Regulation of CFTR chloride channels by syntaxin and Munc18 isoforms

The cystic fibrosis gene encodes a cyclic AMP-gated chloride channel (CFTR) that mediates electrolyte transport across the luminal surfaces of a variety of epithelial cells. The molecular mechanisms that modulate CFTR activity in epithelial tissues are poorly understood. Here we show that CFTR is regulated by an epithelially expressed syntaxin (syntaxin 1A), a membrane protein that also modulates neurosecretion and calcium-channel gating in brain. Syntaxin 1A physically interacts with CFTR chloride channels and regulates CFTR-mediated currents both in Xenopus oocytes and in epithelial cells that normally express these proteins. The physical and functional interactions between syntaxin 1A and CFTR are blocked by a syntaxin-binding protein of the Munc18 protein family (also called n-Secl). Our results indicate that CFTR function in epithelial cells is regulated by an interplay between syntaxin and Munc18 isoforms.

Enantioselective fluorination of organic molecules. I. Synthetic studies of the agents for electrophilic, enantioselective fluorination of carbanions

In order to develop novel methods for electrophilic and enantioselective fluorination of active methine compounds, preliminary experiments were carried out. The N-tosyl derivative 5 obtained from D-phenylglycine was fluorinated with FClO3 or diluted F2 gas to give the N-fluoro-N-tosyl derivative 6. N-tosyl- or N-mesyl-(S)-alpha-phenethylamine 7 or 8 was subjected to FClO3 fluorination to produce the corresponding N-fluoro derivative, 10 or 11, respectively. Enantioselective fluorination of some methine compounds was attempted employing the above N-fluoro agents. Best result was obtained when 2-benzyl-1-tetralone/KHMDS was treated with 10 to produce the fluorinated tetralone 17 in 53% yield with enantiomeric excess (ee) of 48%.

Synthesis of 2-Deoxy-2-halo-L-ascorbic Acids

2-Iodo- and 2-bromo-2-deoxy-L-ascorbic acids (2 and 3) were prepared by facile halogenation of 2-deoxy-L-ascorbic acid (6) with NIS and NBS, respectively. Likewise, chlorination with NCS produced 2-chloro-2-deoxy-L-ascorbic acid (4), but formation of 4 was accompanied by formation of the dichloro hemiketal 9. Direct fluorination of 6 with 1-chloro-4-fluoro-1,4-diazoniabicyclo[2.2.2]octane bis(tetrafluoroborate) (F-TEDA-BF(4)) gave only the difluoro hemiketal 10. A convenient synthesis of 2-deoxy-2-fluoro-L-ascorbic acid (5) was achieved by an indirect route. Fluorination of 2-bromo-2-deoxy-L-ascorbic acid (3) with F-TEDA-BF(4) produced the bromofluoro hemiketal 16 as a mixture of diastereoisomers. Debromination with tributyltin hydride gave 5 in good yield.

The biogenesis, traffic, and function of the cystic fibrosis transmembrane conductance regulator

The cystic fibrosis transmembrane conductance regulator (CFTR) is a cyclic AMP-activated chloride channel that is encoded by the gene that is defective in cystic fibrosis. This ion channel resides at the luminal surfaces and in endosomes of epithelial cells that line the airways, intestine, and a variety of exocrine glands. In this article we discuss current hypotheses regarding how CFTR functions as a regulated ion channel and how CF mutations lead to disease. We also evaluate the emerging notion that CFTR is a multifunctional protein that is capable of regulating epithelial physiology at several levels, including the modulation of other ion channels and the regulation of intracellular membrane traffic. Elucidating the various functions of CFTR should contribute to our understanding of the pathology in cystic fibrosis, the most common lethal genetic disorder among Caucasians.

