Richard J. Gregory

Defective intracellular transport and processing of CFTR is the molecular basis of most cystic fibrosis

The gene associated with cystic fibrosis (CF) encodes a membrane-associated, N-linked glycoprotein called CFTR. Mutations were introduced into CFTR at residues known to be altered in CF chromosomes and in residues believed to play a role in its function. Examination of the various mutant proteins in COS-7 cells indicated that mature, fully glycosylated CFTR was absent from cells containing delta F508, delta 1507, K464M, F508R, and S5491 cDNA plasmids. Instead, an incompletely glycosylated version of the protein was detected. We propose that the mutant versions of CFTR are recognized as abnormal and remain incompletely processed in the endoplasmic reticulum where they are subsequently degraded. Since mutations with this phenotype represent at least 70% of known CF chromosomes, we argue that the molecular basis of most cystic fibrosis is the absence of mature CFTR at the correct cellular location.

Adenovirus-mediated gene transfer transiently corrects the chloride transport defect in nasal epithelia of patients with cystic fibrosis

To evaluate the potential of direct transfer of cystic fibrosis transmembrane conductance regulator (CFTR) cDNA for the treatment of cystic fibrosis (CF), we administered an E1-deficient adenovirus, encoding CFTR, to a defined area of nasal airway epithelium of three individuals with CF. This treatment corrected the Cl- transport defect that is characteristic of CF-affected epithelia. After treatment, there was a decrease in the elevated basal transepithelial voltage, and the normal response to a cAMP agonist was restored. We found no evidence of viral replication or virus-associated adverse effects, even at the highest dose tested (25 MOI). These data represent a small step in achieving long-term improvement of CF lung function by gene therapy.

Phosphorylation of the R domain by cAMP-dependent protein kinase regulates the CFTR chloride channel

CFTR, the protein associated with cystic fibrosis, is phosphorylated on serine residues in response to cAMP agonists. Serines 660, 737, 795, and 813 were identified as in vivo targets for phosphorylation by protein kinase A. The SPQ fluorescence assay revealed that mutagenesis of any one of these sites did not affect Cl- channel activity. Indeed, concomitant mutagenesis of three of the four sites still resulted in cAMP-responsive Cl- channel activity. However, mutagenesis of all four sites abolished the response. One interpretation of these results is that the CFTR Cl- channel is blocked by the R domain and that phosphorylation on serines by protein kinase A electrostatically repels the domain, allowing passage of Cl-. The four phosphorylation events appear to be degenerate: no one site is essential for channel activity, and, at least in the case of serine 660, phosphorylation at one site alone is sufficient for regulation of Cl- channel activity.

Nucleoside triphosphates are required to open the CFTR chloride channel.

The CFTR Cl- channel contains two predicted nucleotide-binding domains (NBD1 and NBD2); therefore, we examined the effect of ATP on channel activity. Once phosphorylated by cAMP-dependent protein kinase (PKA), channels required cytosolic ATP to open. Activation occurred by a PKA-independent mechanism. ATP gamma S substituted for ATP in PKA phosphorylation, but it did not open the channel. Several hydrolyzable nucleotides (ATP greater than GTP greater than ITP approximately UTP greater than CTP) reversibly activated phosphorylated channels, but nonhydrolyzable analogs and Mg(2+)-free ATP did not. Studies of CFTR mutants indicated that ATP controls channel activity independent of the R domain and suggested that hydrolysis of ATP by NBD1 may be sufficient for channel opening. The finding that nucleoside triphosphates regulate CFTR begins to explain why CF-associated mutations in the NBDs block Cl- channel function.

Angiogenesis Is Induced in a Rabbit Model of Hindlimb Ischemia by Naked DNA Encoding an HIF-1α/VP16 Hybrid Transcription Factor

BackgroundHypoxia-inducible factor-1 (HIF-1) is a heterodimeric transcription factor that regulates expression of genes involved in O2 homeostasis, including vascular endothelial growth factor (VEGF), a potent stimulator of angiogenesis. We sought to exploit this native adaptive response to hypoxia as a treatment for chronic ischemia. Methods and ResultsA hybrid protein consisting of DNA-binding and dimerization domains from the HIF-1&agr; subunit and the transactivation domain from herpes simplex virus VP16 protein was constructed to create a strong, constitutive transcriptional activator. After transfection into HeLa, C6, and Hep3B cells, this chimeric transcription factor was shown to activate expression of the endogenous VEGF gene, as well as several other HIF-1 target genes in vitro. The bioactivity of HIF-1&agr;/VP16 hybrid gene transfer in vivo was examined in a rabbit model of hindlimb ischemia. Administration of HIF-1&agr;/VP16 was associated with significant improvements in calf blood pressure ratio, angiographic score, resting and maximal regional blood flow, and capillary density (all P <0.01). ConclusionsThe HIF-1&agr;/VP16 hybrid transcription factor is able to promote significant improvement in perfusion of an ischemic limb. These results confirm the feasibility of a novel approach for therapeutic angiogenesis in which neovascularization may be achieved indirectly by use of a transcriptional regulatory strategy.

