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Chimeric constructs endow the human CFTR Cl- channel with the gating behavior of murine CFTR.

ABSTRACT: The cystic fibrosis transmembrane conductance regulator (CFTR) is a Cl(-) channel gated by ATP-driven nucleotide-binding domain (NBD) dimerization. Here we exploit species differences between human and murine CFTR to investigate CFTR channel gating. Using homologous recombination, we constructed human-murine CFTR (hmCFTR) chimeras with sequences from NBD1, NBD2, or the regulatory domain (RD) of human CFTR replaced by the equivalent regions of murine CFTR. The gating behavior of hmRD and human CFTR were indistinguishable, whereas hmNBD1 and hmNBD2 had subtle effects on channel gating, prolonging both burst duration and interburst interval. By contrast, hmNBD1+2, containing both NBDs of murine CFTR, reproduced the gating behavior of the subconductance state of murine CFTR, which has dramatically prolonged channel openings. The CFTR potentiator pyrophosphate (PP(i)) enhanced human, hmRD, and hmNBD1 CFTR Cl(-) currents, but not those of hmNBD2, hmNBD1+2, and murine CFTR. By analyzing the rate-equilibrium free-energy relationships of chimeric channels, we obtained snapshots of the conformation of the NBDs during ATP-driven dimerization. Our data demonstrate that the conformation of NBD1 changes before that of NBD2 during channel opening. This finding suggests that NBD dimerization does not proceed by a symmetric tweezer-like motion, but instead in an asymmetric fashion led by NBD1. We conclude that the NBDs of murine CFTR determine the unique gating behavior of its subconductance state, whereas NBD2 controls channel potentiation by PP(i).

SUBMITTER: Scott-Ward TS

PROVIDER: S-EPMC2042212 | biostudies-literature | 2007 Oct

REPOSITORIES: biostudies-literature

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Chimeric constructs endow the human CFTR Cl- channel with the gating behavior of murine CFTR.

Scott-Ward Toby S TS Cai Zhiwei Z Dawson Elizabeth S ES Doherty Ann A Da Paula Ana Carina AC Davidson Heather H Porteous David J DJ Wainwright Brandon J BJ Amaral Margarida D MD Sheppard David N DN Boyd A Christopher AC

Proceedings of the National Academy of Sciences of the United States of America 20071003 41

The cystic fibrosis transmembrane conductance regulator (CFTR) is a Cl(-) channel gated by ATP-driven nucleotide-binding domain (NBD) dimerization. Here we exploit species differences between human and murine CFTR to investigate CFTR channel gating. Using homologous recombination, we constructed human-murine CFTR (hmCFTR) chimeras with sequences from NBD1, NBD2, or the regulatory domain (RD) of human CFTR replaced by the equivalent regions of murine CFTR. The gating behavior of hmRD and human CF ...[more]

PMID: 17913891

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Project description:CFTR is the only ABC (ATP-binding cassette) ATPase known to be an ion channel. Studies of CFTR channel function, feasible with single-molecule resolution, therefore provide a unique glimpse of ABC transporter mechanism. CFTR channel opening and closing (after regulatory-domain phosphorylation) follows an irreversible cycle, driven by ATP binding/hydrolysis at the nucleotide-binding domains (NBD1, NBD2). Recent work suggests that formation of an NBD1/NBD2 dimer drives channel opening, and disruption of the dimer after ATP hydrolysis drives closure, but how NBD events are translated into gate movements is unclear. To elucidate conformational properties of channels on their way to opening or closing, we performed non-equilibrium thermodynamic analysis. Human CFTR channel currents were recorded at temperatures from 15 to 35 degrees C in inside-out patches excised from Xenopus oocytes. Activation enthalpies(DeltaH(double dagger)) were determined from Eyring plots. DeltaH(double dagger) was 117 +/- 6 and 69 +/- 4 kJ/mol, respectively, for opening and closure of partially phosphorylated, and 96 +/- 6 and 73 +/- 5 kJ/mol for opening and closure of highly phosphorylated wild-type (WT) channels. DeltaH(double dagger) for reversal of the channel opening step, estimated from closure of ATP hydrolysis-deficient NBD2 mutant K1250R and K1250A channels, and from unlocking of WT channels locked open with ATP+AMPPNP, was 43 +/- 2, 39 +/- 4, and 37 +/- 6 kJ/mol, respectively. Calculated upper estimates of activation free energies yielded minimum estimates of activation entropies (DeltaS(double dagger)), allowing reconstruction of the thermodynamic profile of gating, which was qualitatively similar for partially and highly phosphorylated CFTR. DeltaS(double dagger) appears large for opening but small for normal closure. The large DeltaH(double dagger) and DeltaS(double dagger) (TDeltaS(double dagger) >/= 41 kJ/mol) for opening suggest that the transition state is a strained channel molecule in which the NBDs have already dimerized, while the pore is still closed. The small DeltaS(double dagger) for normal closure is appropriate for cleavage of a single bond (ATP's beta-gamma phosphate bond), and suggests that this transition state does not require large-scale protein motion and hence precedes rehydration (disruption) of the dimer interface.