Efficiency of plasmid delivery and expression after lipid-mediated gene transfer to human cells in vitro

Cationic liposome-mediated gene transfer has become increasingly important in the development of experimental therapies for human diseases, such as melanoma, human immunodeficiency virus infection, cystic fibrosis and alpha-1 antitrypsin deficiency. However, very little is known about the mechanisms by which lipid-mediated gene transfer occurs. We studied the kinetics of plasmid delivery and expression by using this technique. Plasmid entry in the cystic fibrosis respiratory epithelial cell line 2CFSME0-1 as well as in two other cell lines (HeP 2g and HeLa) occurred in 95 to 100% of cells within 1 hr of the initiation of lipid-mediated gene transfer. In hepatic and respiratory cells, transcription of a construct containing the cystic fibrosis transmembrane conductance regulator was observed in more than 80% of the cell population; similarly high levels of plasmid utilization were obtained in studies of HLA-B7 expression in human melanoma cells. Studies directly relevant to current human trials of lipid-mediated gene transfer indicate that plasmid entry, transcription and translation are often surprisingly efficient, and may occur in nearly 100% of human cells in culture when sensitive methods for detection are used. Furthermore, conventional X-gal immunohistochemistry markedly underestimates transfection efficiency during transient gene expression. These studies point to a new mechanistic understanding of the features that limit expression by using cationic liposomes.

Distinct functional properties of Rab3A and Rab3B in PC12 neuroendocrine cells

Rab3A and Rab3B are highly homologous monomeric GTPases that are putative regulators of exocytosis in those tissues in which they are expressed. We have characterized and directly compared the targeting and functional properties of these isoforms in PC12 neuroendocrine cells. Rab3A and Rab3B both targeted to norepinephrine (NE)-containing large dense core vesicles (LDCVs) when stably expressed in PC12 cells, as determined by immunofluorescence and membrane fractionation. Both Rab3 isoforms also bound to recombinant rabphilin-3A in a GTP-dependent manner. The membrane association of rabphilin-3A was modestly enhanced in Rab3B-expressing PC12 cells relative to Rab3A-overexpressing cells. In addition, overexpression of Rab3A modestly inhibited Ca2+-evoked NE release, whereas Rab3B and a GTP binding mutant (Rab3B N135I) markedly stimulated the efficiency of [3H]NE secretion by PC12 cells (i.e. secretion normalized to total cell radioactivity). Expression of Rab3B and Rab3B N135I increased not only the efficiency of NE secretion but also the accumulation of [3H]NE into LDCVs (i.e. the secretory cargo available for secretion). Neither of these effects was attributable to changes in the numbers of LDCVs nor the docking of LDCVs at the plasma membrane. Our results indicate that Rab3A and Rab3B have similar membrane targeting properties and are capable of interacting with the same putative downstream effector; i.e. rabphilin-3A. However, these isoforms are functionally distinct monomeric GTPases with Rab3B stimulating a late step in Ca2+-evoked secretion when expressed in PC12 cells.

Cyclic AMP and chloride-dependent regulation of the apical constitutive secretory pathway in colonic epithelial cells

Epithelial cells of the colonic crypt engage in cAMP-mediated fluid and electrolyte secretion. In addition to participating in electrolyte transport, colonic crypt cells also synthesize and secrete a number of proteins and peptides that play a crucial role in mucosal homeostasis. In the present study we show that cAMP regulates not only electrolyte secretion but also polarized protein secretion in a tissue culture model of colonic crypt cells. We found that apical but not basolateral protein secretion was stimulated by a physiological activator of the cAMP pathway, vasoactive intestinal peptide, as well as by a cell-permeant analogue of cAMP (8-(4-chlorophenylthio)cAMP) at concentrations as low as 12.5 microM. Based on several criteria, we determined that the regulation of protein secretion by cAMP in HT29-CL19A cells occurs via stimulation of constitutive membrane traffic from the trans-Golgi network (TGN) to the apical cell surface. In addition, the regulation of apical protein secretion by cAMP was Cl--dependent with cAMP inhibiting rather than stimulating secretion in Cl--depleted cells. The locus of cAMP action on the secretory pathway is at least in part at the level of the TGN, where it stimulates the sialylation of alpha1-antitrypsin (i.e. one of the identified secretory proteins) in addition to the traffic of secretory proteins from the TGN to the apical cell surface. We propose that a cyclic AMP and Cl--dependent regulation of TGN acidification could modulate both sialylation and secretory vesicle budding at the TGN.