Hypoxia-Inducible Factor-1 Mediates Activation of Cultured Vascular Endothelial Cells by Inducing Multiple Angiogenic Factors

Abstract— Hypoxia-inducible factor-1 (HIF-1) mediates transcriptional activation of vascular endothelial growth factor (VEGF) and other hypoxia-responsive genes. Transgenic expression of a constitutively stable HIF-1&agr; mutant increases the number of vascular vessels without vascular leakage, tissue edema, or inflammation. This study aimed to investigate the molecular basis by which HIF-1 mediates the angiogenic response to hypoxia. In primary human endothelial cells, hypoxia, desferrioxamine, or infection with Ad2/HIF-1&agr;/VP16, an adenoviral vector encoding a constitutively stable hybrid form of HIF-1&agr;, increased the mRNA and protein levels of VEGF, angiopoietin-2 (Ang-2), and angiopoietin-4 (Ang-4). Infection with Ad2/CMVEV (a control vector expressing no transgene) had no effect. Angiopoietin-1 (Ang-1) expression was not detected in human endothelial cells. Ang-4 was also induced by hypoxia or Ad2/HIF-1&agr;/VP16 in human cardiac cells, whereas Ang-1 expression remained unchanged. Recombinant Ang-4 protein protected endothelial cells against serum starvation-induced apoptosis and increased cultured endothelial cell migration and tube formation. Ad2/HIF-1&agr;/VP16 stimulated endothelial cell proliferation and tube formation. Hypoxia- or Ad2/HIF-1&agr;/VP16-induced tube formation was significantly reduced by a Tie-2 inhibitor. These results suggest that HIF-1 mediates the angiogenic response to hypoxia by upregulating the expression of multiple angiogenic factors. Ang-4 can function similarly as Ang-1 and substitute for Ang-1 to participate in hypoxia-induced angiogenesis. Activation of the angiopoietin/Tie-2 system may play a role in the ability of HIF-1 to induce hypervascularity without excessive permeability.

Safety and efficacy of repetitive adenovirus-mediated transfer of CFTR cDNA to airway epithelia of primates and cotton rats.

Gene therapy for cystic fibrosis (CF) will require the safe transfer of CFTR cDNA to airway epithelia in vivo. We showed previously that a recombinant adenovirus, Ad2/ CFTR–1, expresses CFTR in vitro. As adenovirus rarely integrates, treatment will require repeated vector administration. We applied Ad2/CFTR-1 to intrapulmonary airway epithelia of cotton rats and nasal epithelia of Rhesus monkeys. In both species we detected CFTR mRNA and protein after repeated administration and in monkeys, protein was detected six weeks after repeat administration. The vector did not replicate and was rapidly cleared. Despite an antibody response, there was no evidence of a local or systemic inflammatory response after repeat administration. These data indicate that repetitive administration of Ad2/CFTR-1 is both safe and efficacious.

Maturation and function of cystic fibrosis transmembrane conductance regulator variants bearing mutations in putative nucleotide-binding domains 1 and 2

One feature of the mutations thus far found to be associated with the disease cystic fibrosis (CF) is that many of them are clustered within the first nucleotide-binding domain (NBD) of the CF transmembrane conductance regulator (CFTR). We sought to discover the molecular basis for this clustering by introducing into the two NBDs of CFTR mutations either mimicking amino acid changes associated with CF or altering residues within highly conserved motifs. Synthesis and maturation of the mutant CFTR were studied by transient expression in COS cells. The ability of the altered proteins to generate cyclic AMP-stimulated anion efflux was assessed by using 6-methoxy-N-(sulfopropyl) quinolinium (SPQ) fluorescence measurements in HeLa cells expressing mutated plasmids. The results show that (i) all CF-associated mutants, with one exception, lack functional activity as measured in the SPQ assay, (ii) mutations in NBD1 are more sensitive to the effects of the same amino acid change than are the corresponding mutations in NBD2, (iii) cells transfected with plasmids bearing CF-associated mutations commonly but not exclusively lack mature CFTR, (iv) NBD mutants lacking mature CFTR fail to activate Cl- channels, and (v) the glycosylation of CFTR, per se, is not required for CFTR function. We reason that the structure of NBD1 itself or of the surrounding domains renders it particularly sensitive to mutational changes. As a result, most NBD1 mutants, but only a few NBD2 mutants, fail to mature or lack functional activity. These findings are consistent with the observed uneven distribution of CFTR missense mutations between NBD1 and NBD2 of CF patients.

Identification and regulation of the cystic fibrosis transmembrane conductance regulator-generated chloride channel.

Cystic fibrosis transmembrane conductance regulator (CFTR) generates cAMP-regulated Cl- channels; mutations in CFTR cause defective Cl- channel function in cystic fibrosis epithelia. We used the patch-clamp technique to determine the single channel properties of Cl- channels in cell expressing recombinant CFTR. In cell-attached patches, an increase in cellular cAMP reversibly activated low conductance Cl- channels. cAMP-dependent regulation is due to phosphorylation, because the catalytic subunit of cAMP-dependent protein kinase plus ATP reversibly activated the channel in excised, cell-free patches of membrane. In symmetrical Cl- solutions, the channel had a channel conductance of 10.4 +/- 0.2 (n = 7) pS and a linear current-voltage relation. The channel was more permeable to Cl- than to I- and showed no appreciable time-dependent voltage effects. These biophysical properties are consistent with macroscopic studies of Cl- channels in single cells expressing CFTR and in the apical membrane of secretory epithelia. Identification of the single channel characteristics of CFTR-generated channels allows further studies of their regulation and the mechanism of ion permeation.

Cystic fibrosis transmembrane conductance regulator : a chloride channel with novel regulation

Michael J. Welsh,* Matthew P. Anderson,* Devra P. Rich,* Herbert A. Berger,* Gerene M. Denning,* Lynda S. Ostedgaard,* David N. Sheppard,* Seng H. Cheng,+ Richard J. Gregory,+ and Alan E. Smith+ *Howard Hughes Medical Institute Department of internal Medicine Department of Physiology and Biophysics University of Iowa College of Medicine Iowa City, Iowa 52242 +Genzyme Corporation Framingham, Massachusetts 01701