆ 2 ADP). The crystal structure of the nucleotide-binding domain of an SMC protein in complex with the adenylate kinase bisubstrate inhibitor P(1),P(5)-di(adenosine-5') pentaphosphate (Ap5A) suggests that AMP binds to the conserved Q-loop glutamine during the adenylate kinase reaction. Therefore, we hypothesized that mutating the corresponding residue in CFTR, Gln-1291, selectively disrupts adenylate kinase-dependent channel gating at physiologic nucleotide concentrations. We found that substituting Gln-1291 with bulky side-chain amino acids abolished the effects of Ap5A, AMP, and adenosine 5'-monophosphoramidate on CFTR channel function. 8-Azidoadenosine 5'-monophosphate photolabeling of the AMP-binding site and adenylate kinase activity were disrupted in Q1291F CFTR. The Gln-1291 mutations did not alter the potency of ATP at stimulating current or ATP-dependent gating when ATP was the only nucleotide present. However, when physiologic concentrations of ADP and AMP were added, adenylate kinase-deficient Q1291F channels opened significantly less than wild type. Consistent with this result, we found that Q1291F CFTR displayed significantly reduced Cl(-) channel function in well differentiated primary human airway epithelia. These results indicate that a highly conserved residue of an ABC transporter plays an important role in adenylate kinase-dependent CFTR gating. Furthermore, the results suggest that adenylate kinase activity is important for normal CFTR channel function in airway epithelia.

ATP and AMP mutually influence their interaction with the ATP-binding cassette (ABC) adenylate kinase cystic fibrosis transmembrane conductance regulator (CFTR) at separate binding sites

Cystic fibrosis transmembrane conductance regulator (CFTR) is an anion channel in the ATP-binding cassette (ABC) transporter protein family. In the presence of ATP and physiologically relevant concentrations of AMP, CFTR exhibits adenylate kinase activity (ATP + AMP ⇆ 2 ADP). Previous studies suggested that the interaction of nucleotide triphosphate with CFTR at ATP-binding site 2 is required for this activity. Two other ABC proteins, Rad50 and a structural maintenance of chromosome protein, also have adenylate kinase activity. All three ABC adenylate kinases bind and hydrolyze ATP in the absence of other nucleotides. However, little is known about how an ABC adenylate kinase interacts with ATP and AMP when both are present. Based on data from non-ABC adenylate kinases, we hypothesized that ATP and AMP mutually influence their interaction with CFTR at separate binding sites. We further hypothesized that only one of the two CFTR ATP-binding sites is involved in the adenylate kinase reaction. We found that 8-azidoadenosine 5′-triphosphate (8-N₃-ATP) and 8-azidoadenosine 5′-monophosphate (8-N₃-AMP) photolabeled separate sites in CFTR. Labeling of the AMP-binding site with 8-N₃-AMP required the presence of ATP. Conversely, AMP enhanced photolabeling with 8-N₃-ATP at ATP-binding site 2. The adenylate kinase active center probe P¹,P⁵-di(adenosine-5′) pentaphosphate interacted simultaneously with an AMP-binding site and ATP-binding site 2. These results show that ATP and AMP interact with separate binding sites but mutually influence their interaction with the ABC adenylate kinase CFTR. They further indicate that the active center of the adenylate kinase comprises ATP-binding site 2.

A child with progressive multiple tracheal diverticulae: a variation of the Mounier-Kuhn syndrome

Mounier-Kuhn syndrome is a rare disorder characterized by tracheobronchomegaly. Most commonly presenting in adults, a broad spectrum of clinical abnormalities has been described. We report a young woman followed since 4 years of age for respiratory symptoms who was eventually found to have tracheobronchomegaly and multiple tracheal diverticulae.

Demonstration of phosphoryl group transfer indicates that the ATP-binding cassette (ABC) transporter cystic fibrosis transmembrane conductance regulator (CFTR) exhibits adenylate kinase activity

Cystic fibrosis transmembrane conductance regulator (CFTR) is a membrane-spanning adenosine 5'-triphosphate (ATP)-binding cassette (ABC) transporter. ABC transporters and other nuclear and cytoplasmic ABC proteins have ATPase activity that is coupled to their biological function. Recent studies with CFTR and two nonmembrane-bound ABC proteins, the DNA repair enzyme Rad50 and a structural maintenance of chromosome (SMC) protein, challenge the model that the function of all ABC proteins depends solely on their associated ATPase activity. Patch clamp studies indicated that in the presence of physiologically relevant concentrations of adenosine 5'-monophosphate (AMP), CFTR Cl(-) channel function is coupled to adenylate kinase activity (ATP+AMP <==> 2 ADP). Work with Rad50 and SMC showed that these enzymes catalyze both ATPase and adenylate kinase reactions. However, despite the supportive electrophysiological results with CFTR, there are no biochemical data demonstrating intrinsic adenylate kinase activity of a membrane-bound ABC transporter. We developed a biochemical assay for adenylate kinase activity, in which the radioactive γ-phosphate of a nucleotide triphosphate could transfer to a photoactivatable AMP analog. UV irradiation could then trap the (32)P on the adenylate kinase. With this assay, we discovered phosphoryl group transfer that labeled CFTR, thereby demonstrating its adenylate kinase activity. Our results also suggested that the interaction of nucleotide triphosphate with CFTR at ATP-binding site 2 is required for adenylate kinase activity. These biochemical data complement earlier biophysical studies of CFTR and indicate that the ABC transporter CFTR can function as an adenylate kinase.