Accumulation of vitamin C (ascorbate) and its oxidized metabolite dehydroascorbic acid occurs by separate mechanisms

It is unknown whether ascorbate alone (vitamin C), its oxidized metabolite dehydroascorbic acid alone, or both species are transported into human cells. This problem was addressed using specific assays for each compound, freshly synthesized pure dehydroascorbic acid, the specially synthesized analog 6-chloroascorbate, and a new assay for 6-chloroascorbate. Ascorbate and dehydroascorbic acid were transported and accumulated distinctly; neither competed with the other. Ascorbate was accumulated as ascorbate by sodium-dependent carrier-mediated active transport. Dehydroascorbic acid transport and accumulation as ascorbate was at least 10-fold faster than ascorbate transport and was sodium-independent. Once transported, dehydroascorbic acid was immediately reduced intracellularly to ascorbate. The analog 6-chloroascorbate had no effect on dehydroascorbic acid transport but was a competitive inhibitor of ascorbate transport. The Ki for 6-chloroascorbate (2.9-4.4 microM) was similar to the Km for ascorbate transport (9.8-12.6 microM). 6-Chloroascorbate was itself transported and accumulated in fibroblasts by a sodium-dependent transporter. These data provide new information that ascorbate and dehydroascorbic acid are transported into human neutrophils and fibroblasts by two distinct mechanisms and that the compound available for intracellular utilization is ascorbate.

Positron emission tomographic imaging of cardiac sympathetic innervation using 6-[18F]fluorodopamine: initial findings in humans

OBJECTIVES

This study evaluated the safety, efficacy and validity of 6-[18F]fluorodopamine positron emission tomographic scanning of cardiac sympathetic innervation and function in humans.

METHODS

Positron emission tomography (PET) scans, arterial blood and urine were obtained after a 3-min intravenous infusion of 6-[18F]fluorodopamine (1 to 4 mCi, 188 to 809 mCi/mmol) in healthy volunteers, with or without pretreatment with oral desipramine to inhibit neuronal uptake of catecholamines.

RESULTS

6-[18F]Fluorodopamine PET scanning visualized the left ventricular myocardium. Blood pressure increased slightly and transiently. The estimated absorbed radiation dose to the main target organ, the wall of the urinary bladder, was 0.8 to 1.0 rad/mCi of injected 6-[18F]fluorodopamine. By 24 h after the injection, the main 6F-compound in urine was 6F-vanillymandelic acid, a metabolite of 6F-norepinephrine. Desipramine attenuated accumulation of myocardial 6-[18F]fluorodopamine-derived radioactivity and plasma 6F-dihydroxyphenylacetic acid.

CONCLUSIONS

6-[18F]Fluorodopamine produces negligible hemodynamic effects and acceptable radiation exposure at doses that visualize the left ventricular myocardium. Sympathetic nerves take up 6-[18F]fluorodopamine, which is translocated from the axoplasm into storage vesicles, where is it beta-hydroxylated to the fluorinated analogue of the sympathetic neurotransmitter norepinephrine. Therefore, the basis for visualization of myocardium after 6-[18F]fluorodopamine injection in humans is radiolabeling by 6-[18F]fluorodopamine and 6-[18F]fluoronorepinephrine of vesicles in sympathetic terminals. 6-[18F]Fluorodopamine PET scanning provides a novel means for assessing sympathetic innervation and function noninvasively in the human heart.

Syntheses of 2,5- and 2,6-difluoronorepinephrine, 2,5-difluoroepinephrine, and 2,6-difluorophenylephrine: effect of disubstitution with fluorine on adrenergic activity

Synthetic routes to difluorinated analogs of the adrenergic agonists, norepinephrine (NE), epinephrine (E), and phenylephrine (PE) have been developed. The syntheses were based on elaboration of the ethanolamine side chains from the appropriately polyfunctionalized benzaldehydes. The benzaldehydes were prepared from precursor difluorinated benzenes by sequential regioselective lithiations and reaction with electrophiles to introduce hydroxyl and carboxaldehyde functionalities. Binding and functional assay data demonstrate that the 2,6-difluorinated analogs are relatively inactive at both alpha- and beta-adrenergic receptors. These results are consistent with earlier observations that 2-fluoro substitution of adrenergic agonists decreases alpha-adrenergic activity whereas 6-fluoro substitution decreases beta-adrenergic activity.