Project description:The cystic fibrosis (CF) transmembrane conductance regulator (CFTR) is a member of the ATP-binding cassette transporter superfamily that functions as an epithelial chloride channel. Gating of the CFTR ion conduction pore involves a conserved irreversible cyclic mechanism driven by ATP binding and hydrolysis at two cytosolic nucleotide-binding domains (NBDs): formation of an intramolecular NBD dimer that occludes two ATP molecules opens the pore, whereas dimer disruption after ATP hydrolysis closes it. CFTR dysfunction resulting from inherited mutations causes CF. The most common CF mutation, deletion of phenylalanine 508 (ΔF508), impairs both protein folding and processing and channel gating. Development of ΔF508 CFTR correctors (to increase cell surface expression) and potentiators (to enhance open probability, Po) is therefore a key focus of CF research. The practical utility of 5-nitro-2-(3-phenylpropylamino)benzoate (NPPB), one of the most efficacious potentiators of ΔF508 CFTR identified to date, is limited by its pore-blocking side effect. NPPB-mediated stimulation of Po is unique in that it involves modulation of gating transition state stability. Although stabilization by NPPB of the transition state for pore opening enhances both the rate of channel opening and the very slow rate of nonhydrolytic closure, because of CFTR's cyclic gating mechanism, the net effect is Po stimulation. In addition, slowing of ATP hydrolysis by NPPB delays pore closure, further enhancing Po. Here we show that NPPB stimulates gating at a site outside the pore and that these individual actions of NPPB on CFTR are fully attributable to one or the other of its two complementary molecular parts, 3-nitrobenzoate (3NB) and 3-phenylpropylamine (3PP), both of which stimulate Po: the pore-blocking 3NB selectively stabilizes the transition state for opening, whereas the nonblocking 3PP selectively slows the ATP hydrolysis step. Understanding structure-activity relationships of NPPB might prove useful for designing potent, clinically relevant CFTR potentiators.

Project description:Aquaporins (AQPs) facilitate the transepithelial water flow involved in epithelial fluid secretion in numerous tissues; however, their function in the pancreas is less characterized. Acute pancreatitis (AP) is a serious disorder in which specific treatment is still not possible. Accumulating evidence indicate that decreased pancreatic ductal fluid secretion plays an essential role in AP; therefore, the aim of this study was to investigate the physiological and pathophysiological role of AQPs in the pancreas. Expression and localization of AQPs were investigated by real-time PCR and immunocytochemistry, whereas osmotic transmembrane water permeability was estimated by the dye dilution technique, in Capan-1 cells. The presence of AQP1 and CFTR in the mice and human pancreas were investigated by immunohistochemistry. Pancreatic ductal HCO3- and fluid secretion were studied on pancreatic ducts isolated from wild-type (WT) and AQP1 knock out (KO) mice using microfluorometry and videomicroscopy, respectively. In vivo pancreatic fluid secretion was estimated by magnetic resonance imaging. AP was induced by intraperitoneal injection of cerulein and disease severity was assessed by measuring biochemical and histological parameters. In the mice, the presence of AQP1 was detected throughout the whole plasma membrane of the ductal cells and its expression highly depends on the presence of CFTR Cl- channel. In contrast, the expression of AQP1 is mainly localized to the apical membrane of ductal cells in the human pancreas. Bile acid treatment dose- and time-dependently decreased mRNA and protein expression of AQP1 and reduced expression of this channel was also demonstrated in patients suffering from acute and chronic pancreatitis. HCO3- and fluid secretion significantly decreased in AQP1 KO versus WT mice and the absence of AQP1 also worsened the severity of pancreatitis. Our results suggest that AQP1 plays an essential role in pancreatic ductal fluid and HCO3- secretion and decreased expression of the channel alters fluid secretion which probably contribute to increased susceptibility of the pancreas to inflammation.

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Chimeric constructs endow the human CFTR Cl- channel with the gating behavior of murine CFTR.

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Chimeric constructs endow the human CFTR Cl- channel with the gating behavior of murine CFTR.

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