Role of CFTR's intrinsic adenylate kinase activity in gating of the Cl(-) channel

The cystic fibrosis transmembrane conductance regulator (CFTR) is a Cl(-)channel in the ATP-binding cassette (ABC) transporter protein family. CFTR features the modular design characteristic of ABC transporters, which includes two membrane-spanning domains forming the channel pore, and two ABC nucleotide-binding domains that interact with ATP and contain the enzymatic activity coupled to normal gating. Like other ABC transporters CFTR is an ATPase (ATP + H(2)O --> ADP + Pi). Recent work has shown that CFTR also possesses intrinsic adenylate kinase activity (ATP + AMP left arrow over right arrow ADP + ADP). This finding raises important questions: How does AMP influence CFTR gating? Why does ADP inhibit CFTR current? Which enzymatic activity gates CFTR in vivo? Are there implications for other ABC transporters? This minireview attempts to shed light on these questions by summarizing recent advances in our understanding of the role of the CFTR adenylate kinase activity for channel gating.

Processing and function of CFTR-DeltaF508 are species-dependent

Mutations in the cystic fibrosis transmembrane conductance regulator (CFTR) cause cystic fibrosis. The most common mutation, a deletion of the phenylalanine at position 508 (DeltaF508), disrupts processing of the protein. Nearly all human CFTR-DeltaF508 is retained in the endoplasmic reticulum and degraded, preventing maturation to the plasma membrane. In addition, the F508 deletion reduces the activity of single CFTR channels. Human CFTR-DeltaF508 has been extensively studied to better understand its defects. Here, we adopted a cross-species comparative approach, examining human, pig, and mouse CFTR-DeltaF508. As with human CFTR-DeltaF508, the DeltaF508 mutation reduced the single-channel activity of the pig and mouse channels. However, the mutant pig and mouse proteins were at least partially processed like their wild-type counterparts. Moreover, pig and mouse CFTR-DeltaF508 partially restored transepithelial Cl(-) transport to CF airway epithelia. Our data, combined with earlier work, suggest that there is a gradient in the severity of the CFTR-DeltaF508 processing defect, with human more severe than pig or mouse. These findings may explain some previously puzzling observations in CF mice, they have important implications for evaluation of potential therapeutics, and they suggest new strategies for discovering the mechanisms that disrupt processing of human CFTR-DeltaF508.

Curcumin Stimulates Cystic Fibrosis Transmembrane Conductance Regulator Cl– Channel Activity

Compounds that enhance either the function or biosynthetic processing of the cystic fibrosis transmembrane conductance regulator (CFTR) Cl(-) channel may be of value in developing new treatments for cystic fibrosis (CF). Previous studies suggested that the herbal extract curcumin might affect the processing of a common CF mutant, CFTR-DeltaF508. Here, we tested the hypothesis that curcumin influences channel function. Curcumin increased CFTR channel activity in excised, inside-out membrane patches by reducing channel closed time and prolonging the time channels remained open. Stimulation was dose-dependent, reversible, and greater than that observed with genistein, another compound that stimulates CFTR. Curcumin-dependent stimulation required phosphorylated channels and the presence of ATP. We found that curcumin increased the activity of both wild-type and DeltaF508 channels. Adding curcumin also increased Cl(-) transport in differentiated non-CF airway epithelia but not in CF epithelia. These results suggest that curcumin may directly stimulate CFTR Cl(-) channels.

ADP inhibits function of the ABC transporter cystic fibrosis transmembrane conductance regulator via its adenylate kinase activity

ADP interacts with the nucleotide-binding domains (NBDs) of the cystic fibrosis transmembrane conductance regulator (CFTR) to inhibit its Cl- channel activity. Because CFTR NBD2 has reversible adenylate kinase activity (ATP + AMP<==> ADP + ADP) that gates the channel, we asked whether ADP might inhibit current through this enzymatic activity. In adenylate kinases, binding of the two ADP molecules is cooperative. Consistent with this hypothesis, CFTR current inhibition showed positive cooperativity for ADP. We also found that ADP inhibition of current was attenuated when we prevented adenylate kinase activity with P1,P5-di(adenosine-5') pentaphosphate. Additional studies suggested that adenylate kinase-dependent inhibition involved phosphotransfer between two nucleotide diphosphates. These data indicate that the adenylate kinase reaction at NBD2 contributed to the inhibitory effect of ADP. Finding that ADP inhibits function via an adenylate kinase activity also helps explain the earlier observation that mutations that disrupt adenylate kinase activity also disrupt ADP inhibition. Thus, the results reveal a previously unrecognized mechanism by which ADP inhibits an ABC transporter.