Stimulation of Cl permeability in colonic crypts of Lieberkühn measured with a fluorescent indicator

Crypts of Lieberkühn were isolated from rabbit distal colon and the halide-sensitive dye 6-methoxy-N-[3-sulfopropyl]quinolinium was used to monitor changes in cell Cl by fluorescence microscopy. Distal colon from rabbits actively secretes Cl and K when stimulated with prostaglandin (PG) E2 but secretes only K in response to epinephrine. The secretagogues PGE2 and epinephrine each produced transient decreases of the apparent cell Cl concentration in about one-half of the crypt cells. Permeability to Cl was assessed by brief substitutions with gluconate or Br in the bath. After stimulation of secretion by PGE2 or epinephrine, Cl efflux and Br influx were increased but only in the cells that exhibited the decrease in cell Cl at the onset of stimulation. Although Cl efflux during gluconate substitution was stimulated similarly with either PGE2 or epinephrine, epinephrine stimulation led to a lower apparent Cl concentration after 2 min of gluconate substitution. Together these results support the concept that a particular epithelial cell type in the crypts responds to secretagogues and that the Cl permeability pathways differ between the secretory states induced by PGE2 and epinephrine.

Rapid purification of recombinant baculovirus using fluorescence-activated cell sorting

Expression of foreign proteins in the baculovirus-insect cell expression system has been limited by difficulties in rapid identification and purification of recombinant virus. Although the process of identifying recombinant virus has been greatly facilitated by the introduction of vectors that lead to insect cell co-expression of beta-galactosidase with foreign genes of interest, isolation of pure recombinant virus using plaque purification may still take several weeks to months to accomplish. Using a fluorescent beta-galactosidase substrate, we have established that insect cells harboring recombinant virus can be rapidly isolated using fluorescence-activated cell sorting. Pure recombinant virus can then be readily obtained using this cellular fraction, with a pure viral culture generally obtained within 2-3 weeks of insect cell transfection.

Radiation damage of fluorinated organic compounds measured by parallel electron energy loss spectroscopy

The sensitivity of fluorinated organic compounds to radiation damage in the electron microscope was measured by electron energy loss spectroscopy (EELS). Five classes of molecules were investigated with fluorine atoms situated on: (1) an aliphatic chain, (2) an aromatic ring, (3) a trifluoromethyl group on an aromatic ring, (4) a trifluoromethyl group on a heterocyclic ring, and (5) a trifluoromethyl group next to a carbonyl group. The damage dose for fluorine loss was found to depend strongly on the position of the fluorine atoms and on specimen temperature. For poly-substituted fluorine on an aliphatic chain, approximately half of the fluorine was retained at doses in excess of 10(7) e/nm2. At room temperature molecules containing trifluoromethyl groups on aromatic or heterocyclic rings were much more sensitive to fluorine loss than compounds having fluorine substituted directly on the rings. This behavior is consistent with the relatively low chemical stability of the trifluoromethyl group in these structures. The rapid loss of fluorine at low electron doses (approximately 10(4) e/nm2) in the trifluoromethyl aromatic compounds was reduced several orders of magnitude by cooling the specimen to liquid-nitrogen temperatures. An understanding of how specific types of fluorinated compounds suffer damage under the electron beam will be important in their potential application as biochemical or pharmacological probes.

Quantitation of alveolar distribution of liposome-entrapped antioxidant enzymes

Liposome-encapsulated Cu,Zn superoxide dismutase (Cu,Zn SOD) and catalase (CAT) were instilled intratracheally in rabbits, and the temporal and spatial distribution of Cu,Zn SOD and CAT within the lung was assessed at the organ and cellular levels. Specific activities of Cu,Zn SOD and CAT were increased in both lung homogenates and isolated alveolar type II pneumocytes. Peak Cu,Zn SOD activities in lung homogenates and alveolar type II cells were observed 4 h after liposome instillation and returned to control levels by 24 h, whereas CAT activities remained significantly above controls. There were no significant differences in liposome distribution or antioxidant enzyme uptake among lung lobes. The distribution of fluorescently labeled Cu,Zn SOD and CAT was assessed with the use of epifluorescence microscopy and digital image processing to determine patterns of cellular incorporation of liposome-entrapped Cu,Zn SOD and CAT within the lung. Although the mean fluorescence intensity of alveoli from rabbits instilled with liposomes containing labeled Cu,Zn SOD and CAT was greater than autofluorescence observed with either no liposome or empty liposome instillation, fluorescence intensity varied between adjacent alveoli. Both fluorescently labeled Cu,Zn SOD and CAT were located cytosolically, and uptake was not limited to alveolar type II pneumocytes. These results demonstrate that a single intratracheal instillation of liposomes can effect increases in Cu,Zn SOD and CAT activities in distal lung cells, including alveolar type I and type II cells and macrophages.

In vivo imaging of insulin receptors in monkeys using 18F-labeled insulin and positron emission tomography

We previously described a prosthetic group methodology for incorporating 18F into peptides and showed that 18F-labeled insulin (18F-insulin) binds to insulin receptors on human cells (IM-9 lymphoblastoid cells) with affinity equal to that of native insulin (1). We now report studies using 18F-insulin with positron emission tomography to study binding to insulin receptors in vivo. Positron emission tomography scans were performed in six rhesus monkeys injected with 0.3-1.4 mCi of 18F-insulin (approximately 0.1 nmol, SA 4-11 Ci/mumol). Integrity of the tracer in blood, determined by immunoprecipitation, was 94% of control for the first 5 min and decreased to 31% by 30 min. Specific, saturable uptake of 18F was observed in the liver and kidney. Coinjection of unlabeled insulin (200 U, approximately 1 nmol) with the 18F-insulin reduced liver and increased kidney uptake of the labeled insulin. Liver radioactivity was decreased by administration of unlabeled insulin at 3 min, but not 5 min, after administration of the tracer, while some kidney radioactivity could be displaced 5 min after injection. Clearance of 18F was predominantly in bile and urine. 18F-insulin is a suitable analogue for studying insulin receptor-ligand interactions in vivo, especially in the liver and kidney.

Immunocytochemical and functional characterization of Na+ conductance in adult alveolar pneumocytes

The purpose of this study was to document the existence, assess the spatial localization, and characterize some of the transport properties of proteins antigenically related to epithelial Na+ channels in freshly isolated rabbit and rat alveolar type II (ATII) cells. ATII cells, isolated by elastase digestion of lung tissue and purified by density-gradient centrifugation, were incubated with polyclonal antibodies raised against Na+ channel protein purified from beef kidney papilla (NaAb), followed by a secondary antibody (goat antirabbit immunoglobulin G conjugated to fluorescein isothiocyanate). Rat ATII cells exhibited specific staining with NaAb at the level of the plasma membrane, which, in most cells, colocalized with that of the lectin Maclura pomiferra agglutinin, an apical surface marker. In Western blots, NaAb specifically recognized a 135 +/- 10-kDa protein in rat ATII membrane vesicles. When patch clamped in the whole cell mode using symmetrical solutions (150 mM Na+ glutamate), ATII cells exhibited outwardly rectified Na+ currents that were diminished by amiloride (10-100 microM) instilled into the bath solution. Ion substitution studies showed that the conductive pathways were three times more permeable to Na+ than K+. Amiloride, benzamil, and 5-(N-ethyl-N-isopropyl)-2',4'-amiloride were equally effective in diminishing 22Na+ flux into rabbit and rat ATII cells (45% inhibition at 100 microM, with IC50 of approximately 1 microM for all inhibitors). Tetraethylammonium chloride (10 mM) or BaCl2 (2 mM), well-known K+ channel blockers, had no effect on 22Na+ uptake. These results indicate that ATII cells express an amiloride-sensitive Na+ conductance, probably a channel, with a lower affinity for amiloride and its structural analogues than the well-established amiloride-sensitive Na+ channels found in bovine renal papila and cultured amphibian A6 kidney cells.

Regulation of plasma membrane recycling by CFTR

The gene that encodes the cystic fibrosis transmembrane conductance regulator (CFTR) is defective in patients with cystic fibrosis. Although the protein product of the CFTR gene has been proposed to function as a chloride ion channel, certain aspects of its function remain unclear. The role of CFTR in the adenosine 3',5'-monophosphate (cAMP)-dependent regulation of plasma membrane recycling was examined. Adenosine 3',5'-monophosphate is known to regulate endocytosis and exocytosis in chloride-secreting epithelial cells that express CFTR. However, mutant epithelial cells derived from a patient with cystic fibrosis exhibited no cAMP-dependent regulation of endocytosis and exocytosis until they were transfected with complementary DNA encoding wild-type CFTR. Thus, CFTR is critical for cAMP-dependent regulation of membrane recycling in epithelial tissues, and this function of CFTR could explain in part the pleiotropic nature of cystic fibrosis.